MoD Voltmeter - wiring arrangement?

[6SN7WGTB]

Have just acquired an ex-MoD meter, scaled for 500V and with an external shunt - marked as shown.

Does anyone know what the wiring arrangement is for this?

The face suggests it may have some sort of built-in rectifier, and measuring across the meter terminals (carefully using the manual current-limited ranges of a DMM) shows something like 200kΩ in one direction and 500kΩ in the other. Seems a bit odd.

Can anyone advise?

Thanks.

[Station X]

A shunt would convert it to a current meter. Could it be a multiplier resistor? What does it measure resistance wise between the terminals?

[M0FYA Andy]

Read what it says on the dial - 'Rectifier' and 'Use External Unit NFe'. Is it a thermocouple meter?

Andy

[6SN7WGTB]

Originally Posted by Station X

A shunt would convert it to a current meter. Could it be a multiplier resistor? What does it measure resistance wise between the terminals?

Perhaps my use of the word 'shunt' was misleading - it measures 500MΩ series end to end.

[6SN7WGTB]

Originally Posted by M0FYA Andy

Read what it says on the dial - 'Rectifier' and 'Use External Unit NFe'. Is it a thermocouple meter?

Andy

I'm sorry but I haven't come across a 'thermocouple meter' calibrated up to 500 volts. Can you explain?

[6SN7WGTB]

On the face of it, if you'll excuse the pun, I might assume the 500MΩ series resistor is just that and the meter contains a rectifier.

However, excuse me if I'm looking stupid, but it's a really nice meter and I'd rather seek guidance before trying anything.

[ms660]

So far as I can make out the Ernest Turner Model 605 meter movement is 1mA-75 Ohms.

Maybe a 500k (or thereabout) external multiplier ?

The internal rectifier might be copper oxide which might account for the high resistance readings reported across the meter terminals.

Lawrence.

[6SN7WGTB]

Originally Posted by ms660

So far as I can make out the Ernest Turner Model 605 meter movement is 1mA-75 Ohms.

Maybe a 500k (or thereabout) external multiplier ?

The internal rectifier might be copper oxide which might account for the high resistance readings reported across the meter terminals.

Lawrence.

Thank you Lawrence, that would make sense.

(P.S. typo in my post - it's a 500kΩ not 500MΩ multiplier.)

[Radio Wrangler]

The shunt resistor that sets the scaling of a moving coil movement to read current is obviously called a shunt

The series resistor to make a moving coil meter have the wanted voltage scale is unobviously called a burden resistor. A term not commonly used. Not a lot of people know that!

David

[Station X]

Originally Posted by Radio Wrangler

Not a lot of people know that!

David

I certainly didn't!

[Trigon.]

Originally Posted by Radio Wrangler

The series resistor to make a moving coil meter have the wanted voltage scale is unobviously called a burden resistor

I think the vast majority would call it the 'multiplier'.

To me 'burden' is most often associated with current measurement?

Cheers

[russell_w_b]

Originally Posted by Trigon.

To me 'burden' is most often associated with current measurement?

I've always known ammeter and voltmeter range adaptations as shunts and multipliers, and 'burden' for C/T metering on power switchboards (1A metering burden or a 5A metering burden). The ammeter providing the burden being directly connected across the current transformer.

[pmmunro]

6SN7GT,

From you first photograph, its possible to see that this is a moving coil instrument.

The external component is a multiplier in three sections making up the 500Kohms which you measured. This should be connected in series with the basic meter.

500V divided by 500kohms gives 1mA which would be a normal fsd value for an ordinary moving coil meter, not a rectifier instrument as this one is. This neglects the voltage across the meter movement which, as stated above, is often the standard value of 75mV, but that is almost negligeable compared to 500V and would be allowed for in the calibration of either the meter or the multiplier.

A moving coil meter with a rectifier will read the average value of an alternating waveform but normally the required value will be the RMS one, SQRT 2 times the average - for a sine wave. The usual practice is to calibrate the meter assuming that the voltage to be measured will be sinsusoidal, but this might not be the case for a special purpose meter.

The next question is what would you like to do with your meter? If you want to measure sinusoidal voltages up to 500V rms, just connect the multiplier in series. For a lower alternating voltage scale the multiplier value down linearly and trim the exact value against the most accurate meter you have.

If you want to measure direct voltages, you could leave the rectifier in circuit and adjust the multiplier value to suit your needs.This will work down to 100V or less, but below that it would be better to remove the rectifier to avoid non-linearity.

The resistance values which you measure across the meter, without the multiplier, will depend greatly on the voltage applied by your resistance meter, and so may not help in calculations.

PMM

[Silicon]

The scale plate is marked 1967.

Silicon and Germanium rectifiers were available at that time.

If this meter does contain a diode, were there any technical advantages to using alternative devices such as Selenium and Copper Oxide rectifiers?

[Herald1360]

Originally Posted by pmmunro

6SN7GT,

......

A moving coil meter with a rectifier will read the average value of an alternating waveform but normally the required value will be the RMS one, SQRT 2 times the average - for a sine wave. The usual practice is to calibrate the meter assuming that the voltage to be measured will be sinsusoidal, but this might not be the case for a special purpose meter.

....l.

PMM

The RMS to average ratio for a sinewave is pi/2sqrt2 or roughly 1.11 not sqrt2 which is ~1.414.

[6SN7WGTB]

Originally Posted by pmmunro

6SN7GT,

The next question is what would you like to do with your meter? If you want to measure sinusoidal voltages up to 500V rms, just connect the multiplier in series. For a lower alternating voltage scale the multiplier value down linearly and trim the exact value against the most accurate meter you have.


PMM

Thank you for a very full response.

My precise desire is to create something of a conversation piece meter to display the mains voltage, so hence the purchase of a 500V rectifier meter, which therefore would sit at about 50% FSD etc.
Plus it's illuminated, which will add to the 'effect'.

[6SN7WGTB]

Unfortunately, my fears about the seemingly very high resistance has been confirmed by the meter being DoA with the series 500kΩ and brought up slowly on a Variac.

Nothing at all.

[Ed_Dinning]

Nothing to loose, try opening it up and check meter movement directly with a very low voltage and say 10K series resistor. It could be the rectifier (easily replaced with Si diodes

Ed

[6SN7WGTB]

Originally Posted by Ed_Dinning

Nothing to loose, try opening it up and check meter movement directly with a very low voltage and say 10K series resistor. It could be the rectifier (easily replaced with Si diodes

Ed

Absolutely - have suggested this to the seller as an alternative to return etc., as my desire is outcome not originality.

[2000 type]

Not much help, but a little digging for the Nato Stock Number finds that it was used in an item of Marconi Communications Systems equipment who catalogued it as W1S4285SH2-40.