The idea of a new power supply for Commodore 64/128 came when I realized that original power supply for C64 was often the cause of failures in the computer. This is true also for power supply of drive 1541-II. Both these power supplies are linear power supplies and tend to give higher voltage than nominal with heat and age. A voltage higher than 5.5V can often be fatal especially to RAM chips. Original C128 power supply is dfifferent: that is a switching type of PSU and much more reliable. Furthermore C128 PSU have an internal overvoltage protection which can save the computer when PSU fails to regulate the 5V line.
PSU specification
Outputs:
- 9VAC @ 1A
- 5VDC @ 3A (in continuos operation, not just peak)
- 12VDC @ 1.5A (in continuos operation, not just peak)
Inputs:
- 110VAC @ 60Hz / 230VAC @ 50Hz
PSU type: transformer with double secondary + switching DC/DC regulators
Transformer type:
input frequency 50/60Hz
primary input voltage: 110/230 VAC
secondary output voltage A: 20VAC (30VA)
secondary output voltage B: 9VAC (10VA)
5VDC obtained via DC/DC switching regulator
12VDC obtained via DC/DC switching regulator
Overvoltage protection obtained via MC3423 + MCR69. Overvoltage has 3 thresholds:
1) 5.6V with 10ms delay to avoid spurious triggering (on 5V line)
2) about 7 V zero seconds delay (on 5V line)
3) about 14 V zero seconds delay (on 12V line)
PSU has several fuses to garantee safe operation:
1) 400 mA fuse on 110/230 VAC mains
2) 1.6 A fuse on rectified secondary output A
3) 1 A fuse on rectified secondary output A
4) 4 A self resetting fuse on 5VDC line (accidental shorting of connector pins will not kill PSU)
5) 3 A fuse on 12VDC line (accidental shorting of connector pins will not kill PSU)
Additional features:
- by turing ON/OFF your C64/C128/... directly from PSU switch (computer switch always ON) will allow a soft start (voltage will slowly rise), this is nice for old equipment and avoids voltage overshoot
- turn OFF in case of overvoltage triggering (failure of DC-DC converter) is immediate, to block the DC-DC converter before it rises the voltage too high.
First prototype, based on 2 transformers (still in use):
it had issues of triggering overvoltage protection at start up due to voltage overshoot. Solved in later models by adding 1uF capacitor in the switching regulator feedback loop.
