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Commentator

Join Date: Nov 2008
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Wanted: 115-VAC LED Indicator

08/01/2009 8:45 PM

Does anyone out there in WEB ether know of a simple circuit that would use an LED (ultra bright)approximatly T1 3/4 size, that would indicate the presence of 115 VAC. This will be duplicated and placed in an electrical control box that will have 2 115VAC lines to indicate voltage power "on". There is 12v DC available, but it is isolated from the lines. We must keep it that way.

Would really appreciate some guidance.

Thanks,

The mooseman

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Guru
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#1

Re: Need a 115 Volt AC LED Indicator

08/01/2009 11:52 PM

A resistance in series with LED, connected to 12V will work.take care of polarity.

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Commentator

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#2
In reply to #1

Re: Need a 115 Volt AC LED Indicator

08/02/2009 2:10 PM

There are Two lines coming into a building and to the PCB. Each one is 115VAC, Phase 1 and Phase 2. There is 220 Volts across the two lines. I must have an indicator that displays the presents of these voltages. The simple solution would be to have two 115v lamps. Each lamp would monitor a phase. The powers to be want to use a bright LED. They do not want to be spending effort changing burnt out incandescent bulbs. We are talking thousands of these units in the field. To monitor the +12v would take care of one phase(unless the transformer was open).

I suppose we could design 2 small switching power supplies that ran on 115VAC on the line sides( non isolated), but that seems like a lot of money just to light an LED from 115VAC. Perhaps a small capacitor, diode a few resistors and a Zener may work. I just have not seen anything like that. I'm not sure of reverse voltage parameters on these components.

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Commentator

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#3
In reply to #2

Re: Need a 115 Volt AC LED Indicator

08/02/2009 10:54 PM

Would a pilot light work?

I use a lot of telemecanique ZB4 Series of pilot lights

XB4-BVG1 is an example - 48-120VAC

I have a mini LED (5/16 hole mount) that monitors when my "thermonuclear" pool pump is running...(check my old posts... but I digress) I think i got it from allied electronics (google or Bing it)

the telemech. pilot lights are nice and professional looking, with a relatively low buy-in, and lots of colors available.

One light per phase, or they also make a 230V unit...

good luck

JB

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Commentator

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#4
In reply to #3

Re: Need a 115 Volt AC LED Indicator

08/02/2009 11:36 PM

Here is the problem. My client wants an LED. His experience with Incandescent lamps are the burnout problem. If you have an LED indicating power it probably comes from the low voltage side of your system. We must monitor the voltage from the non-isolated high voltage side.

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Commentator

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#45
In reply to #4

Re: Need a 115 Volt AC LED Indicator

08/03/2009 6:37 AM

the pilot lights i mentioned are LED!

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Commentator

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#115
In reply to #3

Re: Need a 115 Volt AC LED Indicator

08/04/2009 12:59 AM

These devices are more expensive than all the electronics, communication sections , power supplies and microprocessors and UL watertight enclosure combined. Sorry can't use. Two indicators $150.00 US.

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Guru

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#5

Re: Wanted: 115-VAC LED Indicator

08/02/2009 11:38 PM

The way it is done for pulsed optoelectronic devices to get their full brightness is to half wave rectify the 115Vac apply it through a resistor that provides the average current to the chosen LED.

For a 20mA LED driven by 115Vac 60 Hz that would be 40% of 115Vac applied through a 2300 ohm 1 watt resistor and the LED to neutral.

Jon

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Commentator

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#10
In reply to #5

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:14 AM

I do not think the reverse voltage specs on the LED will allow this without ample smoke.

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Guru

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#20
In reply to #10

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:18 AM

Mooseman,

What reverse Voltage?

Did you miss this?

"half wave rectify the 115Vac"

Jon

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Commentator

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#117
In reply to #20

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:07 AM

Yup! missed it.

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Guru

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#51
In reply to #10

Re: Wanted: 115-VAC LED Indicator

08/03/2009 9:32 AM

I think he means that you have a rectifrier (stet) diode in series with the LED and the resistor.

But the resistor would overheat. This arrangement gives 50-mA peak current, which is probably correct for most 20-mA average LEDs, but the worst-case resistor dissipation will be just over 3-Watts (~(115x1.05-3)2/2300/2), not the one Watt he suggested.

The rectifier diode will need to be rated at significantly more than √2 x the highest expected RMS current.

If you want lower dissipation, there are a number of possibilities; you could for example transform the 115-Volts to a more usable 10-Volts, or sense the 240-Volts and run the diode from 12-Volts. However, if the indicator is intended to indicate that the unit is safe to access, I wouldn't trust the latter (in case the 12-Volt is de-powered).

From the aspect of cheapness while still driving a suitably peaky current into the diode, my preference would always be to use a series capacitor to set the current and a low-voltage rectifier diode connected head-to-tail around the LED as suggested in #34 (the second diode need not necessarily be an LED). But you may also need a moderate series resistor to protect against inrush currents at switch-on.
And there may still be reasons your customer might not want this...

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#118
In reply to #51

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:12 AM

This may end up a reasonable compromise. It has been used quite a bit (at least it has been well published) and worth a try. I'll go for it.

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Guru

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#127
In reply to #118

Re: Wanted: 115-VAC LED Indicator

08/04/2009 8:30 AM

We can do a lot better if we add one resistor and one capacitor. Can you afford this? If so, please state what dissipation you can tolerate for each sensor and I'll try to come up with a circuit (and guidance as to matching the current to your optical requirements).

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#6

Re: Wanted: 115-VAC LED Indicator

08/03/2009 12:07 AM

You should be able to use a 115 vac to 12 vac transformer to power the indicatior led light, the power supply would come from your 115 vac curcuit thus idicating presence of the 115 vac curcuit.

Regards Wally

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Guru

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#8
In reply to #6

Re: Wanted: 115-VAC LED Indicator

08/03/2009 12:19 AM

Wally guest,

The LED is a dc device. Applying transfomers requires fuses and a lot of other hardware and you would still need to design the circuitry to operated the LED.

Op said:

a simple circuit that would use an LED (ultra bright)approximatly T1 3/4 size, that would indicate the presence of 115 VAC. This will be duplicated and placed in an electrical control box that will have 2 115VAC lines to indicate voltage power "on".

Jon

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#11
In reply to #8

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:17 AM

Thanks Jon. You said it!

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Commentator

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#7

Re: Wanted: 115-VAC LED Indicator

08/03/2009 12:12 AM

As Rakesh stated, you can use a resistance in series with the LED.

For the Ultrabright LEDs, the maximum continuous current you want to put through them is around 50mA. For longer life, use less current. Assuming they are white LEDs, they are pretty bright at 20 mA.

So for your 115 VAC voltage you would use a 5.615K resistor.

V=I*R,

(V = Line voltage - LED Voltage)

(115V-2.7V)/20mA = 5,615 ohms.

Since you are only getting half cycle,

(57.5-2.7V)/20mA = 2,740 ohms

The closet standard values are 2.7K and 3K 10%. Going with the higher value won't hurt, just in case you are running at 120VAC or higher.

A diode isn't need for the LED takes care of that. The maximum reverse current on it should be around 60 mA.

It will only be on for half cycle, but at 50 or 60Hz you won't notice any flicker. Just wire from hot to neutral. You could go all fancy put a Zener 1N4148 diode in to limit the maximum voltage to 5 Volts reaching the LED. You could also throw in a capacitor in series with the resistor and this would give your LED a soft-start, further extending the life. If you do put a capacitor in, the resistance of the circuit will change Rc=1/wC (w=2*PI*f) and therefore the original resistor value will be the subtracted from the reactance of cap.

The more components, the higher the cost. I have used the resistor diode alone for many applications. If something can cause the LED to blow, it cause other components to go first.

Take a look at any 120VAC relay (Omron, Tele, etc...) that has an indicating LED. It's just the LED and resistor across the coil. What's funny about that is that on several occations I've had to remove the LED and resistor from one and use it on mechanical limit switches when a replacement inductive or capacitive one wasn't available and the customer needed the machine running yesterday. Some don't believe that mechanical limit switches will do the job of the inductive so the LED lights up and I say, "see, lights up just like the other one when the part is there."

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Commentator

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#12
In reply to #7

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:21 AM

Sorry but a 1n4148 is a small switching diode. Are you guys trying to smoke me? I want to live a few more years.

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#13
In reply to #7

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:23 AM

I can handle the current. It's the reverse voltage of the LED that won't handle it.

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Guru

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#15
In reply to #7

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:47 AM

Greenja,

"So for your 115 VAC voltage you would use a 5.615K resistor."

That is not correct. You would have very dim LED and you can't get an average current of 20mA using that high of a resistance when ac voltages apply current on a part time basis.

"V=I*R, (V = Line voltage - LED Voltage)"

That is not a correct formula for operating a line voltage pulsed dc Optoeltronic device .

40 percent of Vrms divided by the steady dc LED current equals the series resistance that delivers the proper pulsed current for equivalent brightness of the LED if it were operated on steady dc. And you don't have to worry about the small voltage drop of the forward biased LED.

LEDs don't mind being pulsed at a higher current if it is the average of the rated current. If one drives an LED with a pulsed current source, the peak luminance can be higher, albeit that the average luminance will not increase. Run the LED at 60Hz, with a duty cycle of 5% and it'll appear twice as bright to humans. Works better on Blue & Green than it does on Red.

It is a four component circuit. Rectifier, Resistor , LED and a nice panel mount socket for the LED. Anything more would be un-necessary and create potential problems.

"1N4148 diode zener diode."

A 1N4148 is not a Zener it is a switching diode and it's reverse breakdown voltage is 75 Volts at 5 microAmps. The ac supply peaks at more than 167Vac.

Adding more components does not satisfy the "simple" part of the request either.

"a capacitor in series with the resistor and this would give your LED a soft-start"

A capacitor in series blocks dc current. The rectified ac sine wave is the soft start.

"120VAC relay (Omron, Tele, etc...) that has an indicating LED. It's just the LED and resistor across the coil."

The data sheets show ac and dc operated relay coils that operate at various ac and dc voltages and some do indeed have resistors in series with the LEDs. some have built in diodes to grab the inductive kick when the current is removed.

120Vac relay seems to refer to the contacts and not the coils.

Jon

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#32
In reply to #15

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:03 AM

Joh, thanks for the comments.

However, you missed the line that gave the voltage at 57.5 Volts and the resistor value at 2.7K or 3K.

Sorry the 5.1V diode should have been a 1N4733, but this is just an option.

A sine wave is only a soft start if you are able to start at zero crossing.

A relay is rated by it's coil voltage. A 120 VAC relay means the coil voltage is 120. As for the contact A, B or C they are in VAC, VDC and horsepower.

As for the cap, it's not an electrolytic (I should have stated that) X2. The sine wave gets rectified by the LED. As for simple solutions, some people like to put in safeties whenever you are introducing line voltages to your controls, especially if microcontrollers are involved. This isn't necessary in this case, but it's something that should be known, better safe than sorry.

Get a relay and take a look at it, it's across the coil, not the contact. When the relay is energized the light comes on. That is the easiest solution. Nothing is simpler and it works, it's been used in industry for many many years.

All you need is a resistor and your LED.

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Guru

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#36
In reply to #32

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:52 AM

Greenja,

Thanks.

Makes sense. Though I was wondering about the reverse voltage / current on the LED in the ac application.

Jon

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#41
In reply to #36

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:25 AM

The reverse current on the LEDs are in the 50mA to 60mA range, the resistor limits the current, forward and reverse to 20 mA so it's not a problem. You could simply wire up and mount the relays anywhere if you want to go that route. Not only that, but you could use the contacts on the relay to wire into an alarm or flashing beacon if power to that circuit fails.

...or double redudancy using the contact to turn on the Ultrabright and resistor in series that is wired from line to neutral.

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Guru

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#42
In reply to #41

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:50 AM

Greenja,

Thanks.

Sounds like special LEDs if they have reverse current at that level.

There are some the have dual LEDs in one and produce 3 different colors depending on dc polarity or ac so they could just as easily pop a reverse diode in there.

Jon

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#43
In reply to #42

Re: Wanted: 115-VAC LED Indicator

08/03/2009 5:34 AM

Those are special LEDs, not the Ultrabrights you are probably using. Ultrabright Specs

If you connect two LEDs to an AC supply (resistors used of course), in opposite directions and with a slow enough frequency, you will see one light turn on when the other light is off. These mult colored LEDs are nothing but two LEDs of different colors wired up as above.

As long as the reverse voltage does not exceed the rating, 5 V for the CML in this case, then the current is less than 100uA. Some are confusing this current as the actual reverse current the LED can be subjected to. This is only the leakage current.

If this was not the case, no LED would last in these kinds of applications. It's a diode, just like any other, high resistance when reverse biased. Exceed the reverse voltage and large current will flow destroying it.

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#120
In reply to #41

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:24 AM

Relays? Did you ever do a failure analysis of a system with a relay. The fail rate is that of the relay.-- very high!

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Guru

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#129
In reply to #120

Re: Wanted: 115-VAC LED Indicator

08/04/2009 8:47 AM

Depends on the relay and the environment. Telephone exchanges had a failure rate (per relay) in single-digit FITs. But I don't really see how they address your basic problem.

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Guru
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#130
In reply to #120

Re: Wanted: 115-VAC LED Indicator

08/04/2009 9:15 AM

Sorry, I was just suggesting an unvoiced alternative. While a poorly designed relay system can be the bane of technicians, they can be more reliable than you think.

But you have both the responsibility of choosing the design and knowledge of all of the criteria that this design must meet, we do not. We on the other hand can't seem to agree how much power a non-linear circuit dissipates or that not all LEDs produce the same amount of light for a given current.

If I were you, I'd either go with an existing pre-packaged indicator. It will have it's own mounting hardware and environmental tolerance criteria. Or I'd go with the combination of a capacitor, resistor, reverse bias diode and the LED. Then buy a sample lot and test for suitability. Repeat until satisfied.

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#49
In reply to #7

Re: Wanted: 115-VAC LED Indicator

08/03/2009 8:46 AM

An L.E.D. is NOT an inductive load that requires a soft start due to high inrush currensts, and it can be run at 100% its rated current with no ramp; in fact you can PWM the supply voltage and it will give you a proportional wattage, but every pulse will sink the same current while ON.

Read post #9, it is the proper solution for OP's circuit.

Yahlasit

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Guru

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#9

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:03 AM

Here is very simple solution I used to replace neon pilot lamps that kept burning out.

Use a 1N4003 diode and a resistor in series with the LED (the LED usually isn't rated to block 115vac, so the diode blocks for you). Find the rated current for the LED, then R=115vac*root2/(led amps). R watts is (led amps)^2*R. Be conservative with the watts, even double for long life.

Experiment with R around the value for various brightness levels.

Resistor watts will be surprisingly large.

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Commentator

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#14
In reply to #9

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:36 AM

This seems to be the best so far. I was hoping to avoid all that heat, but looks like I'm stuck with it. I just thought of a disastrous affect. The new package is in a Poly carbonate, water tight box. One of those electrical boxes with a clear plastic lid. UL of course. There is no way to get the heat of Two 2 watts out of the box. and it must be waterproof. The rest of the circuit in the box is analog and is somewhat sensitive to heat (tracking) I guess I'm still looking for a good solution.

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Guru

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#16
In reply to #14

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:54 AM

OK, lets get creative. We need an equivalent impedance, so: Z=1/jwC, w=2*pi*60, if led current is .02 amps then we need a Z~8K, or C=0.3 uF. No power loss! (OK so I ignored the LED)

Haven't tried this, but I think the principal should work. Cap should be non polarized, 200V rated. We can either run it through Spice or some such program or run out to the bench. Bench is faster and truly practical.

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#21
In reply to #16

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:33 AM

Ok! you have convinced me. I'll give it a try, and thank you and all of you good folk out there. I'll let you how it works out. Now I'm excited. Gotta try it.

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#119
In reply to #16

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:20 AM

I like the bench idea. I've worked with too many spice types that spend days massaging their circuit to get the results they got in 10 minutes at the bench.

I hope I didn't stir up the nest with that statement.

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Guru

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#128
In reply to #119

Re: Wanted: 115-VAC LED Indicator

08/04/2009 8:42 AM

No-resistors is very low-dissipation, but almost certain to be unreliable in use unless you limit inrush currents* . This could be done losslessly with a series inductor, but I suspect series resistors would be more economical, and the resistor dissipation could readily be contained below 200-mW at 20-mA mean LED current.

The trouble with SPICE is that it won't tell you whether the LEDs are bright enough in your environment - nor for this case will it answer questions about practical robustness.

*
Are the capacitors designed to be shorted instantaneously to 200-Volts?
Will the diode protect the LED against reverse Voltage if the circuit is connected instantaneously to 200-Volts?
So you would need to connect and disconnect to the mains several thousand times to be certain such a design is safe for field use. Better to start with a conservative design.

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#17
In reply to #14

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:03 AM

Mooseman,

The polycarb box dissipates the heat for the circuit that is already there and the combination of the Indicator circuit and Analog circuit can combine to provide a stable temperature above ambient in the box.

Also components for temperature compensation can replace or be added to the analog circuit to provide better thermal stability.

Jon

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#116
In reply to #17

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:06 AM

There is presently no Indicators in this model. Thats the problem. The origional designer eliminated the problem by making it non-existant. Other models have lamps but not air tight enclosures. There are many temp compensiating circuits designed into the system now.

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#141
In reply to #116

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:39 PM

Mooseman,

There are many temp compensiating circuits designed into the system now.

And it still has drift. Can't win all of the battles!

Jon

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Guru
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#18
In reply to #14

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:04 AM

This can work without any heat loss except for diode loses. May be you ll have to do few iteration for selecting AC cap.

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#22
In reply to #18

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:35 AM

I'll play with this one, but it looks scary.

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#23
In reply to #22

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:37 AM

Nothing scary, It is beeing used universally.

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#34
In reply to #22

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:26 AM

This will be more cost effective.

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#40
In reply to #34

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:11 AM

Rakesh,

Good one!

They make LEDs like that with two colors in one device. One polarity provides one color, the opposite polarity another color and ac produces a third color by mixing the first two colors and the effect of the persistency of the eye.

Jon

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#44
In reply to #34

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:18 AM

...this one, I have to see.

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#52
In reply to #34

Re: Wanted: 115-VAC LED Indicator

08/03/2009 9:55 AM

That should work briefly. I do not see anything limiting the current to the LED though. So any spike like signal (contact closure at source peak voltage) will blast through the capacitor toasting the LED. You are assuming that your AC will solely contain the fundamental 50/60 hertz value.

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#64
In reply to #52

Re: Wanted: 115-VAC LED Indicator

08/03/2009 11:14 AM

It works thats for sure. I have made few electric candles.

LED offer constant light in wide range of operating current.One can select capacitor for least current of LED at rated voltage.

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#65
In reply to #52

Re: Wanted: 115-VAC LED Indicator

08/03/2009 11:39 AM

You are correct in principle. That means that the capacitor is not adequate on its own. But a 20-mA diode will tolerate both the occasional 150-mA peak and repetitive 90-mA peaks, so the illustrated circuit should be safe with a series capacitor to control average current and a series resistor to limit peaks. Chances are that a series resistor of about 1.5-kOhm was incorporated in the package of the LED, which should be sufficient for the purpose.

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#66
In reply to #65

Re: Wanted: 115-VAC LED Indicator

08/03/2009 11:43 AM

Yes, having both a capacitor and resistor will make for a much more efficient system and solve my rare but possible surge current scenario of a well timed contact closure.

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#26
In reply to #18

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:44 AM

Rakesh semwal,

Why so many diodes? Won't one do?

The proper sized cap will provide a proper pulsed current too.

What size is it?

Jon

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#30
In reply to #26

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:51 AM

Thats for engineers to figure out why not one diode? why so many.

Put a thought I am sure you ll get it.

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#31
In reply to #30

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:55 AM

Rakesh,

Increased pulse rep rate.

The cap takes care of the voltage drop and provides adequate current.

Am I close?

Jon

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#33
In reply to #31

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:11 AM

Only full AC cycle can pass through the capacitors. Thats why four diodes are there.otherwise it wont work,

Capacitor will drop voltage depending what its reactance is.

XC= {1/2 x pie x F x C)}

Increased pulse rep rate? Sorry thats beyond my head.

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#37
In reply to #33

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:59 AM

Rakesh,

A capacitor in series with a single rectifier in series with the LED provides 60 PPS but the capacitor will charge to the peak Voltage and will no longer work without a discharge path.

The bridge rectifier provides 120 PPS and a two way path for the capacitor.

Thanks,

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#38
In reply to #37

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:02 AM

Yes.

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#24
In reply to #14

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:39 AM

Moose,

I think Number 9 ignores a key component in the calulations. It is a pulsed circuit.

However fooling with resistance will get you there. You will find it to be close to 2400 ohms.

Jon

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#19
In reply to #9

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:15 AM

GW,

A little over eight tenths of a watt for each indicator. So a nice 1 watt resistor for each one will do.

115Vac to 2400 ohm 1 watt resistor to 20mA LED to common, according to the formula in the Optoelectronic Handbook.

Thats equivalent to 45 volts dc through about 2400 ohms to the LED.

It will look bright becaused it is pulsed at a higher current for a short duration of each half cycle and the persistence of the eye likes it.

Jon

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#27
In reply to #9

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:46 AM

Gw,

"Resistor watts will be surprisingly large."

For example?

Jon

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#47
In reply to #27

Re: Wanted: 115-VAC LED Indicator

08/03/2009 8:34 AM

Post 9 is the best of all answers, and didn't forget to multiply the resistor current by the suply volts, thats why he says wattage will be kind of high.

You'll have some heat inside the little plastic box.

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#69
In reply to #47

Re: Wanted: 115-VAC LED Indicator

08/03/2009 12:45 PM

Guest,

The Wattage will not be as high as the incorrect calculation.

He did not take into consideration that the supply voltage is not always there because the rectifier only allows a half cycle to be presented to the resistor and LED. Only the peak of the half cycle is providing significant power to the circuit. The rest of the time there is no little or no current. It provides a nice pulse of light at each peak of the half cycle and rests for the rest of the cycle.

That averages out to about 8 tenths of a Watt and a 1 Watt resisitor can handle that just fine.

The applied Voltage averages out to about 45 volts.

Put a rectifier, 2400 ohm resistor and LED in series and connect it to 115Vac and become a believer.

Jon

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#73
In reply to #69

Re: Wanted: 115-VAC LED Indicator

08/03/2009 1:33 PM

Are you trying to maim the poor fellow?

Apart from the fact that you probably mean a 2400-Ohm plus a diode with > 200-Volt reverse-breakdown diode and the LED in series...

You need to cater for a supply Voltage that is above nominal - by at least 5% which gives about 121-V rms.

Then, roughly speaking, the Voltage across the resistor is reduced 3-Volts for the LED and 1-Volt for the diode. So there is something like 117-Volts RMS across the resistor for half the time. So the dissipation you must cater for is 1172/2400/2 = 2.85 Watts. That is both correct and a great deal more than your claimed 0.8 Watts.

(115-Volts rms for half the time is 115/√2 Volts rms, or 81-Volts - about twice your guessed-at value and nearly four times the power)

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#78
In reply to #73

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:12 PM

Physicist,

I have already said Rectifier, 2400 Ohm resistor and 20 mA LED.

Right now I'm using a 1N5363 rectifier a 2700 Ohm resistor with a red LED with 117Vac applied.

I measured it. it is running on my test board. The LED is as bright as it should be and the 1 Watt resistor does gets hot to the touch as I would expect for a 1 watt resistor dissipating 0.87 Watts.

That tells me that 40% of the RMS value of the 117Vac used to calculate the resistance of the circuit to provide the equivalence of steady state dc to provide the equivalent intensity is correct.

Jon

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#81
In reply to #78

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:51 PM

Your power level would be consistent with 117-Volts peak-peak. But that is not the way AC power supplies are generally specified - which is RMS. (And 115-V RMS at 60-Hz is a standard supply in many countries).

The mean current in your LED with a 117-Volt pk-pk supply would be would be about 6.7-mA, and the peak would be about 21-mA. The LED would be quite bright, even though it is not operating near its optimum.

Please confirm what you are actually doing.

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#87
In reply to #81

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:33 PM

Physist?,

The LED is not using an ac power supply.

It is pulsed dc.

You can't use a typical ac formula to calculate for an Optoelectronic device operating on half wave rectified line voltage or pulsed dc.

lt doesn't work.

Jon

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#89
In reply to #87

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:52 PM

I agree entirely that standard formulae seldom work if you use them blindly. However, both GW and I have re-interpreted the formulae so that they apply correctly to the power in the resistor. (The radiance of the LED is more complex, but that is not yet at issue here)

If you communicate measurements without checking the terminology the problems can be even more severe - mainly because the naive are more likely to believe results that are stated as measured than they are to believe calculations (however correct and carefully documented the calculations may be).
The only way you can have as little as 0.8-Watts in your resistor is if you are not doing exactly what you imply. I'm not certain whether it will be the Voltage source, but that is the most obviously ambiguous item, so we should start there.

In spite of multiple requests, you haven't yet confirmed how you are measuring the AC Voltage. Is it using an AC RMS Voltmeter, or a peak Voltmeter, or a scope, or...?

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#94
In reply to #89

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:30 PM

Physicist?,

I used the data from an Optoelectronic book to do my original calculation for operating my LED pilot lights from 115Vac 60Hz power in 1988.

I am using a Fluke 77 and Fluke 12B and Fluke 8050A. I dont have a Simpson 360.

My Scope broke.

Jon

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#100
In reply to #94

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:58 PM

I never trust anything in books unless I can recreate them from first principles - and if necessary check them with detailed measurements

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#103
In reply to #100

Re: Wanted: 115-VAC LED Indicator

08/03/2009 7:32 PM

Physicist?,

Me too. I did it in the lab at work years ago.

A test engineer provided a modification document for validation on test gear used all over the world. As usual there were errors including the pilot LED was hardly glowing.

There was a 10K Ohm resistor in the rectifier, resistor & LED series and the LED socket was made with a 1.3KOhm resistor for 26 Volts dc. I changed the little 10K for a 1.2K 2 Watt and the tester's Pilot LEDs have been running fine since 1988.

I like it when the books are based on first principles and proved in practical application on the bench.

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#82
In reply to #73

Re: Wanted: 115-VAC LED Indicator

08/03/2009 3:56 PM

Physicist?,

I only had a 200BV rectifier on hand.

If the circuit was dumping an average of 2.85Watts it would have burned my 1 watt resistor long ago.

It averages to 0.87Watts because the power in the circuit is only there during the peak of the half wave.

The half wave peaks are about 165 Volts and the effective voltage is about 40% of that or 66Volts.

When the Optoelectronic device gets its rated current it is the peak of the half sinewave that is doing the work for a short time compared to the time of a full sinewave. The increasing voltage in the circuit provides little luminence in the LED until it gets near the peak voltage to provide a current that adequately illuminates the LED to satisfy the persistence of the eye.

Jon

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#77
In reply to #69

Re: Wanted: 115-VAC LED Indicator

08/03/2009 2:15 PM

Ok, lets get technical.

The LED will conduct current for a half cycle, then rest for a half cycle.

The RMS current heats the resistor, not the average.

We will neglect the forward drop of the LED since it is within the voltage variation of the source.

The power dissipated will be

Allowing for 10% overvoltage( 110% of 115vac), and say 8K ohms,

the power is 1 Watt!

Now, if you use the 2.6K ohms, this increases to 3.1 Watts.

Yes, the power dissipation is significant.

To modify the circuit using a capacitor I would change my R + diode + LED

to C + (LED and diode in anti-parallel)

The argument about what happens for a transient is well taken. I believe the cap will be about .3uF. So lets assume the circuit energizes at the peak of the AC wave form. The step in voltage will give you a momentary high current, but, the energy for the energization will still be less than a normal half cycle. I calculate the energy for the step will be equivalent to dissipating the capacitor energy, 0.5*C*Vp^2 or about 5mJ.

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#83
In reply to #77

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:02 PM

Gw,

I see you have not taken measurements.

They will throw you calculations out the window.

Jon

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#86
In reply to #83

Re: Wanted: 115-VAC LED Indicator

08/03/2009 4:07 PM

If you believe the calculations are wrong, provide correct ones. Until you do, we should for safety assume that you are not measuring the same thing the rest of us are writing about.

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#92
In reply to #86

Re: Wanted: 115-VAC LED Indicator

08/03/2009 5:53 PM

Physicist?

I have done so more than once. I guess you missed the entries.

For pulsed Optoelectric devices run off half wave rectified 60Hz ac mains you calculate the series resistance by taking 40% of the applied ac and use that voltage to calculate as though it were a dc circuit.

If it was dc 20mA and 2400 Ohms would be 48 Volts.

I am using a 2800 ohm resistor and it has 52 Volts across it at 17mA with 118Vac applied. That's 0.88Watts.

I am running the circuit now.

Jon

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#95
In reply to #92

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:32 PM

I would be interested in the developement of your rule of thumb for 40%.

Or verify your entire circuit, give us a sketch.

The Average voltage for a half wave rectification is √2 * Vrms/pi = .45*Vrms

Average does not heat the resistor, it is the RMS.

With a simple resistor the conduction is a half sinusoid. It is not just a little shot of current from the peek, as is the case for a diode bridge charging a capacitor like you use for a power supply.

I know the resistors I used ran very hot. 1/4 watt simply discolored and opened.

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#97
In reply to #95

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:53 PM

GW,

√2 * Vrms/pi = .45*Vrms

Yep, I got about 52 volts across 4800 Ohms.

Jon

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#99
In reply to #97

Re: Wanted: 115-VAC LED Indicator

08/03/2009 6:56 PM

Well, it seems you have confirmed GW's suspicions about the basis for your calculations (and that you are indeed driving 115-Volts RMS into the diode). The mystery remains as to why your 1-W resistor isn't overheating.

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#104
In reply to #99

Re: Wanted: 115-VAC LED Indicator

08/03/2009 8:11 PM

Physicist?,

It is because the 20mA LED only gets the needed current during the high amplitude of the positive half wave. Heating only occurs during that time. That's about 60mA. Then it is naptime until it comes around again.

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#105
In reply to #104

Re: Wanted: 115-VAC LED Indicator

08/03/2009 8:23 PM

The point you are missing is that as soon as the line voltage exceeds the fwd drop of the LED the resistor conducts current. This will occur at about 1 to 2 volts, and that is insignificant on 115vac. Therefore essentially current conducts for the full half cycle.

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#106
In reply to #105

Re: Wanted: 115-VAC LED Indicator

08/03/2009 8:51 PM

GW,

But until it reaches 10mA it has little meaning to the eye of the beholder who sees the LED. And isn't that only about a quarter of a Watt?

As I have said before, the "beef" of the thing is during a short period around the positive peak of the sine wave and that is primarily in the more dc part of the half sine wave.

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#111
In reply to #106

Re: Wanted: 115-VAC LED Indicator

08/03/2009 11:54 PM

You are correct about how much total power is delivered to the LED. But many here forget that the eye gets fooled by the peak intensity of a light. This is why multiplexing a LED array works and why motion picture images appear to be smooth to our eyes.

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#112
In reply to #111

Re: Wanted: 115-VAC LED Indicator

08/04/2009 12:00 AM

Redfred,

Right on !

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#113
In reply to #112

Re: Wanted: 115-VAC LED Indicator

08/04/2009 12:03 AM

I guess I shouldn't have made my last comment OT

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#108
In reply to #105

Re: Wanted: 115-VAC LED Indicator

08/03/2009 9:34 PM

GW,

Most of that current is not about lighting the LED or even giving much heat to the resistor. If it was charging a capacitor or building a field around an inductor it would be significant but it isn't.

The rise before and the fall near the peak is quick and doesn't have a lot of influence in the circuit.

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#110
In reply to #108

Re: Wanted: 115-VAC LED Indicator

08/03/2009 11:48 PM

You are wrong. It is a half sinusoid of current. The LED does not start to suddenly conduct at the rated current. It appears more or less as a diode.

Yes, the luminance is brightest at the peek, but we are calculating power in the resistor.

Assuming the LED starts to conduct at 2 volts (it is probably closer to .7) then the angle of retard for the current is asin(2 /( √2*115)) = 0.7 deg. This will be trimmed off both ends of the half sine so the total conduction time is 178.6 degrees.

Previous power calculations are correct.

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#114
In reply to #110

Re: Wanted: 115-VAC LED Indicator

08/04/2009 12:15 AM

GW,

You make it sound as though the LED is going to be brightly lit for just about a whole half cycle.

It won't.

The subject of this thread is operating an LED on line voltage and has to do with rectifying, current limiting and pulse width and duration for illuminating the LED to a brightness that sticks to your eye when the current is gone.

Put your components on the breadboard and poke around with your O'scope and get back to me when you have figured it out.

Jon

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#121
In reply to #114

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:57 AM

No, I said the current waveform is a half sinusoid.

Yes, I have scoped it, and sized it, and run about 200 units.

No, I did not say the luminance was bright for a half cycle. It starts dim, builds, peeks at the crest, then tracks the sinusoid back down to zero. You can measure the luminance with a photo transistor. A scope probe across the resistor verifys the voltage wave form (and current) across the resistor.

And yes, there is a flicker. If you move the lamp it gives a strobe effect.

(Someone made a comment about a 1/4 inch diameter carbon composition resistor. That is typically 1 watt!)

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#133
In reply to #121

Re: Wanted: 115-VAC LED Indicator

08/04/2009 1:47 PM

GW,

I mentioned that I was using a 1/4 inch resistor on my breadboarded circuit.

It certainly wouldn't handle the nearly 3 watts someone said would be produced when they were using a formula that doesn't apply to halfwave circuit.

Shifting your eyes allows you to see a strobe effect. I see a lot of cars trucks buses and even traffic lights using strobed LEDs now.

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#136
In reply to #133

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:19 PM

As the logic doesn't convince you, but you don't say where you see a flaw, let's see if experiment will sort this out...

It's a 2.7-kOhm nominal resistor.

Try running it with series diode into the LED from a DC supply (so no argument about RMS etc.),with similar airflow to the situation when it was driving AC. Check both Voltage and current with the supply at 50-Volts (to check the resistor value stays constant), and also see if the resistor "feels" the same temperature you had when driving the LED.

So long as the current remains below 25-mA, increase the supply in (say) 5-Volts steps (still monitoring both Voltage and current so we know the actual power rather than the theoretical). If I was doing it I would make certain there was a screen between myself and the device under test until the current stabilised (and move and/or turn off quick if the current started to change once I was nearby).

(If you have enough meters, you could monitor the Voltage across the resistor itself as well as the applied Voltage and the current...)


The

Theory is equivalent to

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#153
In reply to #136

Re: Wanted: 115-VAC LED Indicator

08/04/2009 9:45 PM

The equivalency comment was about LED brightness using the pulses of half wave rectified ~115V 60Hz.

115Vac > Diode (drops ~0.7) > 2800 Ohm 1 Watt Resistor > 20mA Red LED peak pulsed to 60mA (drops ~3Volts) > ac return.

DC

Voltage applied: regulated 56Vdc. It's all I had right now.

0.699V across the Rectifier

53.3Vdc across the 2800 Ohm Resistor hot to the touch as one would expect for a Watt dissipating from a 1 Watt. Resistor.

1.72 Volts across Red LED

19.7mA circuit current.

AC

Voltage applied 115Vac

The 2800 Ohm Resistor is hotter to the touch. Vres = Vapplied - (VRectifier + VLED)

So the 40% is subtracted from the 115Vrms or 167Vpeak to give more like ~68Volts to the resistor.

Voltage peak assumed to be 167V - (VLED + VRectifier) = ~163V.

Peak Current across Red LED assumed to be ~60mA ac circuit current.

A little brighter to the eye. That is the purpose of pulsing an LED at higher than its rated current.

Time to get another O'scope.

Jon

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#138
In reply to #133

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:30 PM

"a 1/4 inch resistor"

Is that 1/4 inch long, or 1/4 inch diameter? 1/4 inch long would be around 1/8W. As GW started, 1/4 inch diameter would be in the 1-2W range, depending on length.

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#151
In reply to #138

Re: Wanted: 115-VAC LED Indicator

08/04/2009 5:40 PM

dkwarner,

That was a response to a comment in context with post 121 about a 1/4 inch diameter resistor.

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#139
In reply to #110

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:36 PM

GW,

How about using a high voltage rectifier that doesn't conduct until it reaches 50Volts? Or 2 in series.

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#142
In reply to #139

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:41 PM

You still have the VI power loss, you just shifted it to the diodes.

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#146
In reply to #142

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:58 PM

For the present case it would be counter-productive, as the current flows only when the Voltage is near peak - so the dissipation for a given average current will increase.

However, if you are only after more light for a given average current it might help, as this increases the peak-to-average value, so the quantum efficiency of the LED will be higher (provided that you are not overdriving it). The proportion of charge transferred near the peak current is also increased somewhat (due to the difference in shape between a half-sine wave and a raised square-wave.

Hearses for churches, as they don't say.

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#126
In reply to #108

Re: Wanted: 115-VAC LED Indicator

08/04/2009 8:18 AM

In your circuit, all the current flows through the LED - so all the current contributes to illumination. On the other hand, most of the Voltage is across the resistor - so only a small part of the power is usefully applied.

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#137
In reply to #126

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:20 PM

Physicist?,

The reference was to the less useful lower voltages near zero crossing that provide less than 10mA to the LED and don't provide much illumination.

Correct, only a small part of the power is usefully applied.

That being the power that provides 10 to 60 mA.

Thats the cost of keeping it simple.

A triac and diac circuit could do some wonders too but that would probably induce noise in the neigboring circuits.

Jon

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#144
In reply to #137

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:48 PM

With a half sine wave, currents below 1/6th of peak current only flows for 5.3% of the full cycle, whereas currents greater than 1/6th flow for 44.7% of the cycle.

Put differently, only 1.4% of the total charge is transferred during times when the current is less than 1/6th of peak - not very significant.

So, yes, there are severe power losses, but nearly all the current is reasonably usefully applied.

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#101
In reply to #95

Re: Wanted: 115-VAC LED Indicator

08/03/2009 7:00 PM

GW,

I forgot... "I would be interested in the developement of your rule of thumb for 40%.It is not my rule of thumb."

It was in an Optoelectronic Handbook where it explained using line voltage to operate an LED pilot lamp in place of a Neon or incandescent lamp.

Jon

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#123
In reply to #101

Re: Wanted: 115-VAC LED Indicator

08/04/2009 7:36 AM

Because the forward Voltage of the LED does not change too much with current, the half-sine-wave rule of thumb from the handbook (it should be = average current x peak forward Voltage) would not be too far out when applied to the LED. Using 45% would predict somewhat more dissipation than actually observed (because the Voltage across the LED varies with current). The 40% proposed was probably intended to account for this - but even the sign of the overall error will depend on how hard the LED is being driven.

For a half sine-wave, you could use this same rule for the resistor dissipation:-
Using the 40% would predict a dissipation that is about 13% higher than the actual value.

On the other hand, the "square-of-mean" interpretation of what appears to have been a rather sloppily-written guideline leads to a calculation that gives only 32% of the actual power.

I hope this clarifies matters.

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#145
In reply to #123

Re: Wanted: 115-VAC LED Indicator

08/04/2009 4:56 PM

Physicist?,

Yes.

The 40% figure is about the effective LED brightness.

It is pretty close to other considerations as well.

Jon

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#148
In reply to #145

Re: Wanted: 115-VAC LED Indicator

08/04/2009 5:02 PM

"It is pretty close to other considerations as well"

Yes indeed - provided you choose the correct Voltage for multiplication when calculating the power.

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