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I'm in the process of building a bench top power supply, based on an ATX PSU.

I have nearly finished the build and have a unit which can supply +3.3 V, +5 V, +12 V and a variable voltage between +0.8 V and +30 V. I am going to add a female USB socket but would also like to use the -12 V which comes from the PSU.

Ideally I would like to be able to adjust the negative voltage between 0 and -12 V, but I cannot find anywhere which describes how to go about it. Obviously, if it was a positive voltage I could use a buck converter, but I can't find one which deals with negative voltage.

Does anyone know if they exist.? If they don't exist, are there other options I could try?

Many thanks for any help/advice given.

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  • \$\begingroup\$ Welcome to EE.SE but please note that direct shopping questions will result in fairly prompt closure of your question as per site rules. I recommend that you remove the offending bits. \$\endgroup\$
    – Transistor
    Feb 4, 2021 at 18:13
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    \$\begingroup\$ The -12 V supply was added to help support RS-232 ports --- back in the day. It's a low power rail, generally speaking. So don't expect much from it. Have you considered using the LM337 as a variable regulator? \$\endgroup\$
    – jonk
    Feb 4, 2021 at 18:37
  • \$\begingroup\$ Thanks jonk, that looks like it may be the way to go, provided I can fit another module inside my casing. Things are a little tight in there already. \$\endgroup\$
    – cottonuk
    Feb 4, 2021 at 20:09

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ATX power supplies have a massive current capability on the +12V bus. You could make an inverting switching converter.

Or, for less current and -1.25 to -12V adjustability at a few hundred mA, use an LM337 as jonk suggests:

enter image description here

Typical -12V bus current ratings on a 300W supply are 300-800mA. With a big heat sink you can use all that capability down to the lowest output voltage.

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  • \$\begingroup\$ I have built a small pcb module using the LM337 and added it to my power supply and it works fine. Thanks for the information and advice. I haven't added the 2 1uF tantalum capacitors (as I've had to order some) but the unit works fine and provides a steady output. Are they really required? The supply is coming from an ATX PSU, so are there filters in there or do I really need to add the ones shown in the schematic? Thanks in anticipation. \$\endgroup\$
    – cottonuk
    Feb 12, 2021 at 17:35
  • \$\begingroup\$ I would definitely add the capacitors. C1 can either be an aluminum electrolytic or you can use at least 2-3uF ceramic capacitor with a few ohms in series. For C2 you can use just use a ceramic cap. I'm not a big fan of tantalums, they tend to fail catastrophically unless the current is limited severely and voltage derated. \$\endgroup\$ Feb 12, 2021 at 17:41
  • \$\begingroup\$ Thanks. The current is going to be no more the 800mA, as that's the maximum from the ATX PSU and the voltage will be somewhere between -0.5v and -10.24v, which seems to be the maximum I can get out from the LM337 module. I am using a 1k pot for R2 and a 330 ohm resitor for R1 instead of the 120 ohm. Will this restrict the output? \$\endgroup\$
    – cottonuk
    Feb 12, 2021 at 17:47
  • \$\begingroup\$ The 120 ohm establishes a constant load that exceeds the minimum load current on the regulator (about 10mA) so it's possible that the regulator could go out of regulation (output more voltage) than desired under some conditions. If you add a suitable load like 150 ohms to the output (8mA at 1.2V) it will get pretty hot (almost 1W) at full voltage. \$\endgroup\$ Feb 12, 2021 at 17:54
  • \$\begingroup\$ Sorry for being a bit dumb (I'm a newbie to electronics) but are you saying that I shouldn't use the 330 Ohm resistor, but that I should use a 120 Ohm instead? \$\endgroup\$
    – cottonuk
    Feb 12, 2021 at 18:18

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