Made a Cheep Power Supply using Computer SMPS !

How does it behave with very low currents? Some of these computer PSUs shut down when there's no minimal (for them) current flowing.

This is my home-made PSU.. not very good, it gets a bit warm : )

I only use it when I need a few hundred mA max. It's linear, and can manage 1A on a good day but not at low voltages of course. No current limiting : )

It's fine for op-amp experiments, because it is dual channel.

I think it has LM317 inside. It wasn't my circuit, it was a cheap PCB project from a store, I bought two of them, i.e. one per channel, to which I added the ebay voltmeters.

I use similar (cheap) ATX PSUs as power supplies, and none of the units I've used shut down at low/no load.

I followed jw0752 blogs

Using the Coarse + Fine Control circuit in my new Dual Bench Supply

Oh No! Not another  #@&%# Power Supply

and since it was so easy to follow ... I made did this one.

(BTW John it's been very handy)

I used the case from a computer supply

The cooling fan is controlled by a Digispark clone reading a DS8B20 temperature sensor and switching the relay.

Dallas 18B20 poked into the cooling fins behind the output control transistor

Schematic overview.

Note the current meter goes in the negative lead to allow much greater voltages to be monitored.

Cheers

Mark

I am using a smaller XT power supply to use with my Blade to power my Raspberry PI.

DAB

Nice.  This is encouraging me to open up that Windows 95 computer in my closet, to re-purpose/steal/cannibalize the power supply, to run my lineup of rPi and BBBlack. All those USB wall power supplies create a mess.

Hi Mark,

Looks like you had as much fun as I did.

John

When you want to power multiple RPis, just by a 4-port (or so) USB power supply / USB charger. Much better than to use an 20-year old computer PSU which is probably 10 years of its planned life time... I bought an Anker 5-port USB charger, and it works like a charm.

I used to do this, maybe about 15 years ago with various power supplies. There are a few good things about such a supply:

• Cheap and relatively widely available.
• Large current/power ratings, multiple useful voltages available.
• 50mV peak-to-peak ripple on 5V, 120mV peak-to-peak ripple on 12V as specified by ATX is much better than most cheap supplies.
• ATX supplies provide 5V "standby" supply, often rated up to 2.5A, so can be used without power up the rest of the rails.

But there are also a lot of drawbacks:

• ATX Specification 240W-per-rail limit not strictly enforced by a number of lower end supplies - can go "pop" if overloaded.
• No current limiting - beefy power supply may put your devices at risk if incorrectly used.
• Horrible efficiency at low loads - they're designed to drive computers which draw 40W+ load, so efficiency at low loads is generally poor.
• Some units are unstable at low loads - they're not designed to run load-less, although this is a bigger issue with older AT type supplies which would chirp/cycle at low loads.
• Bad cross-regulation on cheaper supplies - the voltage rails are not independent nor independently regulated - heavy load on 12V with no load on 5V will lead to both rails going out-of-spec even before reaching the "rated" value.
• Fake ratings - cheap Chinese made supplies often have fake wattage/amperage ratings, so you're not going to get the label output. More than that, they often fail due to bad capacitors on the secondary - so ripple will go out of spec.
• Default connectors not the best for certain uses - Molex plugs are ~6A per contact, SATA plugs are about ~3A per contact. No voltage-drop sensing on most rails except 3.3V, so voltage accuracy is not guaranteed.
• Switching supplies do create RF noise - cheaper models which are poorly filtered will interfere with radio circuits and should be avoided.
• Fan can be quite noisy and not thermostatically controlled. They also wear out, so probably need replacement especially in cheaper models.
• No monitoring of output voltages, currents by default. Easily fixed by using your own meter but adds to the cost.
• No easy on-off switch that is immediate for safety - you can build your own, but while some ATX supplies may have one on the primary, it will take a while for the capacitors to drain.
• Outputs are usually NOT FLOATING - the ground is tied to the AC earth, so seriesing the supply with others may not possible.

So as long as the user is aware, then that's all good, but it's no replacement for a benchtop power supply .

- Gough

Hi Allen,

You might find , as it is an older PSU, the issues that Jan Cumps mentions regarding operation at low currents. Then again if it is just sat there gathering dust and you have a requirement....give it a go. You can always add a switch and a resistive load so you can operate low currents.

Rod