Maker Pro
Maker Pro

PC PSU for microcontroller experimenting?



Jan 1, 1970
What steps need to be taken to use a standard (AT?) PC power supply for
experimenting with micros/basic circuits? Hell, is it even safe/advisable?
In the discussion about using two PSUs for devices in the same system, it
sounded like such units won't activate without a suitable load across the
motherboard connector.

I'm new to electronics, so a basic explanation would be appreciated.


rob burns

Howard Henry Schlunder

Jan 1, 1970
Some power supplies will operate without a sizeable load attached, others
will not. For the ones that do work without a load, it is generally true
that the voltage regulation will be inferior compared to the loaded
condition. People always suggest connecting a 1 amp or so load to the 5V
rail to correct for this condition, but since my ATX power supply works fine
without a quiescent load attached, I don't bother. The load doesn't have to
be connected to the motherboard connector, it could be from any of the red
wires to any of the black wires.

I personally find using a computer power supply to be an advisable thing to
do, and so long as you aren't using it in a condition where AC mains
isolation is needed, it is likely a very safe thing to do to. I'm not sure,
but some power supplies may be mains isolated too. If you short circuit the
power supply, you will likely discover that it will create a very unpleasant
spark and shutdown, rather than internally overheat and risk start a fire.
On the other hand, since computer power supplies are capable of supplying a
lot of power, if you make a big mistake in your circuit and not detect it
for a long time, it could easily melt breadboards and cook stuff externally.

Anyway, I personally rarely use my ATX power supply, despite the fact that I
have no bench power supply. I don't care for the noisy fan inside computer
power supplies. The voltage provided by 4 AA size NiMH batteries suits my
needs, and in a case such as [*&terms=12BH348/CS&Dk=1&D=12BH348/CS&N=0 ]
, it is a very convenient and portable power solution.

To power up an ATX power supply, you have to short the green wire on the
motherboard connector to any of the black wires. An AT power supply should
have a separate wire (with likely 4 conductors in it) with a physical switch
attached which will turn it on. All of the black wires are a common ground.
Red wires will likely be +5V, yellow +12V. If you have an ATX power supply,
it will likely have orange wires on the motherboard connector, which are
probably 3.3V. Blue reminds me of -12V. AT and ATX power supplies both
provide a -5V supply as well, but I don't remember what color it normally
is. The colors of the wires are recommended by the power supplies's class
specification, but are not required, so my generalizations may be wrong and
you should measure the various taps before using them.

Howard Henry Schlunder


Jan 1, 1970
apart from the requirements of a minimal load , that kind of power supply can
generate destructive currents. A source for 5V or less for experiments should
be limited to a reasonable current.