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Old 31st October 2006, 06:01 PM   #1
xiaonan is offline xiaonan  China
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Default need help about ammeter

The attachment is my current meter which want to use at high voltage and low voltage as well.

I use the Rsensor and test the voltage on the resistor. Due to the high voltage(V1V2), so I add a voltage divider to get a low voltage input to the differential amplifier. Because the current I want to test is very small, about several hundred nA. So I don't want the current go through the divider is too high. The R1 must be very high(~several G Ohm). And the ratio between left and right R1/R2 should also be the same.

So is there some other solution about the current measurement? And is anybody know how the ammeter works and its schematic?
Thank you very much!!
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Old 31st October 2006, 06:18 PM   #2
Stocker is offline Stocker  United States
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In my post about transistor tester current levels, I am stumbling through the same process. I am looking into using an instrumentation amplifier for this purpose... classic 3 opamp in-amp.

consult google.com for more information...
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Old 31st October 2006, 08:59 PM   #3
xiaonan is offline xiaonan  China
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Yes, I also want to use the instrumentational amplifier at begining, but that need three high voltage amplifiers. I have consult the google many times, and conld not find a better solution.
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Old 1st November 2006, 02:45 AM   #4
Stocker is offline Stocker  United States
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use high value resistors to get the voltage down with a voltage divider.

use low voltage opamps.

?
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Old 1st November 2006, 02:53 AM   #5
xiaonan is offline xiaonan  China
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My meaning is using a voltage divider to reduce(scale down) the input voltage. For example, if the R1/R2=25, and V1=100V, so the input of the amplifier is 100V/25=4V. so I can use common amplifier which power supply can be +/-15V.
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Old 13th November 2006, 07:09 PM   #6
Stocker is offline Stocker  United States
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Smashing finish on a really good speaker. Congratulations on a job well done. This just might be the design to copy.
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Old 13th November 2006, 07:33 PM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
you almost have a bridge circuit there.
Why reference it to earth?

Collect all the 0volt references together and float them to an opamp reference and make the supply to the opamp float with the measuring reference.

Then you can work at any voltage within the insulation range of the equipment.

But, why all the complcation?
Just measure the voltage drop across the sense resistor.
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