VOLTMETER CURRENT DRAIN

freebird1

freebird1

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This may seem a silly question, but what would the current drain be on a VDO analogue voltmeter, on a 12 volt system? I can't get a reliable measure of the resistance, although both meters appear to be fine.

I'm about to fit the voltmeter, but as I won't be staring at all the time, I am wondering whether to fit a switch for it so as to save the juice.
 
Years ago I had an analogue multimeter which, I think, had an impedance/resistance of 10 kilo-ohms per volt. That suggests a pretty small current drain. But it also suggests that the resistance of your meter should be measurable with a digital multimeter. Of course a digital meter should also be capable of measuring the small current.
 
Probably very little if it's a good quality one. I don't know about your particular voltmeter but it's probably of the order of several k? per volt. The old analogue AVO meters used to be 40 k? per volt I think, so for 12v that would be 480 k?. Even if yours was just 1k?/v that would only give you a 12mA current draw.
 
Err

No

It doesn't work quite like that.

The meter itself, the basic mechanism will have a current rating for full scale deflection.

This current will be what the whole instrument draws assuming 12V is fsd. If its less the current drawn will be proportionately less.

Meter probably somewhere between 1 and 10 mA.

If flattening the battery is a concern why not wire the meter to only be connected when the power is turned on. Little point measuring volts if no-one on the boat to see it.
 
As the last poster says. It is a fairly small current that is drawn by the voltmeter. So it would not hurt to have the VM connected when power is on but on the other hand it may be an unnecessary drain to have it connected when you are away from the boat.

Voltmeters for use in electronic testing are always designed to draw as little current as possible because in electronics in drawing current in measuring a voltage you can actually change the parameters of the circuit. ie you reduce the voltage by measuring it. This of course is not a consideration when checking battery volts. So VDO may not have gone to any trouble to reduce the current drawn by their voltmeter. The analogue voltmeters of various designs usually draw more current than digital types although often the digital VM is powered by the battery you are measuring so take some current.

good luck olewill
 
My reading of the resistance is only 782 ohms. That translates to a current of 15mA (at 12v)unless there are other electrical laws that apply in this case. I just wonder if that is a lot for a voltmeter as all references on the internet are talking about megaohms of resistance....

It would be wires across the main battery switch so I can check the voltage of either battery, hence it would be off when the batteries are switched off. I was really debating whether I should fit a small switch with the meter so that the meter only reads when I want to check it. 15mA is not a lot, just a bit less than my echo sounder, but I don't leave that switched on when I', not underway either.
 
[ QUOTE ]
My reading of the resistance is only 782 ohms

[/ QUOTE ] That sounds perfectly reasonable for that type of instrument.

There is a big difference between your application and measuring voltages in electronic circuits where, as William H points out you would be interested in the minimum possible current drain. Good quality, high impedance, analogue meters use a meter movement with a very high sensitivity and therefore have a high resistance. However the meter movement is relatively delicate and may not take kindly to life on board. The less sensitive movements used in low impedance instruments can be much more robust and more suited to life at sea.

As you deduce the current drawn by your meter is not all that great. You have simple choice. Either wire it so that it is registering all the time the battery isolator is closed or via its own switch which could be a push button or or switch which returns to off when not in use.
I would not however wire it across the battery isolating switch as you appear to be suggesting, so that it only reads when the switch is open. It should be wired from positive to negative.

With two batteries it makes sense to wire the meter using a double throw switch with a centre OFF position so that either battery can be monitored at any time.

Fuses of a very small current rating should be incorporated in the battery connections.

There should be some helpful info on the TB Training website
 
[ QUOTE ]
Err

No

It doesn't work quite like that.

The meter itself, the basic mechanism will have a current rating for full scale deflection.

This current will be what the whole instrument draws assuming 12V is fsd. If its less the current drawn will be proportionately less.

Meter probably somewhere between 1 and 10 mA.

If flattening the battery is a concern why not wire the meter to only be connected when the power is turned on. Little point measuring volts if no-one on the boat to see it.

[/ QUOTE ]
So you're saying that the meter has a fixed resistance then, irrespective of the applied voltage? That sounds logical. I stand corrected (and if I'd thought about it a bit longer I should have realised that was the case). Still, I wasn't that far out with my flawed calculation - definitely not enough current there to worry about draining a battery.
 
[ QUOTE ]
So you're saying that the meter has a fixed resistance then, irrespective of the applied voltage?

[/ QUOTE ] The way in which the input resistance of analogue multimeters is quoted is a little confusing until you understand it.

The volts figure you must use is the FSD for each range. Say for example that a meter is quoted to have an input resistance of 10k ohms per volt. What that actually means is that on the 1 volt range it will have an input resistance of 10k ohms, on the 5volt range it will be 50k ohms, on the 10volt range it will be 100k ohms and so on. That is for each range the input resistance is the FSD times the ohms per volt figure.
In this example the meter movement will be 0.1 mA (or 100 µA) that will be the current draw at fsd for each range, at half of fsd the current will be half of that namely 0.05mA (50µA)

Digital meters are a different kettle of fish having a fixed input resistance across all the voltage ranges. Typically that is 1M ohm for all the DC volts ranges for cheapies or 10M ohms for better ones.
 
No thats not what I'm saying.

A moving coil meter works by passing current through a coil of wire in a magnetic field.

By Faraday's rule this produces movement.

By attaching a pointer to the coil the movement can be measured on a scale - a meter.

The deflection of the pointer is proportional to the current in the coil.

A meter is commonly described as having a full scale deflection of "X" mA.

So on any meter there will be a current through the coil of a specific value that equates to 12V across the terminals.

The resistance of the coil is very small so a series resistor is included to limit the current.

Irrespective of the quality of the meter and the value of the resistor it is the value of the current passing through the coil that determines the deflection and hence the reading of the meter and therefore it is this current that is the load that the meter puts on the battery.

Its resistance is not the relevant question - with a multimeter it will vary with whatever scale you are measuring on.
 
I still don't understand what your point is. In my first reply I made a distinction between the VDO meter and a multimeter, and I also mentioned ohms/v as the relevant figure to work with. ohms/v is only another way of expressing current at fsd. As it only has one fixed scale a VDO meter must therefore have a fixed resistance (the combined value of the coil and the built in resistor). From there it's just v= I x R.
 
But is the VDO a thermal gauge, passes currant through a bi metal strip, this moves to operate pointer. Gives a stable damped reading on a moving object.

Brian
 
[ QUOTE ]
But is the VDO a thermal gauge

[/ QUOTE ] With a measured resistance of 782 ohms therefore passing only 15mA at 12volts that's not likely is it?

It does not affect the original question about the best way if installing it anyway.
The argument between srp01983 and Beadle is not really relevant either.
 
A normal moving coil meter has a resistance of somewhere between 10K and 20K/volt full scale and would draw so little current that it could be ignored. It also would have a linear scale.

As your meter has a resistance of 782 ohms and a full scale reading of say 15 volts then as has been said it will draw about 15mA from your battary. In this case it is probably a moving iron meter and probably has a non linear scale where the voltage range you would normally wish to measure is expanded.

A moving iron meter is one where the current driving the movement passes through a fixed coil and deflects a permanent magnet attached to the pointer to give the reading.
This gives a much more robust instrument than a moving coil meter.

All this technology has now generally been 'superceeded' by digital electronic displays which are infinately even less robust.

All this is a bit academic as it is a bit pointless reading the battery with the boat battery switched off and the battery is not under load as you may get an artifically high reading. Can the battery not be connected after the battery isolation switch?

Iain
 
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