LED parallel current question
prof_braino
Posts: 4,313
I inherited a couple ancient bench top power supplies. One made smoke and popping noises, the other one looks like it might have a couple miles left on it. So I got out the old digital volt meter and a fancy Fluke amp meter.
Power supply is set to about 3 volts per the analog needle on the power supply dispay.
The digital volt meter says theis is 3.258 Volts and is steady under no load.
The Fluke 77 DMM is displaying current on the 300mA max input.
A set of LEDs harvested from the same string of xmas lights tested individually draw this much current
LED A) 10.7 mA
LED
8.9 mA
LED C) 9.8 mA
LED D) 9.4 mA
LED E) 6.6 mA
When I connect more than one LED in parallel, the voltage stays the same, but the current doesn't add up.
LEDs A&B = 14.8 mA
LEDs ABC = 15.0 mA
LEDs ABCD = 17.5 mA
LEDs ABCDE = 20mA
LED A appears just as bright when all five are lit as when it was on its own.
The voltage bounces around a little bit, but stays around 3.2 volts. The current for each individual LED and each set of LEDs tested also bounced around a bit, but stays close to the described values.
Based on the individual reading, I expected the combined reading would be closer to 45.4 mA; instead I got less than half. Where am I going wrong? I know the pots on the voltage adjust and fine adjust knows are a bit dirty, does this explain what I'm seeing?
Power supply is set to about 3 volts per the analog needle on the power supply dispay.
The digital volt meter says theis is 3.258 Volts and is steady under no load.
The Fluke 77 DMM is displaying current on the 300mA max input.
A set of LEDs harvested from the same string of xmas lights tested individually draw this much current
LED A) 10.7 mA
LED
8.9 mALED C) 9.8 mA
LED D) 9.4 mA
LED E) 6.6 mA
When I connect more than one LED in parallel, the voltage stays the same, but the current doesn't add up.
LEDs A&B = 14.8 mA
LEDs ABC = 15.0 mA
LEDs ABCD = 17.5 mA
LEDs ABCDE = 20mA
LED A appears just as bright when all five are lit as when it was on its own.
The voltage bounces around a little bit, but stays around 3.2 volts. The current for each individual LED and each set of LEDs tested also bounced around a bit, but stays close to the described values.
Based on the individual reading, I expected the combined reading would be closer to 45.4 mA; instead I got less than half. Where am I going wrong? I know the pots on the voltage adjust and fine adjust knows are a bit dirty, does this explain what I'm seeing?
Comments
LEDs have a very non linear current consumption as voltage increases. They don't light up until a threshold voltage is reached. After that trying to increase the voltage just results in runaway current and a dead LED.
Now, when you parallel LEDs they won't all have quite the same threshold. One will turn on at a little lower voltage than the others and hog all the current. The others never see enough voltage to start to conduct or light up much.
Hence every LED needs it's own current limiting resistor which will cause the current to be shared among the LEDs.
To see the current hogging effect in the extreme put LEDs of different colours in parallel. Say red and blue. Only the red will draw current and light up.
Thevin's equivalent circuits will NOT apply. You get what you get and it is next to impossible to fully explain why - many curves involved and different thresholds. Each LED has its own characteristics.
Be very wary of Meters on mA ranges : The scale says mA, but quite a lot of mV are usually involved, it is common to need 200mV at the full scale, and some I've seen need 2V (!).
If you really want to track current-sharing, put a 1 ohm, or 0.5 ohm in series with each led, and measure the (mV) voltage drop across that.
That means the LED operating condition does not change as more are added. (presumes your power supply is solid).
While I agree with the issue of matching ...
last 2 weeks I re-worked Some commercal LED street lamps . Faulty solder joints, they had LEDs on a Metal PCB and they too were on parallel.. I was kinda shocked too of how they were wired .
24V DC DC > 6X5 LED array . 14.4 or so light up voltage .
In that test circuit, the voltage measured by the meter somewhere inside the power supply is not the same as the voltage across the LEDs. In addition to the internal resistance of the mA meter that jmg mentioned, there are an unseen host of other wires and connections that are dropping voltage. Your result is not surprising. But rest assured that KCL (sum of currents at the LEDs) does add up to the current you read on the meter. And if the LEDs are truly connected in parallel, as in soldered together at their leads, then there will be one voltage across them and the individual currents will correspond to a definite operating point on the I vs V curve of each LED.
OK, I have the digital volt meter across the output of the bench supply, it reads 3.258v. I think this can be considered a constant voltage source? Although it bounces around in the 0.0xx range, is this my problem?
All the LEDs are harvested from the same string of cheap Xmas lights, white. I guessed that the bench supply was a constant voltage, and the LEDs draw as listed in post #1 at 3.258 volts.
There are NO current limiting resistors. Though being that if all his buddies are gone, he'll still only draw what he draws alone. Is this my problem?
The LEDs are in a bread board. When I plug in one or plug in them all, the voltage reads 3.2xxV (pretty much the same), the individual brightness of each LED appears constant whether it is alone or with others. Only the current draw changes, but it doesn't change as expected. The currents don't add up.
My question: Is this what we should expect? I though that the current draw from multiple loads together would be the same as the individual loads alone, added up. I did not expect the last one to increase the current draw by only 3mA, when it drew 10mA by itself.
Yes, use your second meter, to measure the voltage across the mA meter.
Measure the voltage at the LED itself, (after the mA meter) you should be able to adjust the power supply, to get the same LED voltage operating point, and at the same operating point, it should draw the same current (assumes same die temperature).
What colour are these LEDs ? 3.258v is a little high for direct 0-Ohm drive of most LEDs ?
The external volt meter is connected to the terminals of the bench power supply. Since the LEDS are all in parallel, I'm guessing the volt meter connected at the bench suppy terminal is equivalent to the the volt meter at any give LED. Or is the length a wire and the bread board connection a significant factor? Should I solder a a little test board? I do have a ton of these LEDs.
The external Amp meter is in series on the black connector the the bench supply. Since its in series, I think its the same whether its on the black wire, the red wire, or in between any pair of LEDs. Is this correct?
So, maybe I should ride the voltage adjust on bench supply, and readjust to always get the EXACT 3.258 volts before I record the current? So, lets see, the tiny fluctuation in the current draw from the individual LEDs causes tiny fluctuation in the voltage supplied by the bench supply, and this could account for difference I'm seeing?
Ok, I think I got my head around it. I'll give this a shot. This would prove the (low) stability of the bench supply, and explain why the previous owner got rid of it. Cool!
For the experiment:
Connect the external voltmeter directly across the LEDs in parallel, not at the power supply terminals. Yes, all those extra pieces of wire and mechnical pressure connections make a difference. In fact, to put the LEDs in parallel, solder them together with the tightest possible connection.
Your experiment, seems to be to verify Kirchoff's current and voltage laws in relation to the LEDs. Each LED has what is called in EE talk a "constitutive relationship" between its voltage and current. For a resistor the relationship is linear, but for an ideal diode it is exponential, and for a real diode it is a combination of an ideal diode in series with an ideal resistor (plus other stuff like temperature dependence if you want to get fancy.). Sometimes this kind of measurement is done with short pulses so that the device does not have time to get hot. That is why specs on data sheets have to be scrutinized. But those are details.
Start with one LED, and connect your meters to measure current and also the voltage right across the LED terminals. Increase the power supply voltage gradually and record the current and voltage (at the LED and also at the power supply) at each step. To make it less subject to fluctuations, add a fixed resistor like 10Ω in series between the supply and the LED+voltmeter. You are right about the mA measurement--It does not matter where in the loop you put the ammeter. You can do that for a couple of the LEDs to see if they are different. Make a graph of LED voltage vs current. The reading on the power supply is just for curiosity, to see if it differs from the LED voltage.
Now having the graph, the constitutive relationship for each LED, you can start putting LEDs in parallel (solder the leads together in parallel, and the external voltmeter clips right to the point where they join). Do the same experiment varying the power supply voltage and recording external voltage and current. For each voltage you measure on the external voltmeter, look up the current you measured for the individual LEDs at that voltage. Add up the current for the individual LEDs. That should match pretty closely the reading on the ammeter.
If you measure at the PSU terminals, you are on the 'wrong side' of the Amp meter.
Measure instead directly across the LED, and you should get two points on a curve.
I,V for one led, and I,V when all leds are in parallel.