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Constant current power supply, will this work? Answered


 i found some parts in an old computer, since i could not get my hands on an lm317

This is what i used,
1 x L7805cv voltage regulator ( from an old computer)
2 x 660nm 3 watt each running at 2.2v 0.700mA
1 x 10ohm resistor connect to ground and output
1 x 12v 1.5a power supply

I can run 1 or 2 leds and they are pretty bright, only the L7805 gets hot but i assume thats normal.

What do you think of this setup, will it last?

See pictures

Greets JB


It won't work. The resistor would be between Vout on the regulator, and gnd, the middle pin, and your output would be on the middle pin.

BUT the circuit is trying to force 5V to appear across the 10 Ohm resistor, so you have only 5/10 0.5 Amps in the LEDs anyway.

Hi Steve, all is fixed and running good :)

Thnx again for all yr help, i have also given you credits in my youtube movie.


I also had some leftover leds cause the supplier sent me 2 green instead of blue ones, so i made a simple 3 colored desk light.

Do you have a youtube channel, so i can subscribe to you?


HEy ! well done mate. I DO have a channel, but there's not much in it apart from some muddy landrovers with broken parts.


Greets Steve,

Hows this for a parts list :)

5 ST x 401730 resistor 5W 0,47Ohm
1 ST x 184061 Thermisch tape 25st
1 ST x 185900 Coolermaster koelpasta HTK-002-U1-GP
5 ST x 401790 Draadweerstand 5W 1,5Ohm
5 ST x 419222 Metaalfilm resistor 1W 1,5Ohm
10 ST x 419249 Metaalfilmweerstand 1W 2,2Ohm
10 ST x 419214 Metaalfilm resistor 1W 1,2Ohm
2 ST x 444812 potmeter 4W 500R
4 ST x 176001 1,5A LM317T
5 ST x 405256 Koolfilmweerstand 0,5W 0411 1,0kOhm
3 ST x 701343 Mini-Tuimelschakelaar 1 polig
5 ST x 401897 Draadweerstand 5W 10Ohm

I can start building again in 2 days max

Anything i should consider?

Greets JB

Draw me a circuit. There are a lot more resistors in the list than I was expecting......

And my Dutch ain't great ;-)

Circuit will be the one you showed me, but i just got some extra other resistors for other use, but the lm317, 0.5ohm, 1000ohm, and pot of 500ohm i will be using for the driver, first package arrives today, other one tomorrow :)

Only thing is that when turning the pot, i get values from 0.690ma to about 0.8ma, so the pot only adds more amps to the leds instead of lowering

Does this make sense?

Greets JB

Hi Steve,

I got all the parts :)

The circuit contains, (and correct me if I'm wrong)

R1 = 1.803ohm (10ohm with 2.2ohm, parallel connected) (0.694mA)
R2 = 1000ohm
R3 = 500ohm pot

I rather under-run the leds a bit than overrunning them

So, can i start :) ?

Greets JB


The current sensing resistor for the 317 needs to be 1.22/0.7 ohms, to have 700mA in the LEDs

So which circuit is good for me?

the one with the lm317, but i will be using the l7805 or the l7812, or do i use bobblehead Einsteins circuit since he is using a l7805

Use mine ! Einstein's circuit it NOT a constant current driver.

How did you wire your led's series or parallel?
the 7805 getting hot is a normal thing,The voltage difference is causing the heat(12v converted to 5 v).Make sure proper heatsink is used.
Seem's you are wiring up the 7805 voltage regulater in a wrong way, posting a circuit diagram for referance.


Please correct me if i am wrong ,
It seem's from the pictures that pin 2(ground) & 3(5v out) of the 7805 (that he is using) are in contact with each other(i don't think it is a good thing),Which is why i said that he is wiring them wrong.The pic that i posted is for the reference of how a 7805 should be wired

He's actually attemping to make a variation of this, which is a constant current source. It doesn't look to me like he's taking the load from the right pin of the regulator. The reg's a pull from an old power supply, hence the short pins.

trimmable current source.JPG

Hi Steve,

As you mentioned, that im taking taking the load on a wrong pin.

Could you please explain more?


Btw: happy holidays

If you remove the 1K and the 500R pot in my circuit, you can see that the "middle pin" goes to one end of the 0R5, and the other end of the resistor goes to the OUT pin of the regulator. That's NOT what you appear to have wired.

The 0R5 is that a 0ohm resistor, i think they call that the current sensing resistor right

No, like I replied already, its a 0.5 Ohm resistor - its written as OR5 so you don't miss that little point. A zero ohm resistor isn't a resistor, its a WIRE.

Thnx again, appreciate all yr help, it almost seems that you are the only one knowing so much about this.

I will now try to find more parts, by taking apart some more old computers.

When i have rebuild the circuit, i will show you again.

Hope you dont get tired of me :p

You need to find a 317, because otherwise you waste too much voltage in the current sensing resistor.

Do you maybe know the color rings of the 0.5ohm resistor?

I used this site to lookup resistor color rings http://www.camradio.net/resistors.html.

But for yellow, violet,black i get 47ohm, when i lookup green, black, silver i do get the 0.5ohm

You won't find a 0.5 Ohm resistor, for various, complex reasons. The closest you'll get is a 0R47

Thnx, now i understand.

Ive still been busy looking for this lm317

Sofare i have taken somany stuff apart, but i will keep on looking


Yeah i will, but thing is i cant wait, cause i love to be busy building stuff, or to write code.

But 1 thing is for sure, you have been a great help, and learned me new things. And thanx again for your time.

Ill be back :)

Greets JB

Bear in mind this is going to get HOT, and that's why you really need a dimmable supply. There are some nice chips to make your own, but they use surface mounted components, which you'll struggle with.


Yeah, i know

i do have a big alluminimum sheet where i will be placing the leds on, and a 12v 600mA brushless fann. The leds i will be mounting then with screws and ill be using adhesive paste or pads

I have placed an order for the lm317. @ www.conrad.nl but thnx to the holidays i have to wait again.

That circuit with the l7805 is that only a voltage regulator or just a weak constant current driver, cause i tried to look on the internet for

Lm317 led driver circuit VS the l7805 led driver circuit but i found nobody discussing it or what the differences are.

Would you be able to explain the difference between those two circuits?

Greets JB

1.) A voltage regulator is trying to control the voltage between two reference points. We call one the "output" and the other "ground"
2.) A 7805 tries to keep 5V between output and "ground"
3.) Ground is NOT NECESSARILY 0V.
4.) To make a current regulator, we put a resistor in series from the "output" to the "ground" pin.
5.) Now the regulator tries to keep a constant voltage ACROSS the resistor. If it can, then since I=V/R, then an I will flow, subject to the R being stable, that is INDEPENDENT of the supply voltage AND the voltage across the load.
6.) This can ONLY happen if there is enough voltage to go round ;-)
7. ) A 7805 needs to create/see 5V across the R. At 700mA, that means that R has to be around 0.5 Ohms.
8.) The load voltage will be 2 x 2.2V (or whatever)
9.) Total supply volts needed will be 2 x 2.2 +5 +1.5
10) Total dissipation will be total supply volts x 700mA. (Watts)

11. ) Use the 317, and it only needs to see 1.22 V across the reference resistor, so the resistor can have a lower value, and dissipate less power. Likewise, the supply voltage can also be lower.

Finally some good info, from only you again and again :)

And here is another question, for example i have 3 resistors, 12ohm, 25ohm, and 10ohm, all together is 47ohm, do i connect those resistors in series, or do i connect the 3 resistors paralel to get the 47ohm?

Resistors in SERIES add together. resistors in parallel obey this formula 1/R = 1/r1+1/r2+1/r3 .......

In series you get 47 Ohms, in parallel, you get 6.9 Ohms./....

Is there a way i can give you kudos, points, vote, like, or if u ever may need a allround C#, basic stamp, coder just holla, do you have a youtube channel? Mine is

I dont think 1 vote would be enough for all i asked you, they should be adding points like on stackoverflow.com based on every comment

Btw, i might have found out how to run this, i will make a wrong circuit drawing, and a good circuit drawing, so u can see.

And if i click the answer button, i wont be able to comment again, unless i check the unanswer button :p

You CAN, but not in the same chain of answers as the one you've ticked. You can post a new message, that isn't a reply.

Here are some pics of tests that i did, if i did not connect the minus from the powersupply to the middle pin of the l8705 i would get 12 from the ouput of the l7805, and when i do i get a +-5 volt

So i did 5 - 4.4 = 0.6 / 0.7 = 0.857Ohm

But thing is, instead of using an resistor i used a pot, then i checked amps on the leds max could be 890mA, but i adjusted it to 699mA

So im not sure on what ohm the pot is set at
Could be 2 values i can think of

1: 0.857ohm (5 - 4.4 = 0.6 / 0.7 )
2: 7.15ohm (5 / 0.699mA)

Leds have a good temprature, doesnt burn my fingers, even better it isnt hot and same for the l7805, and the pot also feels warm

Pic1: minus direct to power supply minus 12v
Pic2: minus connected to pin2 and to min of e power supply
Pic3: tiny circuit drawings


Hate to say it, but none of those are in constant current mode :-( The resistor is in the wrong place !

Parts are on their way :) when i have them ima build it and show you.

Greets JB

Btw, i do have a mean well lpc-35-700

But that one i will only use for my 6x 3watt 630nm leds
Cause last time i burned out 3 leds, so im kinda carefull to use the mean well, i burned 3 leds when i hooked up 3 red leds, 1 blue, and 1 deep red, the burned out leds was my 1 blue, 1 red, and 1 deep red.

The 2 constant current drivers i wanna build is for
1: 3x 3Watt 460nm leds (should be dimmable to)
2: 2x 3watt 660nm leds

And sofare all test i did by building a test driver myself did not burn any led

Just something i wanna get clear once and for all :)

When you say a resistor of 0R5/0.5ohm i keep on thinking 50ohm
Or is it really an tiny value resistor as in, NOT a 50ohm but 50/100 =0.5 ohm so 0R5 means a resistor that is a hundred times smaller then the 50ohm

So if i would write 50ohm i should write R5
And if i write 0.5ohm i would write 0R5

And from now on to make things easier i will draw a circuit drawing cause 1 pic says more then a thousand words

And 1 thing is for sure, this talk we have will help many others who would have any questions on building a hommade high powered led driver, including lots of details and wise answers you share

That's right. 0R5 means 0.50 Ohms. half an Ohm.

Especially when things are printed, those little '.' characters are beggers to spot, which is why the notation became popular.


Im stil busy opening up home appliances, and now i found the lm340t12, cause its hard to find the lm317