Convert Analog panel voltmeter to ammeter

Thread Starter

naitek

Joined May 31, 2008
15
Hey ;
i have an old analog voltmeter (20v) and i would like ot conver to a 100µA to use for a project.
The voltmeter has a 150Ω internal resistance with an inline 20k resistor so basically reads .13Amps . I dotn know about shunting and am asking to make sure i i dont screw anything up. thanks in advance

-n
 

Senz_90

Joined Jul 11, 2013
70
Hey ;
i have an old analog voltmeter (20v) and i would like ot conver to a 100µA to use for a project.
The voltmeter has a 150Ω internal resistance with an inline 20k resistor so basically reads .13Amps . I dotn know about shunting and am asking to make sure i i dont screw anything up. thanks in advance

-n
Please browse my thread early. maybe you could get the idea from others comment on those thread.
 

alfacliff

Joined Dec 13, 2013
2,458
if the full sscale on the meter is .13 amps, it will not deflect much with 100 microamps. you will need another meter or a shunt with amplifier.
 

Alec_t

Joined Sep 17, 2013
11,402
I don't know where you got your 0.13A figure from, but by my reckoning if the 20V meter has a 20k inline resistor then the meter movement is a 1mA full-scale type. At 100uA it will read only 1/10 scale, so is unsuitable without amplification.
 

Thread Starter

naitek

Joined May 31, 2008
15
I don't know where you got your 0.13A figure from, but by my reckoning if the 20V meter has a 20k inline resistor then the meter movement is a 1mA full-scale type. At 100uA it will read only 1/10 scale, so is unsuitable without amplification.
i made a mistake on the amp math; thanks for the correction. So i would need to amplify my output ( eg 400mv ) through an op amp in order to make the meter read properly?
 

#12

Joined Nov 30, 2010
18,210
It looks to me like a 250 ohm resistor in series with the 150 ohm meter movement will allow 1 ma when you apply .4 volts.
 

Thread Starter

naitek

Joined May 31, 2008
15
i have tried various resistors and i get no movement. I would like to know where t=i can find the correct shunt with amplifier to attempt to make this work. I cant find anything really.

The thing is if i am applying 100mv - 900mv to this voltmeter; i would need to ramp to about 10v in order to see on the meter so an amplification or gain of 10 would be required.

Any help would be awesome. thanks again

-
 

#12

Joined Nov 30, 2010
18,210
That suggests the meter movement may be damaged :(.
I am beginning to get puzzled about this. It seems simple enough from here, but the results aren't happening. I'm about to fall back to seemingly stupid questions.

Did you shake the meter to see if the needle moves?
Is the meter on "zero" with no stimulus being applied?
Have you tried a DMM meter with a diode measuring range on it? That would supply about 1 ma.
Did you remove the 20k resistor?
 

Thread Starter

naitek

Joined May 31, 2008
15
the voltmeter is perfect for measuring 20v ( dc) Vout, but the circuit i was tring to measure was a 300 - 900mv circuit output which i wanted to read on the same meter. So basically i need an ammeter, but would like to know how to conver this 20v meter ( WHICH STILL WORKS with 20v) with 20kΩ resistor put back in.

i cant seem to figure that voltage to current converter figured out so trying to read more and leanr about that.

thanks
 

Brownout

Joined Jan 10, 2012
2,390
Current to voltage involves sensing a current, sometimes using a resistor and feeding an op-amp with prescribed gain to output an appropriate voltage. For example, for a 1 AMP current across a .1ohm resistor, the voltage is V=1A*.1ohm = .1v. Then an opamp amplifier with a gain of 50 gives a 5V output. One may also use a hall-effect sensor for the sensing element. Sometimes, the current carrying wire can be wrapped with several loops around the sensor to get more sense current.
 
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