-5V support pcb ISA ? \ VOGONS

-5V support pcb ISA ?

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First post, by Jackhead

Posted on 2024-08-24, 08:55

Jackhead Online

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I need -5V for a ISA Soundcard.
My problem is, im not a big friend using 30+ years old PSUs on my expensive retro hardware.
So i come up with this little adapter PCBs that you plug into a ISA slot to get transformed -5V to the ISA bus.

The attachment s-l1600.jpg is no longer available

Anyone use this kind of pcb?
How exactly did they work? Any risks?
How they work if you also have VLB slots?

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Reply 1 of 2, by Grzyb

Posted on 2024-08-24, 08:59

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Search for "Voltage Blaster"...
https://www.philscomputerlab.com/voltage-blaster--5v.html

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Reply 2 of 2, by mkarcher

Posted on 2024-08-24, 10:09

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Jackhead wrote on 2024-08-24, 08:55:

How exactly did they work? Any risks?

They take the -12V on the ISA bus and use a voltage regulator that passes some current from the -12V pin into the -5V pin, in a way that at the -5V pin there is just -5V. The remaining 7 volts are converted to heat in the regulator. Think of the regulator as a self-adjusting resistor that removes the excess voltage so just -5V remains.

The 7905 regulator is quite robust. It has short circuit protection and over-temperature protection, so the risk of that chip breaking and passing -12V directly to the -5V line is very low. As long as the power supply does not connect to the -5V line on the mainboard, this PCB is the only source of -5V, so there is no conflict which might damage a second -5V source.

Jackhead wrote on 2024-08-24, 08:55:

How they work if you also have VLB slots?

There is no -5V connected to the VLB part of VLB/ISA slots, so you need to extra hardware to make sure that -5V is injected into the VLB part as well. This adapter hangs over towards the rear of the computer, so it will also fit an ISA/VL combination slot.

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