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Old 11th May 2012, 05:29 PM   #1
akis is offline akis  United Kingdom
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Out of curiosity, if you are using your bench PSU to power some appliance you are experimenting on, and are using the PSU's current meter to see how much current your appliance is drawing, have you ever thought what the PSU's meter will display when the appliance is modulating the current at a high frequency ?
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Old 11th May 2012, 08:16 PM   #2
DF96 is offline DF96  England
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If your circuit is well designed it will have local decoupling so the PSU won't see the current changing much. If there is no local decoupling then the PSU becomes part of the circuit, whether you realise it or not.
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Old 12th May 2012, 07:34 AM   #3
akis is offline akis  United Kingdom
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I guess what I am trying to say is that the current meter of any PSU should be a true RMS meter, and on top it should be capable for very high frequencies.
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Old 12th May 2012, 08:44 AM   #4
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Originally Posted by akis View Post
I guess what I am trying to say is that the current meter of any PSU should be a true RMS meter, and on top it should be capable for very high frequencies.
No, totally pointless.
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Old 12th May 2012, 10:56 AM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by akis
I guess what I am trying to say is that the current meter of any PSU should be a true RMS meter, and on top it should be capable for very high frequencies.
No, and No. The current meter should give a true average - real DC; most do this. It does not need to measure HF because there shouldn't be any HF there.

A true RMS current meter would tell you the heating effect of the current in the PSU output impedance (assumed linear, but it won't be linear); not very interesting. It won't tell you anything useful about the circuit being tested. Assuming the PSU is working correctly with a near-constant output voltage, then to determine the input power to the circuit under test you need a normal averaging current meter - which is what a PSU meter will be.

The correct method is to teach the users of PSUs how to use them properly, and properly design the circuits fed from them. A proper appreciation of the difference between average and RMS, and where each is appropriate, would help too.
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