In the first article in this series, we looked at measuring the voltages while the PSU had no load attached to it. In this article we’re going to give it a number of loads to run. We’ll put a total load of around 364 Watts on the unit. If the unit is rated below that figure then it’ll be fully loaded. If the unit is rated above that figure then it’ll still be fairly heavily loaded.

We’ll load the unit using five small circuits which I’ve set out in circuit-diagrammatical form below. The smaller circuit, consisting of a bulb, a resistor, and a light-emitting diode, needs to be built four times, the larger one, consisting of six bulbs, seven light-emitting diodes, and seven resistors, only once.

Why bulbs, resistors, and LEDs? The resistors are in series with the LEDs to prevent them from overloading. The LEDs are to indicate that the 5V lines are all working; also that the PSU is lighting the Power OK LED as it should. We’re not loading the 5V lines much because it’s ideally the 3V3 and the 12V lines that we need to test under load; because they’re the ones through which the main wattage is drawn from a modern PSU by modern computers.

Which leaves the bulbs: It’s the bulbs that provide the heavy loading. Incandescent tungsten-filament bulbs are notorious these days for using too much wattage and producing too little light and too much heat. Were intending to use those negative properties of them to “waste” the PSUs output. Well be buying products from the automotive industry too, which might help it on its way to recovery; as the bulbs we’re using are car headlamp and tail light bulbs. They are made to work on a voltage of 12 Volts; which means they could be bright and very hot when in operation. – Especially so with the 80 Watt bulbs in the 4 small circuits that test the 12V lines. When you are building the circuits please bear this in mind, as it is possible that looking directly at them for more than a second could result in at least temporary blindness. Also touching them during or shortly after operation could result in severe burns, and if they come into contact with flammable objects or materials they could start a fire. Be warned, and design appropriate safety parameters into the housing for the bulbs when you design the test units built from the circuit-diagrams provided.

Ok so let’s take a look at the first circuit diagram: -

This circuit is built around the P1 motherboard connector socket. The pin-out numbers used correspond to the P1 plug and not the socket, so please bear that in mind. 4 of the 24-Watt car-bulbs load the 4 x 3V3 lines to almost 7.273 Amps each, total almost 29.1 Amps; that’s 96 Watts at 3V3. – The same as the average power-drain from a processor. (These particular bulbs aren’t expected to get particularly bright; but nevertheless could get rather hot, so beware.) The other 2 bulbs provide load of 2 Amps on each of the 12V lines: Total 48 watts. (These two bulbs WILL get both bright and hot. Bear this in mind when designing the circuit layout and component housing.)

The next circuit is fairly simple, and is built around a 4-pin Molex socket. You will need to build four identical of these circuits. Please do bear in mind that the 80 Watt bulb is burning 6 and 2/3rds of an amp: That’s a lot of current, a fair amount of power, and a lot of energy. Energy doesn’t vanish. – Rather it radiates and it changes form> – In this case into heat and light. The bulb will be bright and very hot. (You might even be able to light a cigarette from it given time?) Be careful to protect eyes from it, and beware of the fire risk posed.

Note once again that the pin-out numbers correspond to the peripheral power “Molex” plug and not the socket.

These four bulbs; one on each circuit, will load the 12V lines to another 320 Watts in total; making a grand total of 320 + 96 + 48 Watts load = 364 watts; plus a couple of Watts loading by the LEDs with their series resistors.

What should happen?

When you build and connect the circuits to a known-working PSU; the following should happen:

All the LEDs should light up. If they do it shows that all the 5-volt lines are working and that the PSU is in better health than not. The 80W bulbs will produce a lot of light and heat. All but 2 of the 24W bulbs will be rather dim; but could still get quite hot.

What do these circuits accomplish?

They put a reasonable load on the PSU, so that you can test its performance under near-normal working-conditions.

You could check the output voltages again under load to see if there is any significant voltage-drop under such conditions.

Another thing to bear in mind: -

It is well documented that when some of the cheap and nasty PSUs are fully loaded to specification they can fail or even explode. Therefore if you happen to be testing out £10 worth of cheapo Chinese unknown-label 250 Watt PSU for example; expect it to explode under the full-load that these circuits will give it. If it doesn’t and it still works afterwards then you were lucky. A good-quality 250 Watt PSU will strain and maybe complain in some way, but will easily survive a minute or so of full load.

Yes the above is a bit Heath-Robinson I admit. – But if you’re on a budget you really don’t want to shell out for the proper professional test-loads that can cost anything up to a small fortune. This is the scrub-round-it approach to the matter. – And it works; so why knock it? (Just keep an eye on; or should I say keep an eye off, those hot bright bulbs.)

Now that we’ve loaded it we can do the same measurements that we did in Part 1. The two readings will be slightly different. The difference corresponds to the voltage-drop under load. This amount will depend upon the power-rating of the PSU as well as the quality of the unit.

Ideally the voltage-drop should be less than 5% of the voltage-rating of that rail. If it’s any more then the unit is either wearing out or wasn’t particularly good quality to begin with. If it’s over 10% then replace the unit.