From: Roger Johansson
Subject: Re: How to make/solder a circuit fast?
Date: Sat, 28 Sep 2002 05:36:08 +0200
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firstname.lastname@example.org (The little lost angel) wrote:
>>If you are not interested in DC and slow fluctuations, as it seems you
>>are not, then there is no reason to measure it.
>But I am too, the SMPS will be tested with a changing load, if it
Okay, but you talked about spikes before. This is another need you
have, which demands another solution, a chopper as Bob told you will
work for that.
>>It is maybe not the fault of the voltmeter, if you use cables suitable
>Well, it IS. Because my Fluke measures it correctly. A 5A current
>reads out 5.x mV on the Fluke. I get anything from 20mV to 40mV on
>cheapo ones. I need like 3~4 of them for current readings, and 4
>Flukes would really kill my budget.
I have a feeling you have chosen a less than optimal method in what
you are trying to do, but if you need this kind of simultaneous
precision measurements taken over some time you need a computerized
system for collecting and storing measurement values.
A microprocessor with A/D capacity, and suitable amplifiers is what
you need. Or a PC with suitable ADC:s and measurement amplifiers on an
>>for such low level measurements that voltmeter might work as it
>>should. You have to take special precautions to avoid noise from the
>>environment when measuring such low voltages at high impedance.
>>Keep the cables very short, use coaxial cable, screen off the circuit
>>and voltmeter from electrical and magnetic fields, choose a suitable
>>input impedance (by putting a resistor over the input terminals)
>Erm, I'm measuring the drop over a 1mO shunt for the current. How does
>all this apply?
A more suitable shunt which gives you higher voltages would help a
lot, to begin with.
If you had used a piece of thick cable, as I told you months ago,
which gives you 10 times or a hundred times higher voltages, this
would be much less of a problem.
Is this excercise still about finding out if your power supply is
working correctly, or are you planning an industrial precision