Panel Analogue Meter Circuit

[alin_im]

Hi guys,

I recently found a panel analogue voltmeter, but i can not find a circuit for calibrating it! can you please help me??? Any good articles / circuits to read about this topic ?

[Asmyldof]

You need one or more known quantities.

Usually for analogue the calibration procedure is in the range of 0% and 100% scale optimised by turning the little screw until both are satisfactory (depending on the accuracy of the coils and the printer that made the papers it might be a trade off between either one perfect or both sort of near perfect).

A known quantity can be a very accurate voltage source, but for analogue mV scale will not be needed.

Tuning 0 is easy: Short the two terminals and hold it exactly level, while tuning.
Tuning full scale can be done with a power supply and a decent multimeter, or a power supply with a good enough display of its voltage output.
Then also see how off it is, and choosing what manner of averaging between the errors will work for you.

If it's a spiffy one that's been built for accuracy, you might want a mV scale and a very good eye and might be able to get both ends spot on.

EDIT:
Ah hang on, it says volts, but has no internal resistor, just a 1mA response coil.
So create a known current, by putting a resistor in series, so you can tune the voltage on your power supply to adjust the current, and again use another measurement interface to get to exactly 1mA. If you have some reasonably accurate resistors you could measure voltage across a 1k or 10k type and go for 1V or 10V. Measuring single volts is usually easier on affordable multimeters than measuring tenths of a mA.

[mikerj]

How you calibrate it depends entirely on what you want to measure with it.  Voltage/Current/Power etc.

[alin_im]

You need one or more known quantities.

Usually for analogue the calibration procedure is in the range of 0% and 100% scale optimised by turning the little screw until both are satisfactory (depending on the accuracy of the coils and the printer that made the papers it might be a trade off between either one perfect or both sort of near perfect).

A known quantity can be a very accurate voltage source, but for analogue mV scale will not be needed.

Tuning 0 is easy: Short the two terminals and hold it exactly level, while tuning.
Tuning full scale can be done with a power supply and a decent multimeter, or a power supply with a good enough display of its voltage output.
Then also see how off it is, and choosing what manner of averaging between the errors will work for you.

If it's a spiffy one that's been built for accuracy, you might want a mV scale and a very good eye and might be able to get both ends spot on.

EDIT:
Ah hang on, it says volts, but has no internal resistor, just a 1mA response coil.
So create a known current, by putting a resistor in series, so you can tune the voltage on your power supply to adjust the current, and again use another measurement interface to get to exactly 1mA. If you have some reasonably accurate resistors you could measure voltage across a 1k or 10k type and go for 1V or 10V. Measuring single volts is usually easier on affordable multimeters than measuring tenths of a mA.

Thank you for your response, i tried what you suggested and if I have a 12 kohm resistor in series at 12 volts the a. voltmeter goes to 12 , which means when the current through the meter is 1mA the niddle is fully deflected. The problem is that it is not a linear relationship between the voltage and niddle deflection, because when i put 5 volts in the niddle is at 6V  and if the power supply is set to 1.25V the voltage shown is 2.   

[alin_im]

How you calibrate it depends entirely on what you want to measure with it.  Voltage/Current/Power etc.

I would like to measure voltages from 0 - 12 v

[Asmyldof]

Don't discount the fact that the meter might have a small voltage drop itself, which is a different kind of linear or near-linear to the resistor itself, that might make it slightly unlinear as a total system.

Keep an eye on the voltage across the resistor.

If you want perfect precision of the voltage measurement only a resistor will never give you accuracy at both ends because of the not-exactly-perfect behaviour of the meter itself.
If you want the best possible precision at full range you will need to convert the input voltage to a forced current (code name: VCCS, for Voltage Controlled Current Source).

You can also do some tricks to compensate or the coil's performance, if you want it to run off only the voltage you are measuring, but they will never be fully accurate at the low end.

[PSR B1257]

that the meter might have a small voltage drop itself

Might? It will certainly have a resistance, since I can't see a bottle of liquid nitrogen anywhere 

I would like to measure voltages from 0 - 12 v

That's the application for a simple voltage divider 

[jimmc]

Since it has 'Volts pk-pk' on the scale-plate it is meant to measure AC voltages in conjunction with an external rectifier.
The non-linear scale is designed to compensate for the non linearity of a simple rectifier and series resistor.
Single diode or bridge using silicon or germanium diode(s) is difficult to tell, time to experiment.

Jim

[alin_im]

Since it has 'Volts pk-pk' on the scale-plate it is meant to measure AC voltages in conjunction with an external rectifier.
The non-linear scale is designed to compensate for the non linearity of a simple rectifier and series resistor.
Single diode or bridge using silicon or germanium diode(s) is difficult to tell, time to experiment.

Jim

Yeah i was thinking about that but i thought i could work for DC also...how can i make sure i will not burn it always have a current max of 1mA??? and that is it

[PSR B1257]

The non-linear scale

Only the dB-scale is non-linear, as usual.
The "peak-to-peak" (rather stupid declaration, since it also shows "Batt" - and as far as I know is there no such thing as a AC battery) scale is linear, as usual for a moving coil movement.

always have a current max of 1mA???

Yes, this would be a good idea   

Did you meanwhile measure the coil resistance?

[jimmc]

The non-linear scale

Only the dB-scale is non-linear, as usual.
The "peak-to-peak" (rather stupid declaration, since it also shows "Batt" - and as far as I know is there no such thing as a AC battery) scale is linear, as usual for a moving coil movement...

Please look at the photo in the original post, the pk-pk scale is non linear, the arc from 0 to 1v is much less than that from 11 to 12v.

I agree the moving coil meter is linear hence...

... I have a 12 kohm resistor in series at 12 volts the a. voltmeter goes to 12 , which means when the current through the meter is 1mA the niddle is fully deflected. The problem is that it is not a linear relationship between the voltage and niddle deflection, because when i put 5 volts in the niddle is at 6V  and if the power supply is set to 1.25V the voltage shown is 2.

(The presence of a 'Batt' mark is irrelevant, the meter will have been switched from  normal use with an EXTERNAL rectifier to 'Battery Check'.)

alin_im

If you want to use the meter for DC then you really need to replace the scale with a linear one, however if you want to avoid this then you could try adding a silicon diode (cathode to +ve to the meter. Something like a 1N4148 - not critical) in series with the meter and resistor.
The typical coil resistance for a 1mA meter is 75 ohm and the diode drop approximately 700mV so the resistor will need to be (12-0.7)/.001 -75 = 11225 ohms; try 11k.
Adding the diode will 'cramp' meter deflection at lower voltages in the same way as the scale markings.
The compensation won't be perfect but it will give better results than you measured before.

Jim