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Old 7th October 2012, 04:31 PM   #1
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Default Offset monitoring

Today at the flee market i stumbled upon some new old Jewel analogue zero centered (-100-0-100 microamperes)

ampmeters.

Looking for how to use them it came to my mind they could probably be used as practical offset controlers at the output of

amplifiers.

With an 1k resistor serial connected to it and the whole thing paralleled to the load. Tout va bien. As long as no signal is sent, it
will display offset on a scale of minus to plus 100mv.

Good for setting offset of a new built amplifier. So far, so long.

Though, if we want to display offset during playback, what happens to the coil of the ampeter. I suppose it will burn.

Does anyone has an idea of how to wire the thing without overheating the coil.
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Old 7th October 2012, 04:42 PM   #2
alayn91 is offline alayn91  France
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Hello,

I think you should remove the AC signal, and keep only thye DC voltage.
So, you need a low pass filter at maybe 0.1 Hz and some protection circuit.
Regards.
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Old 7th October 2012, 05:42 PM   #3
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It makes sense, Thank you for ansering.
A big cap across the ampmeter but not polarized?
If my math is not wrong: 1600uF?

Last edited by bobodioulasso; 7th October 2012 at 06:04 PM.
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Old 8th October 2012, 06:26 AM   #4
alayn91 is offline alayn91  France
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Hello,

Don't know, without knowing the voltage across the meter for + or - 100A.
And, you will need top to tail diodes acrooss the meter to protect it.

Regards.
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Old 8th October 2012, 06:30 AM   #5
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I measured ampmeter resistance: 230 ohms
so, for 100mv full scale, serial resistor is 770 ohms anc C is 2200uf?
Is that enough filtering or do i need salen key(s) for steeper slope?

Last edited by bobodioulasso; 8th October 2012 at 06:32 AM.
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Old 8th October 2012, 07:01 AM   #6
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I think i had better to ask how to calculate C as in the formula i use
Fc= 1/2Pi*RC
the ampmeter resistance is not taken into account.
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