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I am trying to decrease the voltage of an ATX PSU to about 11.9 volts or so (it's 12.2v, a bit higher than what I would like for my project).

I did read some stuff about using the voltage sense wire with some resistors, but that only works to increase the voltage, not decreasing it. So I had this idea, of actually connecting the 3.3v sense wire to the +5v wire through a high resistor. Do you think this would work? And if so, what kind of pot range would you recommend?

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  • \$\begingroup\$ The problem I see is that you will be dropping different voltages depending on the load current. \$\endgroup\$
    – mcmiln
    Feb 10, 2016 at 22:09

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I cannot see why connecting the 3.3V sense wire will have any affect on the 12V output. All it will do is try to pull the 3.3V output as low as possible, because the sense wire is feeding back 5V which tells the controller that the 3.3V output is over-voltage. It may even shut down the 3.3v output.

You could use a schottky diode in series with the 12V output. A good schottky will drop about 0.4V to give you 11.8V. What current do you need to draw from the power supply? You need to chose a schottky diode that is suitable for the current.

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  • \$\begingroup\$ Schottky is a good idea, but I'm more interested by why .3V matters. If it's that big a problem then chances are this is near the max operating voltage or there's some majorly sensitive circuitry. In either case I'm wondering if he doesn't need an adjustable LDO he can fix at a lower voltage. It will help with transients as well. \$\endgroup\$
    – mcmiln
    Feb 10, 2016 at 22:15
  • \$\begingroup\$ The load is pretty big, about 10A. I need it for some LEDs. Right now, I am using resistors to drop the voltage, but I was thinking of something a bit more efficient. \$\endgroup\$
    – Radu
    Feb 10, 2016 at 22:57
  • \$\begingroup\$ @Radu why is it so important that it is 11.9V ? How accurate is your voltmeter? When was it last calibrated? \$\endgroup\$
    – Steve G
    Feb 10, 2016 at 23:01
  • \$\begingroup\$ It is important because the LEDs will eat a lot more power (and be less efficient) when they are in overdrive. I also checked with my bench PSU, and 11.9V is where they are the happiest. My voltmeter also agrees with the bench PSU. \$\endgroup\$
    – Radu
    Feb 10, 2016 at 23:11
  • \$\begingroup\$ Anyway, my line of thinking if that the sense wire senses lower than 3.3v, it will increase the voltage. And maybe, if it sees more, it will lower it. This particular PSU (like most of them) has only a 3.3v sense line, and from reading on other forums, that adjusts all the voltages. \$\endgroup\$
    – Radu
    Feb 10, 2016 at 23:16

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