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Old 8th January 2009, 08:56 PM   #1
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Default Measuring resistance while circuit is turned on

I have no knowledge of how a digital multimetre works.

I wonder if I can measure resistance while the line level circuit is turned on and there is no signal going through the circuit, i.e. the voltages between the measured points are within 10mv difference.

I guess that this would not destroy the multimetre and the multimetre would probably give the right readings. However, I don't know if the multimetre would apply a voltage to the circuit or short the circuit that can destroy the circuit then may even destroy itself.

The line level circuit is my EQ network that contains many trimpots to adjust the speaker response. If I need to turn the circuit off before measuring any new adjustments, I would need to wait for a long time because I have a large capacitor bank (with breeding resistors installed). It is rather inconvenient.
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Old 10th January 2009, 03:16 AM   #2
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Hi, although I don't really understand electronically what you describe, I am suggesting this nonetheless: Just short the caps with a low value resistor. Something like 47ohms at 10Watts. Do you have any idea what voltage are these capacitors charged with? that should give you a better idea of what type of resistor to use.

you will not be able to measure resistance with electricity applied to it. you will most likely get a false reading, if anything. As a metter of fact I am convinced you will get a false reading because an ohmmeter measures a resistance, not a voltage, which is what you'd be measuring in this case. there will be a voltage drop developped on that resistor you're "measuring".

what kind of unit is this that you're talking about?
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Old 10th January 2009, 03:25 AM   #3
anatech is offline anatech  Canada
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Hi HiFiNutNut,
The circuit must be de-energized completely before you can use an ohmmeter. It really has no bearing on how much trouble it causes you. You could simply measure the amount of signal is dropped across the control to figure out what you need. Even using an LCR meter that measures using an AC waveform (or pulsed DC for the simpler models) can not be used with an active circuit.

The average ohmmeter works by one of two methods these days. The older was to complete a circuit with the unknown resistance and measure the current through it. This results in a non-linear scale. The second, more recent method is to pass a constant current through the unknown resistance and measure the voltage drop across it. DVMs use this method. 10 mV is enough to create a measurement error. The applied voltage or signal can be enough to saturate any active circuits and disrupt normal operation. The result can be anything from damage to nothing but bad readings. There is no point in making a reading that you know isn't correct.

-Chris
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Old 10th January 2009, 03:30 AM   #4
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Yes, what anatek said.

By the way I meant short the caps after you shut off your circuit, obviously! then read the ohms.
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Old 11th January 2009, 07:01 PM   #5
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Thanks guys for giving me the answer.

I will continue doing it the hardway buy turning off the units and waiting until all capacitors are fully discharged.

Regards,
Bill
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Old 12th January 2009, 02:06 AM   #6
anatech is offline anatech  Canada
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Hi Bill,
Quote:
I will continue doing it the hardway buy turning off the units and waiting until all capacitors are fully discharged.
It's not the hard way.

It's the proper way, and the only way.

-Chris
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