Voltage Conversion Question:

[Charlie]

It seems my Apple II's PSU is starting to cry "Uncle!"

...a combination of age and all the stuff hanging off the computer I think. Time for a new / upgraded PSU.

Today's dumb question:
I have a Meanwell PT65A PSU kicking about: +5v, -5v, +12v outputs... but the darn Apple needs -12v too.
I believe one can use a voltage regulator to flip a +ve to a -ve voltage, but I don't actually know how. My less than stellar Googling has let me down once again, can anyone help?

Thanks.

 

[TheCorfiot]

Hi Charles

I suggest you put the PT65A aside for a BBC Micro and get a PT65B, they are £17 ea from Mouser, and you may need two, One for the GS too.

The PT65B will give you a -12V line instead of the -5V and you can generate -5V from the -12V by simply using a 7905 Regulator and 2 x 33MicroFarad Capacitors.
Beware however the Metal tag of the 7905 must NOT be Earthed or come in contact with the PSU Metal Chassis, it needs to be isolated.

Saves an awful lot of messing about.

 

[Charlie]

Thanks Bas.

Yes, I have a spare PT65A having just upgraded my Beeb's PSU. This one was earmarked for my Master but as my IIe's need is greater I thought...
...still, you're right a PT65B would be a better choice, and I have some MAX660 ICs for the -5v. As it happens my GS has gotten a bit flakey since I replaced the CPU and crystal in my accelerator. Pretty sure it's the PSU so I guess getting two would be sensible.

Thanks again.