# Help with some electronics math...

Hello Everyone!!! I would really appreciate your help fingering something out I think there might be some common math here that I am missing or not seeing.

Today I am working on a light from China that has 25 x 10 watt RGB LEDs on it, controlled by a microcontroller. It came with 2 bad power supplies from a previous repair attempt. Troubleshooting the device with a little walwart 12 volt / 1 amp I can get half of the LEDs to light Red, Blue, or Green using the microcontroller. Cool. However, when it hits the color White, it locks up. Obviously this is because you need all three colored LEDs on at the same time (RGB) to produce the color White, and this increased draw is simply too much for the 1 amp power supply and controller (and 4 fans! hehe). But it is great for testing.

Here's some math I think I can figure out - 25 X 10 watts = 250 watts (per color) 250 watts X 3 colors = 750 watts

4 fans, figure 6 watts? Sub Total = 756 watts Microcontroller = meh. let's just say 1 watt Sub Total 757 Watts

Let's set that 757 Watts aside for now.

The power supply for this unit has 2 (actually 3) outputs. 12 volts @ 6 amps and 24 volts @ 3 amps (the third output is 400v at .5 amp (note the decimal). - What!) . They have this light connected to what is marked 12 Volts (@ 6 amps)

Looking at the capacitors on the LED driver boards and microcontroller, they are rated for 35 volts. They use Surface Mount voltage regulators - not sure I can confirm those - The microcontroller board says 24 volts... However, the manufacturer says it should be run at 12 Volts.

During my testing, I worked up the courage to connect this up to my computer's power supply, which is what? 12 volts at around 40 amps?? Anyway, finally that was enough power for the light to hit the Color White and not lock up. Slowly, one at a time, I added one LED back into the the parallel chain of power. Slowly this added more and more current draw to the power supply... This made my test wires heat up.. My cheap little alligator clip wire... stinky. So I disconnected and have not reconnected since.

That's the background.

My question here is I feel it would be more efficient to run this light at 24 volts but this is against what the manufacturer says. Do they really even know, is another question all together because of other issues I've found with the power supply labeling.

Where I am not clear on is obviously if the LED driver boards are rated for 24 volts. To resolve this I need to look up the numbers on the SMT things, assuming I can even read them and they are labeled. The microcontroller board says 24 volts, so I am not too concerned about that except the 12 volt fans that are connected, I did not notice any regulators near the plugs.

During my testing of 12 volts in, I was seeing 4.8 volts out on the fan connectors.

So that's it, I guess, in a nutshell. lol

Am I right? If I run this light at 24 volts, do I only need 3 amps as opposed to 12 volts and 6 amps?

Is 6 amps even enough? Does the math work out? Meaning is 6 amps @ 12 volts = 756 Watts?

Would 24 volts @ 3 amps = 756 Watts and be easier on my little tiny alligator clip wires?

Originally I thought there was a short somewhere in one of the colors (and there still may!!), but I think I might be able to rule that out because running each color individually does not make the test wires warm. By the way, I only got 12 LEDs connected before I noticed the wires getting hot. Could there really still be a short somewhere?

Amps, voltage, and capacitors scare me and there are plenty here.

What caused the power supplies to go bad? During my testing of the LED boards, the capacitors would store the charges they had when powered up, unless the microcontroller set a bit that 'turned on' that LED. Touching the power wires together when no power was applied would produce a spark and a pop (12 volts at ?? amps). It is my theory that enough of these drivers with stored up power in parallel add up to a pretty big punch of power. ?? amps x 25 = ouch! Fry power supply. What I'd like to see is some super dooper (duper?) diode in line somewhere or something to take that hit and drain it right rather than feed it back to whatever unpleasantly grounds it........

Currently, I have a 19.5 volt @ 3.3 amp power supply (laptop charger) connected. This is the closest I can get to 24 volts. I am afraid to turn it on without "talking" it over with you all first.

I'd like to attach pictures, it is easier to imagine that way. There's one picture.

Thank you for reading and not deleting! I am stumped here. :)

12V @ 750W, that is just under 63 amps, I'm surprised that your croc clip lead didn't vaporize. Are your calculations right in that you need 250 watts per colour?

If they are right, then surely the manufacturer provided some hefty terminals for the supply, or even multiple terminals to share the current?

Double check your calculations and if they turn out to be correct, then make sure you use hefty cables to connect to the power supply. BTW, computer power supplies can provide huge currents but it's spread over many cables (12V is the yellow one). Don't try and pull so much current from just a single yellow cable but rather parallel them (first check all the yellow wires for continuity to confirm that they do in fact all connect to the same point inside the power supply- make sure you do this test with the supply off and fully discharged). Do the same with the grounds (the black wires).

As regards running the lights at 24V, unless the manufacturer specifically says that it's OK to do so, all that you may achieve is lot's of smoke.

Does it have 25 x 10W RGB LEDs, or 25 x R, 25 x G and 25 x B LEDs? In the first case, the 10W rating is probably for the RGB LED module as a whole with all 3 colours lit. [10W for a single LED would be unusually high]. That's still 250W total, which at 12V would be around 21A. No wonder your wires were getting hot.

Don't try to run it from 24V without first identifying the components in the LED constant-current modules.

3.3W per color per module seems far more likely to me (but 25 of them is still a lot of amps...plenty to burn a crocodile clip)

To stop the capacitors retaining charge for long after power down simply wire a 1K resistor across them. This will discharge them in a time given by approximately 3 times R times C where R is in ohms and is 1000 for a 1K resistor and C is in Farads

Some close-ups without camera shake of the individual boards might be handy (identify component markings).

Are you repairing it or is a junker that you're trying to get to light up for your own amusement?

Hi all!

Thank you for the replies, all very good.

I come from the days when 100 watt bulb was 100 watts and forget that LEDs can put out a lot of light too. Watts is a bad rating for light.

So as you say - these are Red, Green, Blue LEDs in one little tiny housing to produce a color, not 10 watts each. hehe

Hopefully this will help, after this post and first reply, I decided to remove the 19 volt power supply and try another 12 volt 3.75 amp supply instead.

Starting from scratch, adding one LED driver at a time (each driver drives 2 RGB LEDs), I made it to 8 LEDs on full white before the controller started puking (due to current draw). Rounding 3.75 up to 4 amps gives me 500ma per LED when it is on full white. This is not totally accurate because this same current is driving 4 small fans, controller, and little led display but close enough. That makes 6 amps per side? ouch.

As someone else suggested, the PC power supply is meant to spread that power across all wire feeds, not just one. The manufacturer also split the power input. There is still another half of this light I have yet to test. Feeling semi-confident this half is working OK I decided to order a 12 volt 10 amp power supply and went to bed thinking that will be enough. Then I wake up and wonder if it should be 12 volt at 12 amps. But how is it they sell these with 6 amp power supplies and they work OK? It doesn't work out... It will take a few days before the power supply arrives. It also has to output terminals. Hopefully they put out 5 amps on each.

Better picture.. I will take one of the LED driver boards tonight, but also as you guys suggested - stick with 12 volts. Probably a great idea.

Thank you again!

Do you understand the relationship between LED voltage and current ? It is not linear at all.

An individual LED junction requires between 2 and 4 voltage to begin conducting, depending on the semiconductor material used. Above this threshhold voltage, the current will rise rapidly. And beyond a certain level, additional current is not productive in terms of light output. There is no point trying to apply more voltage than what is sufficient to drive the LED to the appropriate brightness.

For the purposes of testing, it may be appropriate to use a car battery.

First, thanks to everyone that has helped me out on this project.

I have used the notes here to get me to the point where the light works but using an old PC power supply. It has only 1 power rail which is cool so I can take and combine the yellow wires and the black wires to help carry all the amps without overheating the wires. I did it this way because I am still waiting for the power supply to arrive that I ordered 10+ days ago (tracking information shows it getting closer every day).

This is a typical (eBay item #170884949622) LED power supply surrounded by a cage, they come in a variety of different currents all the way up to 20 amps. Regretting only ordering a 10 amp one before it even shows up. They are very reasonably priced.

The odd thing is the outputs are split up between V-, V-, V-, and V+, V+, V+ (for 20 amp). I've asked the seller if I must split the load between all three outputs - they replied with an internet link to the manual that doesn't work.. They don't know.

Does anyone here know? If a Power Supply is rated for say 20amps and it has V-, V-, V- and V+, V+, V+ as outputs, can I use one set of terminals or do I need to spread it out across all three? Seeing as how they are not numbered (V1, V2, V3, ...) maybe they are all the same and only provided for hookup convenience, not for load distribution.

I suspect that if you follow the traces, you will find that all 3 pairs of terminals connected together. However, even if this is the case, the terminals themselves and their connections to the PCB may not be rated for 20A, so you should run 3 sets of wires between the PSU and your load even if you are not splitting it.

Right on. Will do.

Thank you!

michinyon: For the purposes of testing, it may be appropriate to use a car battery.

...and a fuse!

A car battery can produce hundreds of amperes of current. A short circuit or a blown component on a board could literally start a fire.

Car battery: great idea.

High current: scary and dangerous.

Car battery + fuse: Problem solved! :)

Eddiie: I come from the days when 100 watt bulb was 100 watts and forget that LEDs can put out a lot of light too. Watts is a bad rating for light.

Actually, it's not bad for light, it can be perfectly sensible. It's just that incandescent bulbs were measured not by the wattage of the light they produced, but the wattage of the electricity they consumed. A 100W light bulb did not produce 100W of light, or anything close to it. Because incandescents are so horribly inefficient, only about 20% or so of the electricity is radiated as light; the remaining power was wasted as a lot of heat. So a 100W incandescent would only be good for about 20W of light, if that.

Compare that the the more efficient LED or fluorescent bulbs, which could make 20W of light with only about 25W of electricity. A lot of people are already familiar with the brightness of incandescents, so that's why the packaging has to say "Equivalent to X W lightbulb" on it.

Because incandescents are so horribly inefficient, only about 20% or so of the electricity is radiated as light; the remaining power was wasted as a lot of heat. So a 100W incandescent would only be good for about 20W of light, if that.

I thought figure was closer to 5% efficiency, but that just makes your point all the more.

Lefty

Closing this thread, the 10 amp power supply came in but was still not enough to current the light once all 25 LEDs hit white (75 LEDs). Put a 15 amp supply in and it works just fine.

In the end I left it at 12 volts as everyone suggested and went with large amps. The inputs on the power supply came from the same traces, still split up the load between the inputs because on set of contacts may not have been rated for the full load. (thanks for that note!)

Thank you all for your help! Moving on to the next item..