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Voltmeter advice needed


decauville1126

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Just acquired the voltmeter shown. Works fine and accurate on the lower scale but I need to use it up to 60V DC. So can anyone suggest what size resistor needs to be used? The internal resistance is 400 ohms as marked. No idea what it has come from but is appropriate for what I have in mind.

 

Many thanks for any help.

 

36832061510_65ba608cf3_z.jpgP1050428 by Peter Smith, on Flickr

 

37087079741_2f7b3b52bd_z.jpgP1050429 by Peter Smith, on Flickr

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Hi

 

To get the total resistance needed for a given full scale deflection (FSD) we use ohms law

 

R = V / I where V is the FSD voltage on the scale and I is the current needed by the meter (which is in reality current operated)

 

We know from the scale that this meter has a resistance of 400 ohms and a lower range of 6V, so changing the subject of the formula

 

I = V/R = 6/400 = 0.015 = 15ma

 

The total resistance needed for 15ma at 60v is

 

R = V/I = 60/0.015 = 4000 Ohms.

 

Since the meter itself has a resistance of 400 ohms we need to add 3600 ohms (3.6K) in series with the meter as a "multiplier" to make up 4000 ohms.

 

As a sanity check, the less scientific approach is to say that as the FSD current is a physical property of the meter, and to get the same current at 60V as at 6V requires a resistance 10 times higher, and as the meter is marked 400 ohms, the 60V range should need 400 * 10 = 4000 ohms.

 

Hope this helps

 

Iain

 

PS the original post already confirms that the 6V range is accurate without a series resistor, so we can calculate it as 6V / 400 ohms. Otherwise there would be no way to know the full scale current unless it is printed on the meter. Anyone else reading this who has an unknown voltmeter should be careful testing without a series resistor in case it was designed to use an external one - best to start with a reasonably high value (around 20,000 ohms in series per volt of scale is needed to protect a 50 microamp movement) and reduce it until the meter reads full scale at the correct voltage, and then work out or measure the FSD current.

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Works fine and accurate on the lower scale

 

Peter but is the meter set at zero correctly when you did that check? That large screwdriver slot on the front is to zero set it, or maybe the zero point various if you have the meter vertical or horizontal?

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Hi

 

 

Since the meter itself has a resistance of 400 ohms we need to add 3600 ohms (3.6K) in series with the meter as a "multiplier" to make up 4000 ohms.

 

As a sanity check, the less scientific approach is to say that as the FSD current is a physical property of the meter, and to get the same current at 60V as at 6V requires a resistance 10 times higher, and as the meter is marked 400 ohms, the 60V range should need 400 * 10 = 4000 ohms.

 

Hope this helps

 

Iain

 

 

 

Many thanks Iain. I had to use 2x 2k resistors in series as they are the nearest to hand, and although I realise there is a % error it's near enough. It works.

 

And I also appreciate Clive's post about checking zero when the meter is vertical - I've already discovered what happens when you keep turning the slotted button on the front too far! I use a digital meter to confirm the volts at, say, 5.0, then adjust the button to that. Then power off/on and recheck. Thankfully it's not critical as it will be used to monitor voltage drop to prevent - hopefully - overdischarge of a 60V battery.

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Many thanks Iain. I had to use 2x 2k resistors in series as they are the nearest to hand, and although I realise there is a % error it's near enough. It works.

 

...it's not critical as it will be used to monitor voltage drop to prevent - hopefully - overdischarge of a 60V battery.

 

Er, how big is the battery, because 15mA is quite a considerable current drain for a meter, and you should be able to get something far more sensitive than that without too much difficulty. (500 microamps (0.5 mA) was common for panel meters during WW2, and modern test meters are usually a tenth of that.)

 

Chris (G8KGS)

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I just noticed the 'A.P.' part number. "Admiralty Pattern" so probably off something very substantial!

 

Also, 60 volts at 15mA is 0.9 Watts, so your series resistor will get quite warm and needs to be rated at 1 Watt or more.

 

Chris. (G8KGS)

 

Thanks for that advice - that's why I seek enlightenment!

 

'Proving' resistor is only 1/4 watt, so have ordered a pack of 1 watt ones to come on a slow boat from China.

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Er, how big is the battery, because 15mA is quite a considerable current drain for a meter, and you should be able to get something far more sensitive than that without too much difficulty. (500 microamps (0.5 mA) was common for panel meters during WW2, and modern test meters are usually a tenth of that.)

 

Chris (G8KGS)

 

Probably be seeing how 2x 24V 200ah (ie 8 12v 100ah batteries) does as the control system is parallel/series/series with weak field, so 48V max. This is half the original design but should suffice and give a reasonable run time. Controller is rated at 44 amps. Testing has so far involved 2 12v spare batteries which has proved promising. Luckily I have competent persons to oversee what I'm doing or thinking of doing!

 

I'm very appreciative of the help and guidance I've received on this forum.

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Probably be seeing how 2x 24V 200ah (ie 8 12v 100ah batteries) does as the control system is parallel/series/series with weak field, so 48V max. This is half the original design but should suffice and give a reasonable run time. Controller is rated at 44 amps. Testing has so far involved 2 12v spare batteries which has proved promising. Luckily I have competent persons to oversee what I'm doing or thinking of doing!

 

I'm very appreciative of the help and guidance I've received on this forum.

 

Are you restoring a submarine? :D

 

Chris.

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Are you restoring a submarine? :D

 

Chris.

 

A submarine battery is somewhat bigger than that, but you don't usually get the chance to see them out in the open. This is the battery shop at the old Admiralty Engineering Laboratory in West Drayton, sadly now just another housing estate:

 

attachment.php?attachmentid=129288&stc=1

 

Andy

West Drayton Battery.jpg

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