Rating of PC power supplies?

I'm not familiar with switched mode power supplies so hope this doesn't prove to be a dumb question. I dusted off an old '200 W' ex-PC power supply yesterday and was playing with it with a view to using it for some motor tests. (For the time being I settled on a set of 4 NiCads instead, but I may pursue it later.) Its spec says that its '5V' output is rated at 20A. Arbitrary tests gave me these results:

Current Voltage at load terminals

------- ------------------------- 5.1 A 4.77 7.5 A 4.51

11.0 A 4.20 13.0 A 4.02

I didn't go further, but even at 13A isn't that voltage rather low? I wonder what it would drop to at close to 20A? What is regarded as an 'acceptable' voltage from the '5V' output of these types of supply please?

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell
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Yes, that's correct. You can Google around for information on how to successfully use PC PSUs for general purposes.

Also be aware that there is more than one 5V output, at least if we're talking about a standard ATX supply, e.g.,

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I don't know whether your unit was rated for 20A at *each* 5V output, or for them all put together. They should all be coregulated, indeed they might even all just be in parallel; but it sounds like the issue you're facing has something to do with IR drop between the sense point and the load, so it makes a difference.

Reply to
Walter Harley

Actually I did, Rheilly - just didn't include the correct column in the posted results! Here's the fuller table, which as you rightly say gives the proper picture:

Voltage Current At PSU output At load terminals

------- ----------------- ------------------ 5.1 A 4.96 4.77 7.5 A 4.78 4.51

11.0 A 4.60 4.20 13.0 A 4.50 4.02

My 'PSU output' was at the end of one of the red '5V' output wires, about 30 cm long, the first location accessible without opening the case.

Wiring from there to my load (and back to the PSU 0V cable, a similar distance from case) therefore totaled about 0.04 ohms.

So, much better, with 4.5V at 13A. Not sure how to estimate 20A. Do those numbers look acceptable?

BTW, correct operation only started with a load of something over 1A or so. Presumably a characteristic of this type of PS?

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

I'd parallel all of the black wires as well; forgive me if I'm being redundant here. :-)

Cheers! Rich

Reply to
Rich Grise

One day Terry Pinnell got dressed and committed to text

G'Day Terry Pity you didnt check at the output on the PSU, could have shown up any volt drop in your load leads. The regulation feedback comes from those connections. Then again perhaps I'm wrong (as usual :-)

-- Regards ..... Rheilly Phoull

Reply to
Rheilly Phoull

No-name PC power supplies are notorious for being junk. Buy a lab supply, build your own supply, or buy a PC PS from PC Power and Cooling (they make very good PC power supplies).

Reply to
Guy Macon

Thanks. Yes, there are 3 '5V' outputs, apparently in parallel. I'll check whether connecting them makes any significant difference.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

On Sun, 29 May 2005 07:10:38 +0100, Terry Pinnell put finger to keyboard and composed:

IME most PC PSUs regulate on the basis of a weighted average of the

+5V and +12V rails. If your +12V rail is unloaded, then the +5V rail will also not regulate properly. I'd check the capacitors on the +5V rail (or add an external cap for testing purposes), and I'd measure the +12V rail in all four cases.

- Franc Zabkar

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Reply to
Franc Zabkar

No harm in trying it, so I will! But without any expectation that it will affect any measurements. I'm pretty sure I recall when this PSU was inside my old PC that several of the output 'sets' were floating loose in their plugs, presumably awaiting service for additional hw.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Only one of the output and return terminals will be sensed. Use all releventterminals in parallel to remove harness drop from your measurements.

You can identify the sensed terminals by visual inspection. These will have two wires crimped into a single power pin.

RL

Reply to
legg

Usually spare drive power connectors.

Graham

Reply to
Pooh Bear

I'd expect it to range from 5.1 to 4.9 volts approx. They're normally not too bad. Shouldn't really be more than 200mV out of spec.

Bear in mind that it's a multiple output supply and an absence of load on the 12V output is likely screwing things up.

As another poster noted, these supplies *have* to sense a combination of both the 5V and 12 V outputs so no load on the 12V is bound to be an issue. The 12V is probably high ( due to no load ) and therefore dropping the 5V low.

The -5V and -12V outputs are low current and just rely on the reflected voltage. They won't need loading.

Graham

Reply to
Pooh Bear

That works fine if you have the schematic to calculate the values. ;-)

I suspect Terry's old PSU is likely an AT type btw.

Graham

Reply to
Pooh Bear

Many thanks, and for the other follow-ups. I'll load the 12V output and re-test.

Note that my later post prompted by Rheilly gave the more relevant readings: Current V (At PSU output)

------- ----------------- 5.1 A 4.96 7.5 A 4.78

11.0 A 4.60 13.0 A 4.50

I'd be more than happy with even 4.8V at 15-20A. Crude motor tests apart, that would be handy if I ever build any more stuff from my ancient stocks of TTL. (Don Lancaster's 'TTL Cookbook' recommends +/-

250mV.)

Must say I'm impressed at the performance of these things. I expect there are downsides in hf noise, but they're so light and compact in comparison to a conventional transformer-based equivalent. I made a heavy duty 20A car battery charger years ago, still in use. OK, it's

12 not 5V, but I have to wheel that monster out on an ex-shopping trolley .
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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

On Sun, 29 May 2005 21:49:17 +0100, Terry Pinnell put finger to keyboard and composed:

You *must* load the +12V rail with a dummy load, otherwise you will

*never* achieve proper regulation on the +5V rail.

See this ATX example for an explanation of how a typical PSU regulates:

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The regulation formula is:

5.0V * 3K01/(3K01 + 3K01) = V12 * (3K09||150K||5K6) / (27K + 3K09||150K||5K6) + V5 * (3K09||150K||27K) / (5K6 + 3K09||150K||27K)

This reduces to:

2.50 = V12 * 0.0678 + V5 * 0.327

For this PSU, if the +5V rail increases by 50mV, say, then the +12V rail will fall by 241mV.

If you want to guarantee a stable +5V output, then remove the +12V sense resistor and recalculate the remaining +5V sense resistors such that the midpoint of the potential divider is +2.5V. I have modified several PSUs this way. I have one which I have reprogrammed for +6V, and another for +13.8V.

- Franc Zabkar

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Reply to
Franc Zabkar

Nice post, thanks. As you see from my other reply. I'm off to the shed/workshop to try it soon! Will probably take the easier approach of trial and error loading, until I get a satisfactory minimum, and will report back on results.

When I do get around to opening the case of such a unit, that schematic and notes will be invaluable.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

I have experimented with these a bit, albeit an older variety (AT type) of todays power supplies. What I found is that 5V is usually pretty accurate, while

12V is somewhat flimsy. This particular PS was only sensing the 5V output, relying the others would be more/less close to the design ratio; it can be off quite a bit for +12V and varies a lot with load.

The only output that had to be loaded was the sensing output, that was 5V. My reason to play with these was conversion into 10-15V variable voltage PSU, which involved cutting sensing track from 5V and running it to 12V output via adjustable voltage divider. Loading was now only required for 12V output and fan did a good job of that (required some LC filtering to make it quiet electrically).

Made for a nice encased dirt-cheap hobby power supply with current protection. Some of these PSU's had chromed enclosured, looked fairly good.

Hopefully someone will benefit from this.

Take care with high voltage, can be painfull/lethal!!

--
Siol
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Reply to
SioL

On Mon, 30 May 2005 02:09:08 +0100, Pooh Bear put finger to keyboard and composed:

Not necessarily. The most common PWM controller IC is the TL494 or its equivalent, KA2500B/C. All you need to do is to confirm the voltages at the inputs to the IC's error amp. One input will usually be 2.5V (= internal 5.0V reference divided by 2), the other input will be derived from the +12V rail and/or the +5V rail via a resistive potential divider. All you need to do is to recompute the resistor values in the feedback network so that the target voltage, eg 13.8V, produces 2.5V at the input to the error amp.

The two that I modified were both AT types. I had the schematics for neither. Only very minor reverse engineering was required.

- Franc Zabkar

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Reply to
Franc Zabkar

todays

electrically).

Some

Thanks, very useful.

I've repeated my tests after connecting all but one of the 5V (red) outputs, and all but one of the 0V (black) outputs. This time I was also more careful where I measured my voltage; I used the unloaded

5V/0V wires. Happy to say that I now get pretty solid 5.1 V right up the range. And, like you, I did not have to load the 12V output, only the 5V.
--
Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

As per my reply to SioL, I did not have to load the 12V output, only the 5V.

--
Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

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