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How To Use A Multimeter To Find Proper Current

03/07/2017 8:55 PM

Now they put this info in with the LED, but is there a way to determine proper current for an LED assembly using a multimeter.....and what significance is the relationship of voltage and amperage, so the array is rated at 36v and 3 amps, can you use higher amperage and lower voltage without damaging the light? I know ohms law, but there must be a limiting factor in the amount of voltage and amperage individually?

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#1

Re: How to use a multimeter to find proper current

03/07/2017 9:10 PM

You can't control the current and voltage independently (unless you rewire the array.) If you lower the voltage, you lower the current.

A LED, like any other diode, has a non-linear voltage-current relationship. For this reason, a series resistor is usually used to prevent current run-away. The light output is proportional to the current.

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#2
In reply to #1

Re: How to use a multimeter to find proper current

03/07/2017 9:24 PM

I have a DC power supply that has variable voltage and variable amperage....So if the voltage for the array is stated as 36v and 3 amps for a total of 108 watts, and I have only 30 volts available, can I then raise the amperage to 3.6 amps and expect the same results? How far can you go by varying the voltage and amperage while maintaining the same wattage, without affecting the output?

OK so you're saying no matter how much I raise the amperage, it will not draw any more current than the three amps, and lowering the voltage will also lower the amperage...is that it?

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#3
In reply to #2

Re: How to use a multimeter to find proper current

03/07/2017 9:36 PM

LEDs, like most diodes, are current-mode devices, that is, if they ask for 3 amps, you give them 3 amps and let the voltage go where it may. They expect a constant-current source, not a constant-voltage source. If your supply cannot deliver the voltage necessary to supply the current, you need to use a supply that can. There are supplies made for these modules.

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#4
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Re: How to use a multimeter to find proper current

03/07/2017 9:42 PM

So if I only have 30 volts and 5 amps available, and the light array calls for 36v at 3 amps, then I have to raise the voltage to get full brightness....Is there any way to determine the voltage requirement with just a multimeter?

The reason I'm asking is I have a DC power supply that has a digital readout of volts and amps I would like to use on my 100 watt LED, but it maxes out at 30 volts...now I do have another power supply that I can use with this one that does have variable volts to 48v...both are amperage variable....

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#5
In reply to #4

Re: How to use a multimeter to find proper current

03/07/2017 9:59 PM

Use your multimeter to monitor the current. It probably has 10A range, yes? Start out low and increase your supply's voltage until the current reaches 3A. If your supply can't source enough voltage to reach the 3A mark, then you need to use a supply that can. Try it with your 48v supply. Start out low, raising the voltage until the current drawn by the LED is 3A.

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#7
In reply to #4

Re: How to use a multimeter to find proper current

03/07/2017 10:19 PM

As you can see from the typical plot in #1, as you raise the voltage across a LED, first very little current will flow, and then at the threshold voltage the current increases very rapidly. The knee in the current-voltage curve, the threshold voltage, is determined by the color of the LED. White LEDs have blue or ultraviolet diodes and may have a voltage of 3-5 volts before current starts, multiplied by the number of LEDs in series.

Because the current increases quickly once the threshold voltage is reached, the current needs to be limited to prevent burning out the LED. Small LEDs use a series resistor. Your current limit on your power supply can be used to do this. Set it at 3 amps and slowly increase the voltage. You probably won't see much light at all until you get close to the rated voltage. Light output is proportional to current.

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#6
In reply to #2

Re: How to use a multimeter to find proper current

03/07/2017 10:04 PM

The voltage and current controls on your power supply are likely limit (maximum) values. For example, if you set the amps at 5 and put a wire across the terminals, only 5 amps would flow through the wire. The voltage would be very low. If you remove the wire, the voltage would only increase to the voltage limit setting.

Examples:

https://www.circuitspecialists.com/bench-top-power-supplies-current-limiting

Your load determines the voltage / current relationship. You may only vary one parameter and the load varies the other one.

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#12
In reply to #2

Re: How to use a multimeter to find proper current

03/08/2017 2:42 PM

You really think you can raise that amperage independently of the voltage? Good luck.

Amps for an LED array is definitely a non-linear function of voltage at low forward volts, but as you get away from that "knee" in the performance curve, it straightens out. Things get pretty doggone linear up there.

Maybe you need a server supply, or three radio shack supplies in series?? They make PSU's for those that allow tuning in the volts very smoothly.

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#8

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 10:18 AM

The general rule for LED power supply voltage is to use the typical forward voltage drop for the colour of diodes being used multiplied by the number of diodes in series for the string or substrings in the overall device or fixture.

In your case it uses groups of 10 white LEDs in series with a individual forward voltage drop of ~3.1 - 3.5 volts per LED it would need at least 31 - 35 volts before it will turn on.

Now that said depending on how well the individual LED's (assuming they are white) were made they may show as low as ~2.7 volts or up to around 3.6 volts per LED forward drops which means you may get lucky and your 36 volt LED light array may start working at ~2.7 volts or it may take the dang near full 36 volts to go at all.

No way to know for sure until you hook it up and try given that despite what a typical LED or array of LEDs of any colors forward drop is rated to be it can in fact be a fair amount off from that one way or another.

LED voltage drop by color

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#9
In reply to #8

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 12:03 PM

Well I think they are supposed to be 6000K, which is firmly in the white category I'm thinking....

If I was to wire additionally a cooling fan in parallel with the light that was rated at 12v, then could I just add a resistor on that feed? ...or would I need a different approach?

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#10

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 1:05 PM

If your power supply can only source 30v, you probably won't get to 3 amps and thus won't see full brightness either.

I've used LED's at current levels below their rating with the LED still putting out plenty of light. It's worth trying it out to see if you can get enough light out of the array at less than maximum current. Running the LED at less than rated current won't harm it.

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#11

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 1:28 PM

It all depends.

I have LED 'bulbs' (using the term loosely) that have active circuitry. So if SE has one of these bulbs, you might find that there is an INVERSE relationship between voltage and current.

Below is the data for a 'disc' LED that I use in my RV. It has pins compatible with a G4 bulb. The operating voltage for this LED bulb is 12 VDC to 30 VDC. This unit is produced by Brightech and I have been very happy with their performance and reliability.

On the backside of this LED is a switching power supply to keep the illumination levels constant with voltage. One day, I sat down with a DC laboratory power supply and recorded current as a function of applied voltage. The first graph is a plot of power consumed as a function of voltage. Full brightness happened at about 10.5 volts with a funny drop at about 11 volts. From 12 volts to 30 volts there is no perceived change in the light output. Below 9 volts the LED's become very dim.

The second chart is a function of current consumed for voltage input.

The data I collected and input to Excel is in the chart below along with a picture comparing the 20W halogen bulb to the LED bulb installed in identical fixtures. You will note the slightly higher Kelvin temperature of the LED bulb, but in practice, the eye doesn't notice the difference as much as the camera.

VoltageCurrentPower
90.0410.369
100.0830.83
10.490.090.9441
110.070.77
120.0730.876
130.0650.845
140.0610.854
150.0560.84
160.0540.864
170.0510.867
180.0480.864
240.0350.84
300.030.9
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#13
In reply to #11

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 4:00 PM

Well those must be wired only 2 or 3 led's in series and a pwm controller....these 100 watt lights are wired 10 in series in 10 parallel circuits....Interesting though....is the circuitry visible?

I guess I could use one power supply to feed this pwm and then into the booster and then to the light...then just use the pwm control for the light control.....

http://www.ebay.com/itm/H122-DC-12V-8A-Light-Dimmer-Brightness-Adjustable-Control-For-Single-Color-LED-S-/282321045498?

I have this power supply also, would this work after the pwm control?

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#14
In reply to #13

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 4:55 PM

The circuitry is visible on the backside of these LED bulbs I use in my RV.

You wouldn't be able to use the booster after the PWM control. The whole idea of a PWM dimmer is to drive the LED with the correct current that gives it the right 'color' but only turn it on for a very short period of time, pause, then pulse it again, preferably hundreds of times a second or greater so our eyes don't detect the pulsing. Most booster circuits get VERY unhappy with power that is pulsed at their inputs.

I think you will need a PWM dimmer than can drive the higher voltage of your LED strings. (That is if you want color corrected dimming).

From my experience, LED's maintain decent color rendition with straightforward current limiting as long as you are not trying to woo your date with really low light. A purist would object but most of us are somewhat tolerant of off-color light.

For the best color rendition at really low light levels, PWM dimming is the best way to go. (Unless you want to mess around with old school methods like neutral density filters. Very inconvenient.)

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#15
In reply to #14

Re: How To Use A Multimeter To Find Proper Current

03/08/2017 6:11 PM

Well I was just considering that for informational purposes, in reality the light will probably be utilized as a room lamp and the slight yellowing of the color at lower current levels will not be a problem, it might even enhance the design and be a desirable characteristic...Now in a colored light that might be a different story as I would think the color quality would most likely want to be preserved in most cases....I get the feeling I'm going to end up with a lot of leftover components, anyway every little piece of knowledge helps, thanks...

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#17
In reply to #13

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 11:40 AM

Is that a buck power supply?

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#18
In reply to #17

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 1:51 PM

It's called a boost converter....but anyway I have found a different power supply(yes another one) that will convert from line voltage to 36v dc, exactly what I need to simplify everything....and also a pwm that is also good at that voltage,with a regular knob, also a wall wart for the 12v supply for the fan in the heatsink....I will probably rewire the wart to feed off the incoming 110v so only 1 cord....although I would like to tie the fan into the pwm output so everything shuts off with one knob....Now I just have to design the lamp...

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#19
In reply to #18

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 1:56 PM

Have you also looked at prices on some of the server PSU's on Ebay lately? WOW!

I got me a 2950 W one, and have a Buck-boost converter with its own control (knob), and digital displays for a low low price. Although, I am starting to wonder when the buck-boost PSU controller will arrive.

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#20
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Re: How To Use A Multimeter To Find Proper Current

03/09/2017 3:15 PM

Slo-boat from China no doubt....

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#22
In reply to #18

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 5:10 PM

Don't make a lamp. C'mon, you can buy those just about anywhere. No, make a 100-Watt LED searchlight flashlight!

12 V gel-battery, 12V-36V boost converter. Wouldn't take much. You've probably got most of what you need already in your shop, yes?

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#26
In reply to #22

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 9:59 PM

I already have a super bright flashlight 35k lumens with focus....sorta like that one...

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#16

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 1:16 AM

The answers hereby found make no sense to me.

LEDs must be implemented with series resistors in order to limit the current when powered from voltage sources. Therefore, most of the time your equivalent circuit will be a resistor, where current is propirtional to the voltage. Power is with whatever source you want.

On the other hand, just as other diodes, LEDs can break due to reverse voltage or overcurrent. If you power them with a DC Lab PSU (instead of a VARIAC) you can forget about the reverse voltage. You will only be limited by the temperature inside the component. Get a better power extraction method if you need more light.

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#24
In reply to #16

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 6:05 PM

No, they do not require a series resistor to limit current. You can construct a constant current power source that will vary its voltage depending on the load.

Yes, a series resistor is a convenient way to limit the current, but only when the input voltage is known and stable, and you have few LED's to illuminate. If you start doing this with multiple LED's and multiple strings of LED's the power dissipation can get very high.

Current sources are much more efficient than resistors when going to higher power LED arrays.

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#25
In reply to #24

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 6:12 PM

Yep. The only requirement is that the constant-current source has enough voltage compliance.

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#21

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 4:33 PM

I am truly shocked, S.E. asking a question!!! I don't know exactly what you are trying to do, but to raise the current, you first must raise the voltage or lower the resistance by removing some LED's. That being said, the 2 and 3 watt LED's get quite warm when operating at rated power. Ever feel the ballast area of a 60 "watt" equivalent lamp? I have made some bamboo covered ceiling lamps using 1" square aluminum stock as a heat sink. Even so, forced air cooling is recommended if more than 1 LED per foot at rated power. I have found running them at 75% removes the heat issue, while maintaining far more than adequate light output.

If you really want rated power, you must reduce the number of LED's in series. Just jump them out until you get to 3 amps. If you want all the LED's, you will have to parallel these remaining LED's. You will need a series ballast resistor to "isolate" the two parallel circuits. If you don't do this, the smaller "branch" will draw more current than permissible. In technical terms they will be "wawy, wawy bwite for a wawy short period of time".This resistor will have to be of sufficient wattage to handle the current, the resistance will be determined by the number of LED's remaining. If you provide the number of LED's removed and their power requirements, we can determine the resistor.

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#23
In reply to #21

Re: How To Use A Multimeter To Find Proper Current

03/09/2017 5:35 PM

The LED module SE is using is not user-modifiable. The LED dice are mounted to the substrate and directly connected with bonding wires, then potted with YAG-impregnated optical epoxy.

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#27

Re: How To Use A Multimeter To Find Proper Current

03/10/2017 3:10 PM

WOW, This really sounds confusing.

I have been using, designing with LED's from the first time I saw one. AT first it was simple and we used a different resister on the back of the lights for control systems switches at utilities and manufacturing plants and broke the glass off the burned out bulb and soldered in an led of the proper color (red or Green). Then the company I worked for came out with replacement assemblies at a much higher price, so that made things much simpler.

I have replaced the lighting in my motor home and camper with led lights and have used many around the home.

If I had read this discussion before I did all this, I would never have been able to do what I did.

I would like to say, that none of the comments are really wrong, but most are very naïve. Naïve in the fact that they don't acknowledge the fact that the writer doesn't know everything, he only knows a piece. I would suggest that rather than objecting to someone's comment and stating or hinting that they are wrong, you take a longer view of things, as you may only know a piece of the answer.

One of the problems is that the suppliers are frequently in China and their product also only contains a piece of the answer.

Problems I have experienced: Wall Worts that have totally different characteristics than the information on the case, Led's whose rating is much different from the published data, very non-linear power supplies which caused me to have to add resistors to circuit boards for proper function.

The days of being able to purchase a simple LED and control it with a resister are still here, but most of the time we have to deal with a built in power supply and we must experiment with it to find it's shortcomings and take whatever corrective action is needed.

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#28
In reply to #27

Re: How To Use A Multimeter To Find Proper Current

03/10/2017 9:22 PM

Thermocooler chips and benchtop power supply arrive...

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#29
In reply to #28

Re: How To Use A Multimeter To Find Proper Current

03/13/2017 8:40 AM

I am sorry, why do you need the thermo-cooler chips again? Are you planning on running the emergency LED lighting off the fire in the plant while everyone is evacuating (in their pants) with their hair on fire?

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#30
In reply to #29

Re: How To Use A Multimeter To Find Proper Current

03/13/2017 6:23 PM

Oh well have to redesign a larger light to use the thermocooler chip, my heatsink is too small to handle both the heat from the led and the chip....So I'm going to build two lights, one with just the led and heatsink I have, and a second one with a larger base and heatsink...

The led with the thermocooler chip is just for experimental purposes, I want to see what effect the led temperature has on performance.....

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