Previous in Forum: AC Current Flow Detector   Next in Forum: Turbine Shutdown Procedure Reg
Close
Close
Close
35 comments
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Power Engineering - New Member

Join Date: Mar 2012
Location: the sandbox
Posts: 340
Good Answers: 6

Simple LED Driver

10/21/2013 6:44 AM

I need to build a simple LED setup for the yard, no its not homework, I havent done homework in 20 years, but I havent used any LEDs either. These LEDs are the standard 2v, 20 mA type. Will this work? Maybe I should add a cap at the output of the rectifier, and if so, how to size that?

__________________
BSEE but always learning
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16499
Good Answers: 662
#1

Re: simple LED driver

10/21/2013 6:55 AM

Yeah, but no but.
This is just my gut feeling... no specific experience, but it may get the ball rolling.
Basically it depends on the voltage drop across each LED at 20mA current.
Have a look at the LED data sheet, it may help, or measure the volts drop across a few at 20 mA.
The danger is it will work but will draw too much current, giving short LED life.
A series resistor would help even it out a tad, or maybe a lossy transformer designed for the task.

As an example for a resistor (I take no responsibility for the results calculated here... it's just an illustration of what may work.... it needs testing)
Say for the sake of argument you decide to sacrifice 20volts to give a bit of 'regulation' 20v at 20mA = 1k ohm

Wattage is VxI= 400mW, so call it a 5W 1K resistor
I've overdone the wattage 'cos it's AC and wattages are always quoted at stupid temperature.

You would still need to work out the approx number LEDS required. Obviously you can't just stick in a 1k 5W resistor in series with 1 LED and expect everything to be ok.

So if it was me I'd wire up the 60 LEDS and the resistor and measure the current, if it's too low remove an LED, if too high add a LED...
Not very scientific... no use of CAD or maths... but it will work.

Of course this is all dependant on the 20mA figure being the correct current for the particular LEDS for maximum brightness/vs life.

I would strongly advise testing on a lower voltage and NOT running anything directly of rectified mains without suiatable fusing and earth leakage protection.

Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30404
Good Answers: 819
#2
In reply to #1

Re: simple LED driver

10/21/2013 7:01 AM

So, why not configure the LEDs as the rectifier bridge in themselves, and use the ballast resistor as the DC load, thereby saving components?

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru

Join Date: Oct 2006
Location: Surrey BC Canada
Posts: 1571
Good Answers: 40
#16
In reply to #2

Re: simple LED driver

10/22/2013 12:46 AM

because PIV of LEDS is not so great ? Series diodes blocking end up blowing up the lowest leakage devices. You can get around this with parallel resistors, say 100k each, that dominate the reverse leakage.

Register to Reply
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Power Engineering - New Member

Join Date: Mar 2012
Location: the sandbox
Posts: 340
Good Answers: 6
#3

Re: simple LED driver

10/21/2013 8:00 AM

Thanks Del, thats exactly what i needed

__________________
BSEE but always learning
Register to Reply
Guru

Join Date: Jul 2008
Posts: 1557
Good Answers: 138
#4

Re: Simple LED Driver

10/21/2013 9:00 AM

The easiest method would be to set up a constant value voltage source with possibly some current limiting. That is the path that you and Del are on. That is probably the best way to go.

Many years ago I built several pieces of in-house test equipment with multiplexed LEDs for the display. I was quite surprised by how well the LEDs would run on brief pulses of current and still satisfy the need for the human to think that they are on 100% of the time. This might lower total power consumed (probably not an issue for you), heat generated (might be an issue for you) and LED lifetime (I don't what the pulses did to lifetime but since they ran cooler I suspect that it helped).

I suspect that it is not your best choice to use a 555 timer or other options as a pulser but it would be interesting to see what others suggest.

__________________
Few things limit our potential as much as knowing answers and setting aside questions.
Register to Reply
Guru

Join Date: Feb 2012
Posts: 595
#25
In reply to #4

Re: Simple LED Driver

10/22/2013 11:38 AM

if i remember right and if you re-check your reply it's the heat reducing the brightness of the LED + they don't survive reoccuring peak overvoltage for long (10-s of cycles)
i used my age old opt.pnt. for indoor mood-light as max(d(lumen)/d(ampere)) PS! not the HV-point where the thing goes arcing

__________________
ci139
Register to Reply
Guru

Join Date: Feb 2011
Posts: 7025
Good Answers: 206
#5

Re: Simple LED Driver

10/21/2013 9:34 AM

I doubt you'll get a full 2v at the end of your string with this set up, so you might not get the brightness you're looking for.

Register to Reply
Guru
Engineering Fields - Electrical Engineering - New Member United States - Member - New Member

Join Date: Apr 2007
Location: Wichita, Kansas USA
Posts: 650
Good Answers: 30
#6

Re: Simple LED Driver

10/21/2013 10:44 AM

The first thing that caught my eye was a note on your hand-drawn schematic. You have 20mA x 60 LED's = 1.2A. That would only be true if your LED's were in parallel, the way your LED's are wired, you are looking at series current, and that has to be the same all the way through the string, so if you are shooting for 20mA per LED, that would be your series current.

I think if you do figure out how many LED's to put in series to get your desired 20mA of current, the problem is going to be voltage variation from your AC source. A small change in AC voltage is going to cause a shift in your series current, and I think it will make a noticeable difference in the light output. Ideally, you want a constant current source to drive your string, and there are some simple methods to accomplish this. Another disadvantage of the series string is the "Christmas Tree Light" effect, if one LED goes out (open), you will have no light at all.

If I was doing this, I'd be looking at doing a simple constant current source, and probably wire the LED's in a series/parallel combo, to get the voltage down, and the current up.

Tom D.

Register to Reply Score 1 for Good Answer
3
Guru

Join Date: Jun 2010
Posts: 1296
Good Answers: 102
#7

Re: Simple LED Driver

10/21/2013 10:50 AM

Your total current will be 20mA, not 60 x 20 = 1.2A as shown in your sketch, since they are in series each LED has the same current. But your voltage would be 60 x 2 = 120V.

When dealing with LEDs, you have to think about current primarily, not voltage. Since these are diodes, a small change in voltage, temperature, age, etc. can cause a large change in current, then POOF! That's why many LEDs are packaged with limiting resistors - they use Ohm's law to size the resistor, ignoring the voltage drop across the LED. There are current regulator ICs available, I think, but if you want to do quick-and-dirty, you could:

1. Use a 120/24V transformer. This keeps your circuit voltage within a safe range. Should give you around 30V DC after the rectifier bridge.

2. Wire your LEDs in 4 series strings with 15 LEDs and a resistor per string. Wire the 4 strings in parallel with each other. The resistor should be about 1500 Ω. (I'm assuming your LEDs don't have individual resistors.)

3. After you have it turned on, measure the current and adjustchange the resistors if required.

4. Install a fuse somewhere - probably in the primary in case the transformer fries. And maybe a switch. (I'm ME, but I've watched a lot of EE's. They get to use fuses instead of safety factors . There, I've told you more than I know about LEDs.)

You could use more than 4 series strings with fewer LED's. I picked 4 because 15 LED's x 1.7 V (a typical forward voltage) = 25 V, which is less than what's available. The limit to this would be to put each LED and a limiting resistor in parallel. Then if any one LED burns out, all the rest would remain on.

Edit: I was composing this while others were posting, so there is a lot of repetition with Del and others.

Register to Reply Good Answer (Score 3)
Guru

Join Date: Feb 2011
Posts: 7025
Good Answers: 206
#8
In reply to #7

Re: Simple LED Driver

10/21/2013 11:07 AM

I would have split them up too. a single string is asking for problems.......transformers are cheap. no real need to condition the signal with a cap. LEDS are not highly sensitive electronics

Register to Reply
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16499
Good Answers: 662
#9

Re: Simple LED Driver

10/21/2013 1:06 PM

I like Bigg's answer it answers my concerns about high voltage whilst retainging the simplicity of a resistor adding a bit of regulation.
Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 15196
Good Answers: 937
#10

Re: Simple LED Driver

10/21/2013 2:15 PM

Wow, DO NOT USE YOUR CIRCUIT!

First, 120 VAC is an RMS voltage not a peak voltage. With ideal diodes (0 voltage drop and no reverse leakage current) for your bridge rectifier the peak voltage will be 170 V. So the actual peak voltage will be 2.8V across each LED instead of the manufacturer's specification of 2v @ 20mA. Now by using a simplified diode model version of Shockley's diode equation and assuming the nominal thermal voltage value of 0.026V (an inaccurate assumption for an LED but you got to fudge things sometimes) the anticipated current flow will be about 1.4*10^12 amperes with a 2.8 volt drop across each LED. This brings me to my next point.

Second, you do not show any current protection device (fuse, circuit breaker) on your power distribution wiring. Every circuit has a fuse that will vaporize when currents exceed a safe value. If that circuit protector was anticipated by the designer then a safe failure can occur. This circuit will ignite wire insulation if a power distribution breaker fails to blow.

Third, as the Shockley diode equation shows a diode is a very non-linear device. There must be some current limiting device in series with the diode. In your design a 2.5K 2 watt resistor could be added to handle the extra 50 volts you did not account for and limit the current flow. This approach will still cause an annoying problem when you find that some of your LED will be brighter than others as normal deviations from 2V @ 20 mA happen.

Sophisticated yet simple in concept current measuring and controlling switching supplies are used in many commercial LED replacement bulbs today. These devices can be made to respond to an SCR dimmer waveform but to explain how to build that circuit I will be obligated to bill somebody.

The LED will flicker at a 120Hz rate. If you wish to smooth out that flicker then a 2.7 uF 200V capacitor could be across the bridge output.

__________________
"Don't disturb my circles." translation of Archimedes last words
Register to Reply
Guru

Join Date: Feb 2011
Posts: 7025
Good Answers: 206
#11
In reply to #10

Re: Simple LED Driver

10/21/2013 2:53 PM

who needs fuses??

Register to Reply
Guru

Join Date: Oct 2006
Location: Surrey BC Canada
Posts: 1571
Good Answers: 40
#17
In reply to #10

Re: Simple LED Driver

10/22/2013 1:11 AM

Your model is incorrect. LEDS have very significant series resistance.

Without even knowing the part number of the LED proposed we have jumped to many possibly erroneous conclusions.

In the past I have used LEDs to replace 120vac neon lamps. I replaced the series resistor the neon used with a resistor more suitable for the LED. In addition I used a 1N4004 diode that provided increased reverse blocking capability. Yes there is a 60 Hz flicker with half wave rectifier, and the peak current is above "rated", but the average and RMS are both below the rated DC current. Your eye interprets the LED as bright, and unless you have movement, you can't see the flicker. Good enough for a pilot lamp.

Strings of Xmas tree LED lamps typically flicker, and they have multiples in series similar to what you propose. I have used a full wave rectifier ahead the string and you then get a 120HZ flicker that is less noticeable.

Adding a capacitor ahead of the string of LEDs will reduce flicker, but it also increases the average voltage, and hence power dissipated.

So, what are you trying to achieve? Xmas tree lamps, lawn lamps, a bright lighting panel? The end use will determine to a large extent the quality of light you want, and the complexity of circuit rquired.

No one has yet suggested a high frequency current regulated chopper circuit using an inductor to smooth the current ripple - this will be more efficient than seres resistors that typically dissipate watts of power.

Register to Reply
Guru

Join Date: Feb 2012
Posts: 595
#27
In reply to #17

Re: Simple LED Driver

10/22/2013 11:53 AM

the outdoor UV LED-s tempt huge amount of species you never seen before , perhaps some could help pollen your greenhouse , although the other half may eat it off along the veggies

__________________
ci139
Register to Reply
Guru
Engineering Fields - Electrical Engineering - Analog and Digital Circuit Design Engineering Fields - Electromechanical Engineering - Transformers, Motors & Drives, EM Launchers Engineering Fields - Engineering Physics - Applied Electrical, Optical, and Mechanical

Join Date: Jan 2008
Location: NY
Posts: 1208
Good Answers: 119
#12

Re: Simple LED Driver

10/21/2013 4:34 PM

Are you trying to build a single light source or a distributed array of small landscape lights?

Register to Reply
Commentator

Join Date: Jul 2005
Posts: 65
Good Answers: 4
#13

Re: Simple LED Driver

10/21/2013 11:12 PM

I can't believe no-one has yet to mention the danger of electrocution in this circuit!

Forget (for now) the danger of starting a fire...

If a child or dog (or you) were to somehow come in contact with just one of the wiring connections toward the 'hot' end of your circuit while barefoot in the yard (possibly even with shoes), they could be killed from current passing through their body (much less than the 20ma current that lights an LED). This is a very serious mistake in design!

Before taking anyone's suggestions or approvals on your design, study well how to be safe for yourself and others with 120VAC line voltage. If you don't do that, buy something off the shelf that will work for you.

Register to Reply
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16499
Good Answers: 662
#18
In reply to #13

Re: Simple LED Driver

10/22/2013 1:48 AM

I can't believe you haven't read post #1 (or #10)

Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Commentator

Join Date: Jul 2005
Posts: 65
Good Answers: 4
#29
In reply to #18

Re: Simple LED Driver

10/22/2013 12:13 PM

Actually, Dell, I read all the posts, including #1 and #10. The experience I have with people who have little understanding of electronics is they want to just implement a simple, straight-forward design and be happy. That is quite commendable, as we all want to make our dreams come true.


The posts had yet to let the experimenter know the danger of death. This needs to be stated first to inform those who obviously do not know, then to scare off anyone who is not serious about doing their homework to be sure their dream will not become a nightmare.


I could not, in good consciousness, keep from chiming in; I am sorry if you were offended, and I have a hard time believing you do not agree with the importance and necessity of my post.


Keep adding your suggestions, many of them are good answers.

Register to Reply
Guru

Join Date: Feb 2012
Posts: 595
#28
In reply to #13

Re: Simple LED Driver

10/22/2013 12:03 PM

look at the bright side it keeps the wild boar off and is probably the only effective such (as reported by my passed off neighbour - it died on a different circumstances however)

__________________
ci139
Register to Reply
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Power Engineering - New Member

Join Date: Mar 2012
Location: the sandbox
Posts: 340
Good Answers: 6
#14

Re: Simple LED Driver

10/22/2013 12:28 AM

Yes, this would be fed from a 15A circuit breaker, but a fuse would trip quicker.

This would be a single fixture with heatsink, not strung all over the yard. It would be fed to NEC 2011 code with proper SOW or SOOW cord,

or liquid tight metallic flex, correct connectors and "bell" (waterproof) boxes. Not accessible to kids or pets or wife.

I will do a complete circuit redesign.

Thank you all for your generous help!

__________________
BSEE but always learning
Register to Reply
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16499
Good Answers: 662
#19
In reply to #14

Re: Simple LED Driver

10/22/2013 1:52 AM

Forget a 15A circuit breaker! Work out the wattage at 15A x 120V ... thats 1800watts enough for 4 electric kettles plenty to start a fire burn your house down. You nead an earth leakage circuit breaker.

Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Associate

Join Date: Oct 2011
Posts: 38
Good Answers: 7
#30
In reply to #14

Re: Simple LED Driver

10/22/2013 10:32 PM

First of all, for safety reasons, you should use a transformer to isolate both sides of your circuit from the mains. Especially important in any outdoors usage.

Second, for safety reasons, you should ground one side of your resulting secondary circuit, usually the negative side after your bridge rectifier.

Using a 115/24 transformer is a good idea as they are readily available and inexpensive.


RadioShack® 25.2V CT 450mA Std Chassis-Mount Xformer

Model:273-1366 | Catalog #: 273-1366 $7.99

and this Bridge rectifier:

Model: 276-1146 | Catalog #: 276-1146

Full-Wave Bridge Rectifiers for Power Supplies

Price:$2.99Rated 4 amps, 50 Peak Inverse Voltage (PIV). Ideal for power supplies.

With this capacitor across the bridge:


Model: NTE NEV47M50CB | Catalog #: 55047282

NTE NEV47M50CB - CAP-47uF 50V RADIAL

Price:$0.51

This will produce a low ripple voltage of about 32 volts to power four strings of 15 Leds each in parallel. A series 100 ohm, 1/2 watt resistor in series with each string will help limit the current in each string to about 20ma.

These will work:

Model: 271-1108 | Catalog #: 271-1108

100 ohm 1/2W 5% Carbon Film Resistor pk/5

Price:$1.49

And finally, a fault protection fuse:

Model: 74-5FG125MA-B | Catalog #: 55048553 Price:$2.94

74-5FG125MA-B 5x20mm 125MA Miniature Glass Fuse

With this fuse holder for mini fuses:

Model: 74-FC5-B | Catalog #: 55048604

74-FC5-B Double-fluted Fuse Clip

Price:$1.61
This should give you a low cost solution, about $18 plus the cost of your LEDs and wire, and AC plug.

Hope this helps

Dan A

Register to Reply
Guru

Join Date: Oct 2009
Location: sometimes Wales,UK.. was Libya, now Oman!
Posts: 1715
Good Answers: 116
#15

Re: Simple LED Driver

10/22/2013 12:45 AM

I was in Tandy's/Radio Shack the other day (in Oman)... just browsing, and I noticed that they still have those build it yourself kits.... and I noticed one for flashing LED's.

Maybe that might help!

__________________
The square root of nothing is what you make it!
Register to Reply
Guru

Join Date: Feb 2008
Location: Van Nuys, CA
Posts: 563
Good Answers: 33
#20

Re: Simple LED Driver

10/22/2013 3:01 AM

"I need to build a simple LED setup for the yard"

Little 20mA LEDs don't put out much illumination. What exactly do you want to do with them "in the yard"?

A.C. power supplies are so cheap and readily available, it really isn't worth the effort of designing & building one from scratch for one-of-kind projects. That leaves you with building a simple constant-current DC-DC converter.

LED Driver Circuits

Register to Reply
Guru

Join Date: Jun 2006
Location: Ottawa Canada
Posts: 1975
Good Answers: 117
#24
In reply to #20

Re: Simple LED Driver

10/22/2013 9:53 AM

Where did you get that kuul app?

__________________
If it was easy anybody could do it.
Register to Reply Off Topic (Score 5)
Power-User

Join Date: Jan 2009
Posts: 250
Good Answers: 7
#21

Re: Simple LED Driver

10/22/2013 6:37 AM

Forget that additional rectifier bridge and resistors.

Just put those 60 +/- LEDs in series. Keep the correct polarities. An inverted LED will be blown out. Adjust the LED qty to have a decent brightness.

LEDs are highly non-linear, as pointed out. So the current will not be sinusoidal but a pulsed one. LEDs just love to be driven by a pulsed current. Light output efficiency is higher with a pulsed current (junction is kept cooler).

I can't see why this should more dangerous than a xmas tree lamp string, provided you use proper isolation. Best thing is to keep the whole thing out of reach.

There will be flickering. You have to tell how much is tolerable.

I have used this arrangement many times.

brgds

Snel

Register to Reply Off Topic (Score 4)
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#22

Re: Simple LED Driver

10/22/2013 7:08 AM

A few D's & D's:-

Do only use isolated power supplies, switched mode ones are very efficient and relatively cheap nowadays, needed if the LEDs are on a long time.

Search on Youtube for LED Tutorials, also on the web and especially on instructables.

Generally speaking a LED needs a series resistor to drop the power to a level the LED can handle. A reasonable through current for starters with cheap LEDs is between 10 and 20ma. Some can handle far more and are far brighter and a lot more expensive.

LEDs can be connected to each other serially over a higher voltage, but always via a resistor.

LEDs of different colours are more difficult to handle as their intrinsic voltage levels are all different, so a 3 LED string of blue LEDs needs another resistor value (for the same voltage) than say a 3 LED string of red LEDs. It is therefore easier to keep different colours on different strings, even if you start using RGB LEDs....

As a starter, I would recommend only using 5 volt power supplies of say 1 amp. Once you are further on, you can start using say 24 volt supplies with say 5 amps....

For the yard, you need to have a place that is dry to have the mains power supplies.

Always think safe.

Make sure that you even install a residual earth leakage breaker on the mains side to make possible accidents less deadly!

Learn Ohm's Law before starting. Buy a cheap Multimeter and learn how not to damage it! But if you do, immediately buy a new one....

Above all have fun and learn.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Guru

Join Date: Feb 2011
Posts: 7025
Good Answers: 206
#23

Re: Simple LED Driver

10/22/2013 7:46 AM

I'd just like to make a point about this thread from it's inception. "for the yard" is a weak description. It's hard to give a short or detailed response to something so vague. clearly the Op was not building a conditioned power supply for sophisticated electronics so in this case the OP wanted to "keep it simple". running 120V right off of a bridge is pretty simple, this is true but I doubt most here would have gone this route whether this small system was to illuminate a tree, walkway or was merely intended as general lighting.

I'm not complaining, I'm just stating if someone asks a fairly clear and complete question, they are far more likely to get a quick, useful response to their query. a little more time describing the application would have yielded an entirely different crop of replies. and of course this statement has universal application on all forums, not just this one.

Register to Reply
Guru
Hobbies - DIY Welding - Wannabeabettawelda

Join Date: May 2007
Location: Annapolis, Maryland
Posts: 7291
Good Answers: 424
#26

Re: Simple LED Driver

10/22/2013 11:45 AM

What works nice in this situation is a current regulating diode (CRD). Basically a CRD is a two terminal device that consists of a JFET transistor with a small resistance in the source leg and feedback to the gate.

You can put one of these in series with your series sting of LEDs and it will maintain a fairly constant current as long as there is sufficient voltage across the device ( a few volts or so, up to a maximum of 50 V or even 100V.

Here is one from Mouser for $2.48

http://www.mouser.com/ProductDetail/Semitec/S-183T/?qs=sGAEpiMZZMtjJbtednZihh523TK1gNO%252b

If you look at the 18 mA device (E-183 on the chart), you will see you get reasonable current regulation between about 3 and 30 V.

This permits a fair amount of voltage ripple to exist on your rectifier-capacitor DC supply. I recommend you use a capacitor on the output of your rectifier to smooth out the ripple. Unloaded, rectifier-capacitor power supply powered directly from the mains will have about 170-180 VDC on it. Once you put a load on it, the voltage will drop some. (See Schade's curves SCHADE, O. H. Analysis of Rectifier Operation. Proc. I.R.E., July, 1943, Vol. 31, No. 7. and a more recent coverage is in http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/images/archive/311pet23.pdf )

You can see by the graphs below, the output voltage of your depends on the ratio of source resistance to load resistance (LED's + CRD) and the size of your filter, frequency (60 Hz/ 377 radians per second), and the load resistance. In general, larger the capacitor, and lower the source resistance, the higher the output voltage will be. Vc/Vm refers to the ratio of the output voltage Vc to the maximum value of the ripple voltage (~177 vdc).

Watch your power dissipation. You want to keep the voltage across the device as low as possible but high enough for good regulation. The leaded device is rated for 300mW. That means at 18 mA, you want to get the voltage across the device to be about 16 VDC or less (on average). You may have to experiment with the number of LED's and the size of the filter capacitor to keep that voltage down under 16 VDC.

Good luck with your quest.

Register to Reply
Guru

Join Date: Nov 2012
Posts: 633
Good Answers: 13
#31

Re: Simple LED Driver

10/23/2013 5:41 PM

Try search on 'instructables.com' for led bulb conversion....uses a cell phone switching supply to drive large lumen led to replace florescent bulb.

Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1386
Good Answers: 31
#32

Re: Simple LED Driver

10/23/2013 9:20 PM

I have been running several DIY LED lighting systems for about 2 years.

I use 3.2 volt LEDs from Digikey (.26 to .32 a piece), either 9 or 6 volt Switch Mode Transformers (9 to 12 dollars), and 18-2 Thermostat wire from Lowes.

I have used no current limiting resistors because the Switch Mode Transformers have been quite voltage stable.

If using a 6 volt system I put 2 LEDs in series with as many branches as I need.

For a 12 volt system I put 4 LED's in series with as many branches as I need.

I home build a dusk to dawn switch using a diode, a P Channel MOSFET, a photo-resistor, and a 100k resistor.

I fuse with small .25 to .5 amp GMA fusses.

So far so good.

I also have several 12 volt solar powered systems running. One for about 2 years the other for about a year.

If you build a solar system without a dusk to dawn switch then a resistor will be required because unless you are using a regulating controller the LEDs will see the charging voltage during the daylight hours and will burn out prematurely.

I have also tested a simple N Channel MOSFET based circuit that will switch in and out a current limiting resistor for use with solar powered systems. I think it will work well in application.

I have a PDF file showing how to build the system using either point-able assemblies made from 1/2 inch schedule 40 plumbing end caps, 90 degree elbows, Ts, and grey plastic electrical conduit or light bars using just the conduit. This includes all the part numbers for delivery by US Mail anywhere that interfaces with the US postal system. If you want a copy send me a private message with your email address.

Easy to build, Cheap, and Robust.

Gavilan

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1386
Good Answers: 31
#33

Re: Simple LED Driver

10/23/2013 9:23 PM

I have been running several DIY LED lighting systems for about 2 years.

I use 3.2 volt LEDs from Digikey (.26 to .32 a piece), either 12 or 6 volt Switch Mode Transformers (9 to 12 dollars), and 18-2 Thermostat wire from Lowes.

I have used no current limiting resistors because the Switch Mode Transformers have been quite voltage stable.

If using a 6 volt system I put 2 LEDs in series with as many branches as I need.

For a 12 volt system I put 4 LED's in series with as many branches as I need.

I home build a dusk to dawn switch using a diode, a P Channel MOSFET, a photo-resistor, and a 100k resistor.

I fuse with small .25 to .5 amp GMA fusses.

So far so good.

I also have several 12 volt solar powered systems running. One for about 2 years the other for about a year.

If you build a solar system without a dusk to dawn switch then a resistor will be required because unless you are using a regulating controller the LEDs will see the charging voltage during the daylight hours and will burn out prematurely.

I have also tested a simple N Channel MOSFET based circuit that will switch in and out a current limiting resistor for use with solar powered systems. I think it will work well in application.

I have a PDF file showing how to build the system using either point-able assemblies made from 1/2 inch schedule 40 plumbing end caps, 90 degree elbows, Ts, and grey plastic electrical conduit or light bars using just the conduit. This includes all the part numbers for delivery by US Mail anywhere that interfaces with the US postal system. If you want a copy send me a private message with your email address.

Easy to build, Cheap, and Robust.

Gavilan

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Guru
Engineering Fields - Instrumentation Engineering - New Member Hobbies - Automotive Performance - New Member Technical Fields - Education - New Member Fans of Old Computers - TRS-80 - New Member Hobbies - Musician - New Member

Join Date: Jan 2008
Location: Tucson, AZ
Posts: 1331
Good Answers: 30
#34
In reply to #33

Re: Simple LED Driver

10/22/2014 8:18 PM

When you don't know what you're doing...DON'T...because it's better to be SAFE than dead SORRY.

__________________
...and the Devil said: "...yes, but it's a DRY heat..!"
Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1386
Good Answers: 31
#35
In reply to #34

Re: Simple LED Driver

11/10/2014 11:31 PM

Thanks for great advice Cuda. I'll try not to fry myself with my fused 12 and 6 volt 30 milliamp adventures.

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Register to Reply 35 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

1capybara (2); 70AARCuda (1); Andy Germany (1); bigg (1); Brave Sir Robin (1); brich (1); BruceFlorida (1); ci139 (3); Dan A (1); Del the cat (4); Fredski (4); Gavilan (3); geraldpaxton (1); GW (2); mjb1962853 (1); pantaz (1); PWSlack (1); redfred (1); Snel (1); tdesmit (1); WhiteHorse (2); Yusef1 (1)

Previous in Forum: AC Current Flow Detector   Next in Forum: Turbine Shutdown Procedure Reg

Advertisement