high current regulated psu

Thread Starter


Joined Nov 22, 2007
Hey there, I'm working on a personal project, a large LED light board, I have a bunch of shift registers driving a bunch of darlington arrays, afaik this part of my circuit is working a-ok (driven by an atmega).

Now the board is going to have over 75 parallel circuits of LEDs, which means I have a rather high current circuit. I'v started building my own Power supply for this shindig, and my schematic for it is:

My problem is the specifics... I'm not sure of the resistors to be using.

for the technical details:
Transformer drops the voltage from 240v to 15/12v
Bridge Rectifier is connected to the 15v, and outputs @ ~13.4V DC

the weird thing now is my 7805 on the one side is pushing out 4.2v, and , with a 3ohm resistor in place of r1(got this idea from the 7805 datasheet), my output on the second supply is 12v.

I've been trying to do this on my own, but now that things are finally starting to come together and I can test things with the multimeter, I believe I may have either missed a step or doing it all wrong.

I would really appreciate any help anyone can offer for this, if you need more #'s just let me know.

Oh.. and: The purpose of the two circuits is to seperate the high current LED board side, with my shift-reg/microcontroller (low-current) side.


Joined May 16, 2005
Mr. Meval is correct, as usual; get rid of R2.

For calculating R1...

Vbe for the MJ2955 is 1.5V. DC current gain of same depends on collector current. What numerical value would you assign to "a rather high current?"

Thread Starter


Joined Nov 22, 2007
Ah I guess this is where it could get tricky, the current can change to be anywhere from 0 to ~5 amps. As its a 5x5x3 grid of LEDs, each independently driven.

lol @ the transformer, you are right, and I must have made that when I was not in a sober state :)

I saw the R2 being added in another schematic I had found, and it even had it all listed as R1=1ohm R2=10ohm, which differs greatly from the one located in the datasheet.

As for my 4.2V problem, what could cause the output to be lower then .. expected? I figured being a regulated supply, it shouldn't change depending on the load, So I haven't put much more then a few mA load on it, but it still only spits out 4.2V


Joined Jan 10, 2006
Where's the smoothing capacitor after the rectifier..??? Without that you have all sorts of problems. If you put a scope after the rectifier you would find not a smooth DC output, but a 100 positive half AC cycles every second (assuming 50hz mains) with a peak of around 20 volts. Putting them thru the regulator will cut off everything above the 5 volt mark, but the part of the cycle that drops to zero will still be present... hence your low reading.
Adding a smoothing capacitor of several thousand Microfarads and at least 25 volt, will fix those problems. The first thing you will notice is the voltage after the rectifier should go up to around 20 volts (with little load) and the output from the regulator should now be smooth, and as close to 5 Volts as the regulators are designed for.

The second regulator may either be faulty, the wrong type (ie a 7812, or 7905 etc etc), or the ground pin may not be connected (as the output voltage seems suspiciously close to the input).


Joined Nov 23, 2007
It doesn't need to be very exact, it will just alter how much power is dissipated by the regulator vs the transistor. Let's say your worst case is 5A. At 5A the MJ2955 has a DC gain of say approx 28. Let's give you an extra 250mA through the regulator. The base will provide 5A/28 = 179mA leaving 71mA to go through the resistor. As the base-emitter voltage is about 1.3V so we want 1.3V to produce 71mA thus need 18.2 ohms. The closest to this would be 18 ohms. I threw together the above but with a 15 ohm resistor (I'm out of 18s) and ran it off my 13.8V supply. Works perfectly, only 0.01V change from no load to 1 ohm load! Regulator will get hot. Transistor will get even hotter. USE BIG HEATSINKS! Your supply rail is higher than mine, use BIG heatsinks!

It is preferable to use the 12V winding of your transformer if it has sufficient current rating. Remember AC current is root 2 times dc current. Also if you put both devices on the same heatsink (good for temperature compensation) insulate at least one from the heatsink with nylon bushes & mica or silicon rubber washers. ;) As Gadget said, add a large cap after the bridge. Should fix your under voltage issue. For 5A you may want a fairly large one, 4700uF would be a reasonable choice but you might get away with 2200 since your supply is much higher than your regulated output.

Thread Starter


Joined Nov 22, 2007
gadget/rmack, thanks for your help!
I have them currently mounted on a pretty hefty heat sink, and have the silicon insulators

I'll rig it up with the caps and see how it works!
I had left them out until later as someone I was talking to said it would depend on my readings (the right caps to use...) but I guess this is exactly whats going on, my readings are messed *grin* And thanks a lot for the descriptive "technical" bits rmack

As for using a higher amp regulator (ie. LM2677), it's slightly two parts, I'm limited in my parts I have around here, and shipping those things in, although small, could prove to be twice the price of the current setup, being a student, the 20-30$ difference is kind of nice, but after the hassle I've been having with this, I may need to consider it for my next project.