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I am using a computer PSU to power a 92 watt(12V 6A) peltier device.The PSU has a rating of 450W,it has got 2 rails of 12V ,one delivering 15A and another delivering 16A.However when I connected my peltier device the current drawn from these rails are not more than 1.6A ,is there a problem with the psu or am I wrong somewhere?

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    \$\begingroup\$ A computer power supply sometimes needs a dummy load on the +5V rail before it can deliver full power. \$\endgroup\$ – Dave Tweed Nov 7 '14 at 23:42
  • \$\begingroup\$ @DaveTweed the resistor is only needed to turn on (load) but does not regulate the current on rails. The power supply is rated while summing all the rails in the power supply not just one rail. To get the proper current you need to sum up the power rails in the supply. One rail may have lower current delivery than the other. \$\endgroup\$ – Chetan Bhargava Nov 10 '14 at 7:45
  • \$\begingroup\$ @ChetanBhargava: See Bruce Abbott's answer below. Many computer power supplies regulate their voltages based on a single output, often the 5.0V output. All of the other output voltages track that output. The dummy load gets the PWM driver circuit operating in a mode that allows all outputs to deliver their full current while remaining in regulation. \$\endgroup\$ – Dave Tweed Nov 10 '14 at 12:01
  • \$\begingroup\$ @ChetanBhargava I didn't need any load to turn on the PSU,I just shorted the green wire to ground. I think the current supply for each rail is independent,because in the ratings it is given 12V1-15A and 12V2-16A,and the total wattage of the PSU is 450W so I think,The rails are independent of each other. \$\endgroup\$ – Arunim Bhattacharya Nov 11 '14 at 13:29
  • \$\begingroup\$ @ArunimBhattacharya Older power supplied needed load to tun on. ATX power supplies are switched-mode power supplies (info at en.wikipedia.org/wiki/Switched_mode_power_supply); they must always have some load to operate properly. The power resistor is there to "waste" energy, which will give off heat; therefore it should be mounted on the metal wall for proper cooling (you can also pick up a heat sink to mount on your resistor, just make sure the heat sink doesn't short circuit anything).... \$\endgroup\$ – Chetan Bhargava Nov 11 '14 at 21:28
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The power supply's job is to supply 12VDC regulated- it's up to the Peltier to draw enough current at that voltage.

Assuming your power supply voltage is actually 12V (and assuming that you have not put something in series like an ammeter that would excessively drop the voltage)-- so the voltage across the Peltier device is actually 12V +/- not much, the Peltier simply isn't drawing the current you think it should.

Perhaps the Peltier is designed to operate on a 13.8V automotive "12V".

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Most computer PSUs have a single transformer which is pulsed at high frequency, with multiple output windings that produce several voltages. Regulation is achieved by varying the on/off ratio of the pulses (Pulse Width Modulation). Increased current draw makes an output voltage sag, so the PSU compensates by increasing the PWM ratio to keep the voltage (almost) constant. However if only one output is loaded down then the higher PWM ratio will cause voltages on the unloaded outputs to go up, eventually triggering the over-voltage protection circuit.

The easiest solution is to simply put enough load on the other outputs to keep their voltages down. Drawing 2-3A from the +5V output is usually enough to get at least 10A from the +12V output. You can use a 2Ω resistor, which draws 2.5A at 5V. It should be a wirewound type rated for 20W or higher, and remember that it's dissipating ~12.5W so it will get hot!

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  • \$\begingroup\$ Thanks for answering,but I am not quite sure if that is true for my PSU,it is by some no-name company ,I don't even know if it conforms to ATX standard.I will surely try out the dummy load thing and will post back. \$\endgroup\$ – Arunim Bhattacharya Nov 11 '14 at 13:21

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