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Old 19th May 2002, 11:10 PM   #1
Kevinbd is offline Kevinbd  United Kingdom
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Lightbulb Measuring PSU supply line impedance?


I am novice at electronics but do want to build a good amp.

Just wondering if anyone could tell me how to measure supply line impedance of the PSU?

Can it be done with a good meter? Or do you need certain equipment?


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Old 20th May 2002, 01:15 AM   #2
Kevinbd is offline Kevinbd  United Kingdom
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Default Just wanted to add something

Obviously psu impedance varies with frequency, unless it's a very good regulator which can be flat, say upto 100Khz.

If i wanted to measure psu impedance at 20Hz,
would i inject a 20HZ sine or square wave at an appropriate voltage level into the power amp then measure it with a meter?
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Old 20th May 2002, 03:55 AM   #3
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Not sure exactly what you want to measure. You can measure the impedance of any power supply by feeding a signal through a large value resistor (making it a constant-current source of sorts)and a cap to prevent DCfrom frying the source. Then measure the voltage on the supply rail, but you need a way of measuring only the signal frequency, and not ripple, hum, whatever.

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Old 20th May 2002, 08:56 AM   #4
UrSv is offline UrSv  Sweden
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I would also expect that you mean output impedance which you can measúre for DC impedance by measuring the output voltage at one specific current and then at another and then calculating the impedance as dU/dI. So say that you have 40 V out with 2 A load and 38 V out with 4 A load then the output impedance is (40-38)/(4-2) = 1 Ohm as far as I can tell (it is the voltage drop over the internal output impedance).

Also you might want to investigate an article in Elektor a few years back which deals with measuring dynamic performance of PSUs by switching a load in and out for the PSU and measures recovery, spikes ect by simulating this more real life load (for some apps of course). Interesting as far as I remember.

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