I got an ATX computer power supply (HEC-250AR-T) that I wanted to use
as a fixed voltage power supply at my workbench.
I plugged it in, pressed the on switch, and- nothing. The fan did not
come on and there was no voltage on the output.
What has to be done to get it to work? Is there a jumper that has to
be put on one of the wire sets? Is there a web site? Google did not
help me; maybe I just did not use the right search words.
Connect the green wire (PS_ON#) to ground and your ATX power supply
will come on.
These power supplies are very sensitive to having their outputs
shorted to ground. If you so much as brush one of the DC outputs
across a ground, the PS is fried. I have gone through about five this
year. There may be a simple fix for this, but since these are nearly
free for me I just trash them.
I also have problems using these power supplies for my steppers. I
think the energy from switching the steppers on and off gets conducted
back through the power supply and affects the 5v logic side of the
supply voltage to the stepper driver. My plan is to use the ATX power
supply for logic only, and get a nice low voltage high current linear
unregulated supply for the steppers. I am driving a CNC milling
machine, so when the steppers get extra pulses I loose position and
have major problems.
- James B
ATX supplies use a POWER ON signal from the motherboard to turn on. The +5VSB
voltage is available whenever the power supply is plugged in (and the switch
on the back is turned on), and this powers some logic on the motherboard that
monitors the momentary power button on the PC case. When you press that, the
motherboard makes the POWER ON signal active, and the power supply starts.
I think the POWER ON signal is at TTL levels, but I do not know if it is
active high or low.
If I were you, I'd steer well clear of computer power supplies for anything
except driving a computer.
The big problem with them, other than the fact that they're not adjustable
and lack the useful ammeters and voltmeters you find on a laboratory power
supply, is that they are switch mode supplies. This means that, rather than
using a bog standard isolating transformer, bridge rectifier and some other
simple, rugged components, they use a complex mess of high frequency
circuits and inductors to regulate the output.
The principle is still a bit of a mystery to me, but it's something to do
with applying raw mains (or whatever you're supplying it with) to the
inductor and switching it on and off at the correct rate to produce the
required output. Some models apparently don't even use an isolating
transformer, which means that if they do fail, you might end up with mains
voltage on all the output lines. Definitely not good. Of course, if they
fail, they might also dump mains into all their internal components, and can
sometimes explode quite violently if they go wrong (that's the cheap ones -
better models have safety cut off devices)
Another reason they're useless for robotics applications is that they must
never be used to drive inductive loads, like any kind of motor, including
steppers. This is because the added inductance buggers up the power
supplies' internal inductor, and causes bad things to happen. Shorting them
out has similarly catastrophic effects.
They do have some advantages, though. The best (and one reason I do plan to
use a similar device for some very specialised circuits in my own robot) is
that, because of the charge-pump way the inductor works, they can easily
produce output voltages greater than their input voltage, or with opposite
polarity, and operate with almost any input, ac or dc. There isn't really
any other practical way of doing this. Second, they are very small compared
to conventional power supplies because they don't need a massive transformer
(though, as mentioned above, this can lead to added mains hazards).
The charge-pump effect also means that they could theoretically allow a
battery to continue to supply it's specified potential when the emf of the
cells themselves has dropped much lower than this value, so the system could
run for longer without recharging (though certain types of battery might be
permanently damaged by this).
Finally, on PC switch-mode supplies, there is a signal sent back into the
supply from the motherboard to shut the supply off immediately if it
produces an incorrect output, to prevent damage to the motherboard. If you
still plan to use the supply, you'll need to find a way to provide this
Hope this helps.
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