# voltmeter and ammeter in circuit

#### macson66

Hi, I'm hoping for a little advice as I'm a rookie at electronics. I'm building an amp (TPA3255 from 3E with PS: LRS-350-48) or at least I would be if the amp were to ever leave the port in China (virus time!!). In the mean time I'm building a chassis vintage style, as in I have a couple of old Bakelite meters and a chickenhead knob so thought it would be fun to make the whole thing look old - just my thing. It would be nice if I could get the meters to work with the amp so my question is does the diagram (excuse bad drawing) make sense? If so what size Shunt would I need (I can't work it out), given I'm running at 48v (maybe a little less 45v?) for the Ammeter? Would I need to add a resistor? Any help appreciated.

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#### macson66

....and

Forgot to add, the Ammeter says 240v so could it still work with 48v? Accuracy of meter is not important as it's just part of my fun? project.

#### Mooly

Paid Member
The voltmeter looks ready to go... try it on a 9 volt battery and make sure its OK. The 4ma/12.5k scratched on the back suggests it presents a load of around that value... so no problem there.

50 volts full scale and 4ma work out at 50/0.004 which is 12500

The ammeter has two options and you need the 90 millivolt one. What that really means is that the meter is a 90mv voltmeter in native form or a 240 volt meter with an internal resistor. A moving coil ammeter is simply a voltmeter with a shunt.

So that to me reads as two ranges, 90mv full scale and 240 volt full scale.

So you connect it across a shunt that will give 90 millivolt drop at the maximum current you want it to show at full scale.

So if that is 5 amps then you need a shunt of R=V/I which is 0.090/5 giving 0.018 ohms. Very low as it should be. Try a small piece of hookup wire a few cm long for that.

#### JMFahey

0.18 ohm is available ready made in 5W dissipation, check an electronic parts supplier.

Not sure about your drawing showing a grounded Ampere meter; proper use is connecting it "floating" (NO connection to ground) IN SERIES with +V wire, across the shunt resistor.

#### macson66

Lovely, thanks - will 'float' the ground. Plenty to be going on with, thanks again.

#### Mooly

Paid Member
0.18 ohm is available ready made in 5W dissipation, check an electronic parts supplier.

I think we wanted 10 times lower value than that... as a guesstimate of course if we are looking at say 5 amps max current.

And it all depends whether you want something accurately calibrated or just something that moves We don't really know the max current draw of the amp either.

#### macson66

... just something that moves.

#### Mooly

Paid Member
So perhaps make the shunt resistor higher in value so that you get decent volt drop at lower output levels and add a hotshot (lol, don't you just love spell checkers ) shottky high current diode across the resistor to stop any volt drop going to high. A clamp.

Thanks Mooly.