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LED Driver Help - 24 Leds, Red, 2.1-2.4 Drop, 700ma? Answered

Hey everyone

Been scratching my head for awhile now about this one. I believe I got the wrong drivers so I'm trying to figure out which one to get.

I purchased these

And I got this driver to run them

But like I said, I'm running 24 in series not 18 and this drivers output is 680ma not 700ma

I was going off the advice of a friend but I think he's wrong and I wasted my money (I can use them later though I'm sure)

My question is
Which LED Driver would be appropriate for these?
If I increase the current (amps) would the wavelength change? It's just a tad lower then I wanted (more curious then anything)
Can I alter the driver I got to accomidate the leds to save me money
For future reference, how can I figure this out for myself

Thanks guys, really hoping to hear back from you!


Sir I designed 20w led driver @750mA. but the value of current is going decrese w.r.t time. how i can troubleshoot


4 years ago

  • Twenty four LEDs at  2.1V drop needs about 44VDC
  • Twenty four LEDs at  2.4V drop needs about 58VDC
  • The Driver is 36V to 75V covers your worst case Min / Max VDC range
  • and delivers 680 ma,  which is a perfect current
  • I never run LEDs at peak current anyway.
  • The LEDs will be a tiny bit cooler
  • There should be no wavelength change.

This driver and the leds should work together quite fine. This is a constant current driver that will deliver around 680ma (maybe 700, maybe 660) as long as you make sure the Voltage drop is between 36V and 75V.

The Voltage drop:
You just have to count LEDs for that.
18 white leds 3.2-3.4Vx 18 = 57V-61V, so that's fine.
24 red leds: 2.1-2.4V = 50.4V-57.6V, so that's fine as well, could even add a couple more (about 5 or 6).

Can it be modified?
Theoretically yes. You would have to find the sensing resistor, make that 10% smaller and make sure that the extra current can be suffered by all other components as well. Is it worth it? No! As far as brightness goes, 680ma is almost at 750mA, and the LEDs last longer. If you need more light, add another LED in series

Using it in parallel with another driver?
No. Usually works with linear regulators, but not with more complicated switching regulators  like this one.

LEDs this bright need cooling, their back has to be attached to some heat sink.

Other problems:
The back that goes to the heat sink is often connected to one of the LED's legs. So, if you connect more than one LED in series and then put them on  a heat sink, you'll create a short.
The easiest solution: Put each LED on its own aluminum star pcb and then put them on a heat sink.

More LEDs:
You can add a few more red LEDs to this driver. But for more than 30: Like you suggested, two drivers on totally independent circuits. Basically, as far as electricity is concerned you are building two separate lights.

Different color:
No. LEDs change their color slightly when driven with a higher current, but if that's really noticeable, it means the LED is getting too hot and won't last too long. 

The blinking:
I assume, you got a bad connection or a short somewhere (probably the heat sink problem (see above)

To test:
If you have a (cheap) multimeter:
See if your multimeter can test the LEDs. disconnect the power supply for the LED light and test one LED. Setting the multimeter on Ohm, touch both sides of the LED with the multimeter cords (leads). The LED should glow slightly in one direction and do nothing the other way around. Test all LEDs this way, to make sure they're all soldered in the right way. If that is correct, connect one side of the multimeter to the heat sink and then touch each LED leg. There shouldn't be any connection.
If you don't have a multimeter:
You can test each LED with a 3V button cell. It should light up. If you use something more powerful (like a power supply, add a small resistor or a 3V lightbulb in series).
Test like above. For the heat sink testing, use a non-LED-flashlight with two batteries and two cables. Connect one cable to the heat sink, and if you touch the heat sink with the other cable, it should light up. If you touch any leg of any LED nothing should happen. Don't use anything more powerful than a 3V flashlight. Also, the light bulb works as a resistor.

You only need to post a question in one area. No need to post in both the Q&A and the Forums.

But like i said in your Forum post. This is all pretty straight forward. High power LEDs like that need drivers. So you but a driver designed to run them. The drive you link to is designed for 10 to 18 3W LEDs. So if you want to run 24 of them you'll need to break them up between 2 drivers. Also the wavelength is the color of the LED. Changing voltage and current of the LED isn't going to affect the color ony the brightness.

Don't alter the driver and don't try to run more than 18 LEDs off it. It may work for a while but you will damage the driver.

Deleted the forum topic

Would I solder the two drivers in parallel while I wait for the new ones?
What is the equations so I can calculate which drivers I actually need?

Finding the proper voltage output, ma's etc. It's all very new to me.

Thanks for the response, I hope to hear from you again soon! Very exciting stuff!

No equation. You look at the drivers and the specs tell you how many LEDs they can handle. Nothing to calculate there. When you're dealing with high power LEDs like that it's a whole different ball game to dealing with your basic 2V 20mA LEDs.

If you want 24 LEDS and you have to use more than 1 driver from a single power source, then yes, wire the drivers in parallel as long at the power source can handle the amperage needed. In this case the driver you linked to needs 36V to 75V DC and will draw 680mA. So a pair of them wired in parallel will draw a little less than 1.4A.

Oh snap, I gotta hook up the drivers to power supplies haha didn't even cross my mind.

I couldn't find the 20mA requirement for the LED's, that makes my life a lot easier thank you.

As for heat I'm expecting a large amount, any cheap heatsink suggestions?

Thanks again!

20mA is what a standard LED draws. 3W LEDs use more power. In this case they need more voltage. If you want to know how much current the LED will draw divide the power by the voltage. In this case if the drivers if offering 36V then the driver will provide about 8.3mA. See the driver offers a constant voltage and current so the LEDs don't burn out and resistors are not needed.

As for heatsinks consider what kind of configuration you want the LEDS to be in. Rather than buying individual heatsinks for each light maybe they can all be put onto a solid strip of aluminum. Also consider whether there will be good airflow for them. If so a solid piece of aluminum may be fine and you won't need a finned heatsink.

ok so I need a 70~volt driver to keep them going at about 20mA. What is the equation to figure that out? I don't remember it from Physics class.

I still need a power supply, I'll have to find a old PC supply I guess and wire it up

As for heat sinks, it's 48 lights on a small 1ft x .5ft square so I'll need to add fins to the back of the aluminum pcb board. (seperated into two circuits)

Ok we got a little off there. The drivers plug into the wall and output between 36 and 75 volts. There is nothing to figure out there. Look at the specs on the drivers it tells you all you need to know. They are made to support between 10 and 18 3W LEDs.

Now we start this line of thinking with you using only 24 LEDs now your saying 48. How many LEDs are you trying to power? If it is 48 then you will need 4 of those drivers to power them all.

You'll just need to look around far a rather large heatsink. It' won't be easy and it won't be cheap either.

Sorry, 48 is the total. My bad

So i got the drivers today, I soldered them on and this is what happened

1 driver- The lights flicker on and off, not all of them turn on (maybe one or two) and when I unplug the driver the lights slowly dim then get really bright and turn off.

2 drivers- most of the lights don't turn on and when unplugged nothing happens and the drivers slowly release there capacitors

So I'm confused, why are the lights flickering and not turning on properly? I hear a slight squeel noise from the drivers but I presume that's just the frequency

I checked the voltage and these drivers should work fine as red led's use almost 50% less power then white.

(I have a PCB board made out of aluminum which has 48 lights seperated into two series of 24 lights each with it's own + and - connectors)