ISA Voltage Regulator Cards?

stripey357 asks: "I'm trying to install a Dialogic Gammafax card in Pentium-based SCO unix box, and I've run into a problem. The power supply for this box doesn't supply -5v to the ISA, and that's the voltage the Gammafax wants. Do they still make ISA voltage regulator cards that can reg -12v down to -5v, or would it be easier to replace the power supply?"

Comments

Possible solution...

[cr0sh]

Not sure if this will work or not...

If it is just that card that needs it, and you know which pin it is on the ISA card, you might be able to do it yourself, with a little work:

The power supply should have a -12 volt line (or even a -5V line, which is easier) - find the pin(s) on the ISA card that need the voltage, trace each one back to a solder pad (or you might have to scrape the trace). Cut the trace with an xacto knife to keep voltage flowing into the motherboard (unless you know that it is routed properly - better safe than sorry). If you need to scrape the trace, do it with the knife as well. Apply some solder, add the wires you need, bring 'em together at one point, and hook up the -5V if you have it from the PS, or if you only have -12V, used a -5V regulator (sorta like the 7805, which is a +5V regulator - they make the opposite, can't remember the part number).

Of course, this invalidates all warranties, and it could cause other problems - it is a hack, plain and simple, I am just offering it to you as a possibility and learning advice - don't blame me if your motherboard/fax card/power supply blows up...

I hope this helps... Good luck!

7905 - 3-Terminal Negative Voltage Regulator

[Black Acid]

Do they still make ISA voltage regulator cards that can reg -12v down to -5v?

You're looking for a 7905 regulator, provided the card does not require more than 1A. These regulators are quite common LSI devices used in hobbyist electronics, they can be found at Radio Shack. Several firms manufacturer 7905's, here are some datasheets:

• Fairchild
• Micrel
• Microsemi

Installation is relatively simple. Pin 1 connects to ground, pin 2 to -12V, and pin 3 to the -5V line (this is the output). 7905's are often in a TO-220 package which allows you to screw on a heatsink if needed. Be sure to apply thermal grease to increase heat conductivity.