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PC Power supply and hacks

For posterity and myself, I'm leaving some tips and tricks of PC Power Supply Unit (PSU) whcih is an SMPS (Switched Mode Power Supply). There are a variety of uses of a +12V, +5V and +3V DC power supply like lighting up an LED strip or powering a raspberry pi.


There are various colored cables in a typical ATX 12V SMPS. I'll list out the various color lines and what they mean,

Sr. NoCable colorNumber of cables in a PSUUse
1Greenexactly one (1)Wake up signal from motherboard.
Pressing PC power button makes this signal carry wake
up signal to PSU to start.
Green needs to be touched with the any ground to make the
SMPS start. For self-starting PSUs, green needs to be connected
with one black all the time.
2Blueexactly one (1)-12V
3Purpleexactly one (1)+5V standby. When power supply is on standby mode (not
on by signalling green), this line can give 1-2 A current.
4Grayexactly one (1)Power good signal. When PSU levels has reached specification,
it turns to +5V level. It takes a few millisecond to reach correct voltage levels.
5Brownexactly one (1)+3.3 Sense signal. Generates signals to determine voltage drop across +3.3V rails.
6BlackmanyGround (0V reference)
7Redmany+5V
8Orangemany+3.3 V
9Yellowmany+ 12V (Majority of power come from this rail)

Some Caveat,
  • Power supplied by various rail depend upon the particular model. For example, the image below is a spec sheet of Corsair Power Supply, VS model (cheapest one).
    Note how, the VS model only supplies 25A on +12V rails and 14A on +5V rail. It's not advisable to exceed the current rating but there are other ways of using the remaining power on other rails.
  • Switched mode power supplies need a minimum load across the output for it to start, otherwise the voltage will not be stable or it won't start.

Comments

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