Barney's Cape Computer Club 6809 Microsystem: PSU

Barney built a very neat PSU section using all kinds of bits he had lying around.

When I started testing, I found that the 5V rail fell to 4V under very light load (this is after I replaced the big cap which had dried out).

The 5V supply is regulated using a Boschert 3T12AP switchmode regulator which is good for 12A. +/- 12V is via linear regulators, one for -12V and two for +12V (split between the floppies and the main system).

The +/- 12V section ("P-200") comes from an SWTPC system he had before.

At this stage I decided to scrap the PSU and move to a modern-ish switchmode supply. A quick search revealed that I had:

• Autek A55H1-04M: 5V 5A, 12V 2A, -12V 0.3A, -5V 0.3A
• Delta DPS-101BP: 3.3V 16A, 5V 20A, 12V 3A, -12V 0.3A
• Golden Power GPS25-03: 5V 6A, 12V 2.5A, -12V 0.4A (I have two, can dedicate one to the floppies)

The C3Bus makes provision for 5V, 12V, -12V and also -5V. This dates back to when Dynamic RAM was called 4116 and needed -5V. Even then it's silly, it would have been easier to regulate the -12V (used for RS-232, this was way before they invented the MAX-232) down to -5V, DRAM needs very little current at -5V (around 200 uA per chip). Barney didn't bother with a -5V supply at all. The Autek has all four, so all other things being equal we could get the PSU up to C3 spec... but will the Autek work? Let's see...

The Boshert regulator is good for 12A at 5V, but I doubt the system needs that much (or that the transformer could deliver that much). The two FD-55B floppy drives need a maximum of 0.6A each at 5V, that's over an amp already... the 5A from the Autek might be a bit marginal. But there's a bigger problem. The floppy drives also need a maximum of 0.9A each at 12V, as does the Motorola M68MDM1 monitor. That's a total of 2.7A (peak, a 2.5A supply could probably handle things). The Autek only has 2A. Out.

Since we don't need -5V, the Delta looks good. Lots of current at 5V, ignore the 3.3V... Now the thing with switchmodes (well, some of them, the Golden Power is stable at no load) is that they need a minimum current to work right. I was hoping I could load the 3.3V rail, but the Autek needs 675mA (4R7+2R7) on the 5V rail to stabilise (650mA using 2 x 3R9 resistors was not enough).

The Delta is a better choice than 2 x GPS25-03, it has more 5V and less 12V and that matches this system better.