Author Topic: how can you test a modern PSU?  (Read 782 times)

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Online legacy

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how can you test a modern PSU?
« on: April 24, 2018, 08:49:05 pm »
if a power supply provides 2A@12V, to test you simply need to verify it provides 2A without voltage drop on the load side, so you can simply plug a rheostat of 48Watt as working load.

But what about a PSU that provides 20A@12V? (common modern PC's PSUs)

How can you verify it can really provide 22A to the working load?

Can't find a 500Watt rheostat for sale, do I need to steal some gear from the near power plant?  :-//
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Offline BradC

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Re: how can you test a modern PSU?
« Reply #1 on: April 24, 2018, 08:49:53 pm »
kettle elements and a bucket of water?
 

Online legacy

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Re: how can you test a modern PSU?
« Reply #2 on: April 24, 2018, 11:20:10 pm »
kettle elements and a bucket of water?

LOL. :-DD :-DD :-DD :-DD

bu seriously, how can you test 22A@12V ?
You need a laboratory equipment to do so, exactly what?

2A@12V are easy to be verified: 12V/2A can be tested with a 5 ohm rheostat, which can be set to 6ohm.
2A@22V .... 0.5 ohm rheostat? no way, wrong, can't be done, and it doesn't exist, so ....
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Online legacy

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Re: how can you test a modern PSU?
« Reply #3 on: April 24, 2018, 11:24:15 pm »
also, my multimeter has a limit to 10A on the ampere-meter, with 22A it will literally burn from the inside.
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Offline Dielectric

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Re: how can you test a modern PSU?
« Reply #4 on: April 24, 2018, 11:27:49 pm »
They make such a thing as a "programmable load".  Here's an example in the 300W range:
https://www.tequipment.net/BK8500.html?v=0

Not super cheap, but not break-the-bank expensive and you might find a used one. 

If you're doing this as a hobby, you can get some big wirewound resistors from the usual sources.  Look into the dummy loads that ham radio guys use, they put an element in an oil bath for big stuff.
 
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Online legacy

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Re: how can you test a modern PSU?
« Reply #5 on: April 24, 2018, 11:43:14 pm »
Thanks! Perfect!
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Offline BradC

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Re: how can you test a modern PSU?
« Reply #6 on: April 25, 2018, 12:21:36 am »
kettle elements and a bucket of water?

LOL. :-DD :-DD :-DD :-DD

bu seriously, how can you test 22A@12V ?
You need a laboratory equipment to do so, exactly what?

I'm not quite sure why you're laughing. Kettle elements, toaster elements, fan heaters and combinations of have all been used for dummy loads since long before I was born. 44 2400W 240V kettles in parallel will give you 0.545 Ohms which will give you 22A@12V. 50 odd 2000w fan heaters will do the same thing for less money.

Alternatively, 4 12V headlamp bulbs and a tail light bulb in parallel will do the same thing for much fewer devices to wrangle.
If you are clever you'll use a couple of toaster elements and re-configure them to give you a much lower resistance so you need fewer of them, but still.
 

Online nctnico

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Re: how can you test a modern PSU?
« Reply #7 on: April 25, 2018, 01:55:11 am »
They make such a thing as a "programmable load".  Here's an example in the 300W range:
https://www.tequipment.net/BK8500.html?v=0

Not super cheap, but not break-the-bank expensive and you might find a used one. 
A DC load is a good way because sometimes these can also do pulses and they are not a purely resistive load.
A couple of high power resistors is also an option but ofcourse you can't regulate these very well.

There are quite a few DC loads on Ebay. Those of TDI (transistor devices) can be bought for a reasonable amount of money on Ebay sometimes. I restored one earlier this year: http://www.eevblog.com/forum/testgear/dynaload-dlf-100-100-1500-(100v-100a-1500w-dc-load)-teardown-repair/msg1381740/#msg1381740
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline C

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Re: how can you test a modern PSU?
« Reply #8 on: April 25, 2018, 05:59:30 am »
You could make a trombone slide.

Just two parallel conductors with a shorting bar.

If you use conductive balls or rollers for shorting bar, you could adjust while hot. You can use more then one also.

And you have option of using pipe for parallel conductors with cooling liquid.

Linear conductors lets you add a raw resistance scale.

Using a spiral for conductors lets you save space with many turns making up length.

Should note that this is just a constant current test and not a test of dynamic change.

« Last Edit: April 25, 2018, 06:11:06 am by C »
 

Offline C

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Re: how can you test a modern PSU?
« Reply #9 on: April 25, 2018, 06:35:45 am »
legacy

You might want to look at

Quote
Linear Technology  Application Note 104
Load Transient Response Testing for Voltage Regulators
Practical Considerations for Testing and Evaluating Results
Jim Williams

http://www.analog.com/media/en/technical-documentation/application-notes/an104f.pdf
 

Online legacy

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Re: how can you test a modern PSU?
« Reply #10 on: April 25, 2018, 11:15:44 pm »
You might want to look at
 

yes, very appreciated lecture! Thanks. So kind  :D
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Offline C

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Re: how can you test a modern PSU?
« Reply #11 on: April 26, 2018, 02:25:35 am »
legacy

 You might want to think on how to test the tester.

Here you have a shunt that you can control.

If you build two as separate things and put them in series, you could use one to test the other.

As you think of an improvement, you can test it using one against the other.
If the improvement is better, change both to match.

The power supply is a dynamic thing.
The tester is a dynamic thing.

To really know that both work properly, you need a bunch of dynamic tests.

You might think of war where you try to kill the other,
Good sensing would let you stop at the edge of the kill.

Your 20 amp power supply could be fine with a static 20 amp load yet die with 5 amp pulses at some pattern in time.

You will never know with out testing.
C
 

Offline alanb

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Re: how can you test a modern PSU?
« Reply #12 on: April 26, 2018, 02:31:25 am »
Look at this from Mikes Electric Stuff.

 

Online rhb

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Re: how can you test a modern PSU?
« Reply #13 on: April 26, 2018, 03:31:42 am »
A "trombone" load made using some carbon arc rods from the welding supply should do the trick.  You might need a few in parallel to get the resistance low enough.

I clearly need to measure some rods.  In an oil filled container it would make a great general purpose adjustable passive load.

Thanks @C!
 

Offline T3sl4co1l

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Re: how can you test a modern PSU?
« Reply #14 on: April 26, 2018, 03:49:55 am »
I have a luxury: for a past customer, I designed and built a dual channel, 0-18V, +/-60A controlled source/load.  So, of course I keep one on hand so I can test anything they can.

Test a 12V supply?  No problem, plug it in and crank up the amps!  Load voltage and current are displayed on meters. :)

If you aren't so fortunate, any random load resistance is helpful.  If you need a lot cheap, try looking inside industrial hulks at the scrapyard -- there may be power resistors inside!  However you do it, you'll want a collection of assorted values, so you can mix and match for whatever loads you like.  Tip: use a resistor network calculator to find the optimum network for a given maximum load.

Tim
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Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline C

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Re: how can you test a modern PSU?
« Reply #15 on: April 26, 2018, 04:19:51 am »
I see a possible problem with some thinking on  "trombone" load.

Think some are thinking very high resistance rods.

Think of what this gives.
1. Very high heat output per unit of length.
2. Very large change in resistance per unit of length.

I would think both are not a good thing.

Using lower resistance rods
1.  low heat output per unit of length.
2. Small change in resistance per unit of length.

#1 is less thermal resistance change.
#2 lets you create a better raw resistance scale.

The down side is the longer length. This is why I mentioned the spiral and the conductive balls for shorting bar.

You need to think of two pipes or rods coiled in a flat spiral. You have an air gap between the two pipes or rods in the spiral. You can roll the ball around the spiral to adjust the resistance.

You can get a very long length in the form of the spiral.
You could use steel for the rods/pipes. Smaller size increases resistance & also lets you have a greater length in same size spiral. Think of A 50 foot long rod/pipe in a spiral, not that large.

Now if you think, a conductive ball will have some contact resistance. You could have one ball be course adjust and more balls down the spiral giving a finer adjust.

And it is not hard to create the spiral. Just bend three rods between two plates. Remove extra rod when bent. Adjust starting point of one rod in center to get gap the total length.

The problem with the spiral is increased heat output the shorter the length used.

Hope this helps some.

C

 

Offline Geoff_S

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Re: how can you test a modern PSU?
« Reply #16 on: April 26, 2018, 01:14:11 pm »
There are some options in between the high-end commercial load testers, and the low-end DIY resistive loads.  Google for the SkyRC BD200 for example - a commercial product for discharge testing of batteries which will do 30A/200W and isn't terribly expensive.
 

Online legacy

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Re: how can you test a modern PSU?
« Reply #17 on: April 27, 2018, 02:53:21 am »
I also have to test a few switching PSU units, whose label (on the primary) only reports "240VAC@1A". It's dual output, 5V and 12V, but the max Ampere capability, per channel, is unknown.

Got a lot from some sellers on eBay  :)
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