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Old 26th February 2012, 10:34 AM   #1
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Default mA meter drags the circuit down

I have a 0-1mA D.C. Milliamperes meter by the Beede Electrical instrument Company Inc that I wanted to use as a testing meter for tube bias. I measured the meter and it measures roughly 45 ohms. I thought maybe this was too low and that I needed some type of circuit to help so that when I used the meter I would obtain the correct reading without pulling the circuit down. I used a TL082 op amp in which to drive the meter thinking that it would raise the impedance of the meter as seen by a circuit. It didn't seem to work as I wanted it to. Measuring tube bias in a circuit with a 10 ohm resistor from cathode to ground with a standard VOM meter I saw .350 volt across the 10 ohm resistor and with the meter and circuit it dragged it down to about .300 VDC across the 10 ohm resistor.

Any ideas?
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Old 26th February 2012, 10:52 AM   #2
SY is offline SY  United States
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Are you trying to insert the meter directly in the cathode circuit or put it across a sensing resistor that's already in the cathode circuit?
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Old 26th February 2012, 11:48 AM   #3
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Sy,

Thanks for the reply.

I was putting the meter across the 10 ohm resistor as I do the VOM meter.
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Old 26th February 2012, 12:11 PM   #4
M Gregg is offline M Gregg  United Kingdom
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If you put the meter across the 10 ohm you have resistors in parallel so yes it will pull it down.
You need either a high resistance multiplier and read the voltage or a small value shunt in series with the cathode resistor.
Link:
http://sound.westhost.com/articles/meters.htm
This may help..

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M. Gregg
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Old 26th February 2012, 12:20 PM   #5
DF96 is offline DF96  England
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I have an idea. You show us the circuit, so we know exactly what you did with a TL082. Then we will think about where things went wrong. The alternative may require us to read your mind; we are good, but not that good!
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Old 26th February 2012, 12:40 PM   #6
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Meter circuit
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Old 26th February 2012, 12:46 PM   #7
DF96 is offline DF96  England
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That looks OK, but is that what you built? Check your wiring, and the opamp pinout, very carefully.
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Old 26th February 2012, 12:49 PM   #8
SY is offline SY  United States
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OK, so what you want to do is apparently to convert your milliammeter into a voltmeter. You can do that with a simple series resistor- see the Rod Elliott article for the calculations- no opamp needed. Your cathode resistor is 10R, so the voltage across it (0.350V) means you have 35mA. Where do you want that on the milliammeter? A convenient spot is 0.35mA (but you can make that anywhere you like). The total series resistance in the meter circuit is R = 0.35V/0.00035A = 1k. You have 45R already, so the series resistor will need to be 955R.

The 1k total meter resistance shunted across the 10R sense resistor will perturb the reading a bit, but not much.
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Old 26th February 2012, 12:50 PM   #9
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Just put 955 ohms ( 1k - Ri of your meter) in series with the meter.
Then it forms a voltage meter with max. reading of 1 V.
Now you can place this tester directly across the 10 ohms cathode resistor.
Full readind 1 V represents 100 mA.
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