C64 & 1541II HD Power Supply

   Heavy Duty Power Supply for the Commodore 64 and 1541-II  

with overvoltage protection.

Designed by ready


The following are rather tips than real instruction on how to build a power supply. If you need help ask me (danieleredivo(at)tin.it). Be careful especially with high voltage part, 110/220VAC can kill you! It is expected that you know about soldering and electronics for building this project.

  • Output specifications:
  • The power supply outputs are 5VDC, 12VDC and 9VAC.
  • Mains transformers specifications:
  • Transformer 1 - 110V/220VAC to 9VAC / 10VA
  • Transformer 2 - 110V/220VAC to 16 – 24VAC / 35VA

5VDC output:

5VDC is generated using LM2576-5 switching voltage regulator and can deliver up to 3A.

5VDC stage has an overvoltage protection based on MC3423 (overvoltage crowbar sensing circuit). The overvoltage threshold can be adjusted using R1 and R2. In my project I used R1= 6.3 kOHM and R2=5.88 kOHM, providing a trip at 5.4V (Vtrip=2.6*(1+R1/R2), but similar values can be used. I considered that R1+R2 should be around 10kOHM. Consider that Vtrip should be no more than 5.5V (5.5V is usually the lowest of the maximum voltages allowed by the ICs inside the computer, usually RAM ICs). It is good to test the Vtrip by using a variable voltage supply and ensuring that it trips at the wanted threshold. When the MC3423 detects an overvoltage, it opens the SCR crowbar (MCR69), which causes a short between 5V and 0V, the 3A fuse blows and the power is cut. Put capacitor C4 as close as possible to R1,R2: this helps to prevent false triggering of the overvoltage protection.

The embossed traces should be kept as short as possible to reduce electromagnetic interferences. Output capacitors should be of low ESR type.

In order to reduce the output voltage ripple, put a 100uF low ESR capacitor as close as possible to the power supply output.

My power supply had a maximum ripple of 90mV peak-to-peak.
Without the output capacitor, the ripple can go up to 200mVp-p and more.

12VDC output.

The same principles explained for the 5VDC stage are valid for 12VDC stage. Only the overvoltage protection changes.

12VDC uses a 13V zener for overvoltage crowbar sensing circuit, since 12VDC does not require such precise triggering threshold. Keep C3 close to the zener diode.
Make sure you test all outputs in loaded and no-load conditions before powering you Commodore equipment. Buy power resistors for these tests.
Put self shrinking tube around 110/220VAC wires, it is important these do NOT get damaged.

Make sure you have the correct voltage at the correct pin.

My prototype heavy duty power supply:


As you can see i forgot the heat shrinking tube around HV wires.

Red LED: 12VDC indication. Green LED: 5VDC indication. You might want to use a larger box.


Bill of materials:

  • - 1 x transformer 110V/220VAC to 9VAC / 10VA
  • - 1 x transformer 110V/220VAC to 16 – 24VAC / 35VA
  • - 1 x FB3510 bridge diode
  • - 1 x LM2576-5 3A Step-Down Voltage Regulator
  • - 1 x LM2576-12 3A Step-Down Voltage Regulator
  • - 2 x MBR360 Shottky diode
  • - 2 x 100uH inductance
  • - 1 x R1,R2 series (R1=6.3kOHM, R2=5.88kOHM) 0.25W
  • - 1 x MC3423 overvoltage crowbar sensing circuit
  • - 2 x MCR69 SCR crowbar
  • - 1 x ZPY13 13V zener diode 0.25W
  • - 2 x 100 uF 50V electrolytic capacitor (C7,C8). Note the rated voltage of 50V!!
  • - 2 x 1000 uF 25V low ESR electrolytic capacitor
  • - 4 x 100 uF 25V low ESR electrolytic capacitor
  • - 2 x 0.4A fuse if 220VAC or 0.8A fuse for 110VAC
  • - 1 x 4A fuse + fuse holder
  • - 2 x 3A fuse + fuse holder
  • - 1 x 1kOHM 0.25W
  • - board connectors
  • - one common heatsink for volt. regulator (or two separate)