Power Supply

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When working with electronics, you always need one basic thing; power. This power supply is great for powering all kinds of electronic projects. It produces a well filtered, variable 1.2-30 volts at 5 amps. It is easy to build and the parts are realitively easy to find.


This is a schematic of the Power Supply


Total Qty.
C1114000uF or 10000uf 40 VDC Electrolytic Capacitor
C21100uF 50Vdc Electrolytic Capacitor
C310.1uF Disc Capacitor
C410.01uF Disc Capacitor
R115K Pot
R21240 Ohm 1/4 W ResistorSee Notes
U11LM338K 1.2 to 30 Volt 5 Amp Regulator
BR1110 Amp 50 PIV Bridge Rectifier
T1124 V 5 Amp Transformer
S11SPST Toggle Switch
MISC1Wire, Line Cord, Case, Binding Posts (for output)


  1. The regulator comes in a TO-3 case and MUST be used with a LARGE heatsink. You may want to mount a small fan to blow air across the regulator (I did).
  2. The filter capacitor is large. It won't fit on any board so bolt it to the case.
  3. You can, of course, add a volt and amp meter.
  4. Since this project operates from 120 VAC, you must include a fuse and build the project in a case.
  5. R2 may need to be decreased to 120 Ohm if you experience voltage drift at light loads. 240 Ohm may not load the output appropriately on some regulators. The datasheet for the LM338K does specify 120 Ohm (I suggest you use a 1/2W unit) so you may just want to use 120 Ohm and not bother with the 240 Ohm resistor showin the parts list. This has been discussed on the forum.

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Power Supply
Tuesday, January 20, 2015 5:43:46 AM
can i used 24 v 10 amp transformer with this circuit.
Hassam Balouch
Power Supply
Tuesday, November 12, 2013 11:37:31 AM
I have made this successfully but i did some changes by using LM350T volt Regulator and 10k pot .
Power Supply
Thursday, June 27, 2013 11:29:09 PM
Hello again. I commented on this LM338K-based power supply. back in 2008 when I built a dual version for my G-Scale Model Railroad. It's still working great with no problems! It's great circuit. It's very stable and easily puts out 5 amps at up to 30 volts. The amperage has been checked with a DC clamp meter and suitable dummy load. Thanks again. I really like your website here, too. Regards.
Jason Timbrook
Power Supply
Monday, February 27, 2012 1:12:09 AM
Hello, I've never wired up anything, but as a working tattoo artist, have run across the need to. What I am looking to build is a power supply with a 3 way toggle that could run 3 separate tattoo machines, one at a time. Preferably 0-18v @ 2a. Could someone please help me come up with a parts list and wiring diagram for something like this? It would be much appreciated. Thanks in advance!
Power Supply
Monday, August 01, 2011 8:48:32 AM
I want to build a power suplly inwhich 220voltage (vac) is used and variable voltage 0-----30 volts will outper(DC) and the minimum output current is 1 amp Please help me to bulid this circuit diagram I WILL BE VERY THANKFUL TO YOU.
Power Supply
Tuesday, March 01, 2011 8:55:09 AM
I have given up trying to locate a 24V 5A transformer. they do not exist. the one with the lowest amps I can find is a 10 amp. will that be ok?
Power Supply
Friday, December 10, 2010 5:33:46 AM
R2 gets super hot, and the pot get in fire. These 2 resistors are connected from the positive to the negative rails, WHY? It burns the resistor across the rails. I am sure this circuit diagram is right, as I saw it from many difference places, but why the pot burned? I am sure I have a right setup.
Adam Kellerson
Power Supply
Sunday, June 20, 2010 6:55:42 PM
More a curiosity question than anything. The LM338 schematic shows an input voltage of greater than 28V while you have a transformer outputting 24v spec'd out. (assuming 120Vac input) ?
Power Supply
Sunday, February 14, 2010 11:47:34 PM
I have a couple 13.8 vdc power supplies that were used for ham radio; I'd like to build an aux. project box to use as a 1.2 to 12 vdc, 5A Variable Power Supply. If I include everything to the right of the bridge rectifier in the above circuit, am I on the right track? ... Thank you very much for your reply!
Mike Tanner
Power Supply
Wednesday, January 20, 2010 6:12:41 AM
Hi Aarron What pin on the 2n3005 is the IN, OUT and ADJ. The data sheet for the 2n3055 does not give this kind of info. Am I to assume that the IN is the Collector, the OUT is the emitter, and the ADJ is the base. I have tried to wire this up but it is not working. I may have burn something up or have it wired up wrong. Thanks Mike
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