Isn't it exactly the same as some ISA board mentioned elsewhere on the vogons, hosting 7905 regulator, taking -12V from ISA connector and feeding -5V back into connector?

vstrakh wrote on 2023-01-11, 09:36:

Isn't it exactly the same as some ISA board mentioned elsewhere on the vogons, hosting 7905 regulator, taking -12V from ISA connector and feeding -5V back into connector?

Yes, Necroware's Voltage Blaster.

But as far as I understand he wants to connect the PSU's -5V rail directly to the related pin on the ISA slot.

I am no electrical engineer but I'm pretty sure that voltage should go through some filtering/regulation and maybe even protection circuitry first?

appiah4 wrote on 2023-01-11, 10:25:

Yes, Necroware's Voltage Blaster. […]
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vstrakh wrote on 2023-01-11, 09:36:

Isn't it exactly the same as some ISA board mentioned elsewhere on the vogons, hosting 7905 regulator, taking -12V from ISA connector and feeding -5V back into connector?

Yes, Necroware's Voltage Blaster.

But as far as I understand he wants to connect the PSU's -5V rail directly to the related pin on the ISA slot.

I am no electrical engineer but I'm pretty sure that voltage should go through some filtering/regulation and maybe even protection circuitry first?

Usually the PSU's regulation and protection should be fine for -5V. If you're extra cautious you can add a capacitor between the ISA pin and ground to reduce ripple but that'll most likely fix a nonexistant problem.😉
If you do so keep in mind that the negative side of the cap goes to -5V and the positive side to ground.

I’d recommend a voltage blaster or pico atx-at adapter. (See pcb projects in sig)

But yeah, you could also just hook -5v straight to the pin.

If you use a regulator, The full regulator circuit usually includes some filtering caps be sure to use those.

If your psu has -5v how come your slot doesn’t? Did the mobo not route it? Or did the atx-at adapter you bought not route it?

I am actually using a pico atx-at adapter, that's where the -5v is being generated.
My issue is, the main board is some industrial type, and doesn't have a normal power connector, and it's missing any input for -5v, so I need to get it to the ISA some other way.

I am no electrical engineer but I'm pretty sure that voltage should go through some filtering/regulation and maybe even protection circuitry first?

This is what I was wondering.

I wonder, is -5v used for any system components on an AT class system?

If it's not, then the only hypothetical "damage" would be caused on the ISA cards using -5v.

And the few soundcarda who use -5v do require it for their opamp amplifiers, mainly, I suppose.
Which in turn won't suddenly die if the voltage is off by 1-2v.

Anyway, I would insert a little fuse in the cable. Just for the sake of safety.

A few extra caps may not hurt, either.

See De-noising and capacitors

Just be careful doing so (polarity of electrolyte caps), remember that you're working with -5v, not +5v.

Minus isn't really negative/ground per se, it rather mainly means that you're working below the defined ground voltage.
It's a floating ground, rather.
- Sorry, I don't know how to put this into words.

RockstarRunner wrote on 2023-01-11, 09:21:

My question is, can I take -5V directly from my PSU, and connect it to the B5 pin on ISA? Is it safe, or do I need something inline with that?

It is safe. -5V is often the least powerful supply rail anyway, and you usually don't have anything in line with all the other lines, like the +5V line (you could draw 20 amps on it on many supplies) or the +12V (even 10A there would yield 120W, which, applied at a bad location, could start a nice fire). Most mainboard do have some filter caps on all supply rails, but they are optional, as long as the rail is only used by an ISA card, which is supposed to has local caps for filtering anyway. You definitely do not need any kind of regulator. The -5V from any PC-type supply is already regulated. If the -5V is supposed to go along very thin traces on the main board, putting a 1A slow-blow (T) or medium-blow (MT) fuse in series with it might prevent trace damage in case of a short circuit. But I expect your board to either have no trace at all at B5, or have a trace as wide as all the other ISA power traces.

Thank you mkarcher and everyone else who offered info, I feel confident to go ahead now.

I agree with MKarcher. The -5v line is rarely used by comparison to the other lines.
And in addition, the devices that use them don't really draw current as such, but use the voltage difference for special purposes.
Like rail-to-rail operation in certain opamp applications.

That being said, be careful with old XT motherboards.
It *could be* that certain old DRAM chip types needed multiple voltages, including -5v.
In such an application, a separate voltage regulator might be worth considering, at least.
But that's a special case. It's not related to the AT mainboard in this thread.

Edit: I was still writing when the last reply was sent, my bad. 😅

RockstarRunner wrote on 2023-01-11, 17:13:

I am actually using a pico atx-at adapter, that's where the -5v is being generated. My issue is, the main board is some industr […]
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I am actually using a pico atx-at adapter, that's where the -5v is being generated.
My issue is, the main board is some industrial type, and doesn't have a normal power connector, and it's missing any input for -5v, so I need to get it to the ISA some other way.

I am no electrical engineer but I'm pretty sure that voltage should go through some filtering/regulation and maybe even protection circuitry first?

This is what I was wondering.

Ah, gotcha. Put a meter on the pin and measure to your power pins. If floating, go for it.