Help with multi LM317 in parallel

dethlore · Jun 11, 2012

I'm trying to set up a circuit to drive some custom LED lighting in my office space and need some help getting the LM317s working properly.

I have 5 different color LEDs and will be using about 1A Max for each string of switchable LEDs (50 LEDs).

My problem is while testing my LM317s in parallel, I have 1 with an output of 46mA, and the rest are outputting only 6 mA. All LMs have 23 Ohms on Adjust to Out pins.

The power source is 13.8V up to 25A.

Thanks for any help you can give!

justinjja · Jun 11, 2012

Draw up a circuit schematic, its hard to tell what is wrong from only what you have said.
With 23 Ohm resistors you will only get around 50-60mA output, for 1A you would need 1.25 ohm resistors
also 1A sounds like a lot, what led are you using?

This is what it should look like:

Bionic-Badger · Jun 11, 2012

I don't think you have enough voltage. The LM317 drops about 3V, leaving you with 10.8V for the LEDs. If each strand has 10 LEDs (5 strands of 10), and each LED needs a minimum of 1.7V (for red), you won't have enough voltage to turn the strand on.

dethlore · Jun 11, 2012

Your schematic is exactly what I have... except I was going to run the LEDs of each strand in parallel and up the Amps and not rely on so much volts.

I was using 23 Ohms just for testing 2 or 3 LEDs in parallel on each strand. I don't want to push 1 Amp each strand until I have the things working properly...

justinjja · Jun 11, 2012

LEDs won't work well in parallel, it will also give your circuit very bad efficiency.
What LEDs are you using?

dethlore · Jun 11, 2012

I have built a 9 LED circuit that runs all in parallel with a single 18650, but not using an LM3XX, but regardless, I don't see why the LMs are giving only 6mA except one at 60mA?

I'm using 5mm LEDs. Red, Blue, Green, White, and UV. All clear. 3 rows of 5 colors of 5 sets of 10 = 750 LEDs total.

So... if these need to be in series... would it be better to use a 120V circuit?

justinjja · Jun 11, 2012

dethlore said:
I have built a 9 LED circuit that runs all in parallel with a single 18650

The problem with that is, if one led is more efficient than the others,
It could end up running at double the current you intended,
and the less efficient diodes run at half the current you intended.

Also if one dies when they are in parallel, you could start a chain reaction and kill them all.

dethlore said:
I don't see why the LMs are giving only 6mA except one at 60mA?

are they running different color leds?
different color leds have different voltage/current requirements

dethlore said:
3 rows of 5 colors of 5 sets of 10 = 750 LEDs total.

post a pic of what you have so far, it is diffictul for me to visualize this...

dethlore · Jun 11, 2012

Makes sense why to not parallel... but it seems easier to get high amps than it does high voltage in this application.

The different output on the LMs are tested with the same loads... and all the LEDs are rated for and should be running the same current. so the difference doesn't make sense to me.

I'll try to get a better schematic drawn for the entire application tonight.

justinjja · Jun 11, 2012

Don't 5mm led run at like 50mA or somthing like that?
Or is it 20mA?

dethlore · Jun 11, 2012

I thought they would be higher than the 3mm, but the manufacturer says they're 20...

justinjja · Jun 12, 2012

Alright here is what you need for 20mA led's

The voltage needs to be atleast 5(or the number of led's in series)*(individual led votlage) + 5v (for lm317 and resistor voltage drop)

by adding a 50 ohm resistor to each string you can help ensure that all led's run at the same current.

also a single properly heatsinked lm317 can run upto 100 strings of 20mA led's

dethlore · Jun 12, 2012

justinjja said:
Alright here is what you need for 20mA led's

The voltage needs to be atleast 5(or the number of led's in series)*(individual led votlage) + 5v (for lm317 and resistor voltage drop)

by adding a 50 ohm resistor to each string you can help ensure that all led's run at the same current.

also a single properly heatsinked lm317 can run upto 100 strings of 20mA led's

So here's my original idea... which was to have 3 levels of lights, under desk, on desk, above desk. Each level has 5 different strings of 50 LEDs that I want to control brightness separately on.

Now, instead of parallel 50 LEDs like originally thought, I could use your last drawing and with 13 volts have 4 LEDs in series (5V for LM317/resistors, 4*2(LED voltage)) parallel with 12 rows (almost 50 LEDs) per string for... efficiency? if one set of 4 went out, it would still blow the rest, right?

Things · Jun 12, 2012

Why not just buy some RGB LED strip off eBay? It's extremely cheap and has built on current limiting resistors, all they need is 12V.

Dan

dethlore · Jun 12, 2012

Things said:
Why not just buy some RGB LED strip off eBay? It's extremely cheap and has built on current limiting resistors, all they need is 12V.

Dan

I suppose I can do that instead of using 5mm LEDs... but I still want to be able to switch on each color on each level and control brightness as well.

Just all on and all off is easy enough to do... but I want more!

justinjja · Jun 12, 2012

IMO running 50 diodes in parallel is bound to have problems, but who knows...

As for efficency, with your 13V psu the circuit you drew would have about 15% efficency
running 4 led's in series would give you about 60% efficency.

Things · Jun 12, 2012

Something like this: New 5M 150 LED Waterproof RGB 5050 Strip + free remote | eBay

There are thousands of hits for the stuff on eBay, and you can control it with an IR remote.

Just watch out that you get one with combined RGB LED's, not separate ones.

I used 15 metres of it to light up the front of my house for xmas:

Dan

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