Circuit for converting ammeter to voltmeter.

[blkfrd]

I have a shunt ammeter that has about 0.3 ohms of resistance and is made up internally with an electromagnet (a metal post with wire wound around the post and both ends of the wire connected to external terminals). The needle sits in the middle and fully deflects in either direction for about 2A of current depending on the direction of current flow.

I want to convert it to a voltmeter with an 8 to 16V range. A 12V stable reference power supply came to mind using a SEPIC DC-DC converter powered by the same battery whose voltage I am measuring. The SEPIC supply is connected to one side of the ammeter and a current limiting resistor and the battery is connected to the other side.

When the battery voltage is +/-4V different than the 12V reference and with 2 ohms of circuit resistance, 2A would be flowing and the ammeter would be fully deflected in one direction or the other. Current and subsequent ammeter deflection would be linear for < +/-4V.

Problem here is that a SEPIC converter can't sink current. It can source current, but not sink current. I could add a current sink transistor switch and resistor possibly that turns on only when 12V < battery voltage.

4 switch Synchronous DC-DC converters can source and sink current, but this solution is expensive and i'm trying to find the lowest cost solution.

What do you guys recommend? I thought of using a Power Op Amp, but I think it would require a V+ and V- power input. I'm a bit rusty on Op Amps so I can use some help though.

[pebe]

It depends on what the sensitivity of the basic movement is, once you have removed the shunt.

If it is 250uA, or less, for full scale deflection then it's possible. If not, forget it.

[blkfrd]

It depends on what the sensitivity of the basic movement is, once you have removed the shunt.

If it is 250uA, or less, for full scale deflection then it's possible. If not, forget it.

There is no shunt in the ammeter. The ammeter itself is the low current shunt path in parallel with the main current path. The application relied on the resistance of small gauge wire for the ammeter path vs the large main wire where the brunt of the current flowed. It takes 2A to fully deflect from center 0 in either direction.

I can make this work in the lab with lab power supplies so I know it is possible. It works well actually. I just need a supply or supplies that can both source and sink up to 2A of current. The 4 switch synchronous buck/boost converters that are available will do it, but the price point is high so i'm investigating other circuits.

[I_Daniel]

I think there is a bit of a misconception by you of how a meter works. What is connected between the two poles of Your meter?
An ammeter has a parallel resistance and a Volt meter has a series resistance.
An ammeter can be considered to be a volt meter which reads the voltage across this resistor which in many cases is a piece of resistance wire.
This resistance is dependent upon how many micro- or milli-amperes gives a full deflection on the meter which in turn is dependent upon the internal resistance of the meter.

You can always remove whatever is between the + and - poles of the meter. Use a very large say at least 5M Ohms Linear variable resistor in series with the meter and slowly decrease the Ohms until you get a fullscale deflection.
This means the series resistor limits the current tthrough the meter.
To get a 16 Volt full deflection reading you will have to have a regulated variable power supply to use the method I have given you.
To calibrate the meter you will required a variable Zero to say 20 volt regulated power supply. This power supply MUST also have a volt meter. Your calibration is thus also dependent upon the accuracy of the power supply and it's meter you use for the calibration.

This is not infallible so I accept no responsibility if you damage your meter by overloading it. But if you use the variable power supply and set it to zero to start off with you should not have any problems.

[blkfrd]

I think there is a bit of a misconception by you of how a meter works. What is connected between the two poles of Your meter?
An ammeter has a parallel resistance and a Volt meter has a series resistance.
An ammeter can be considered to be a volt meter which reads the voltage across this resistor which in many cases is a piece of resistance wire.
This resistance is dependent upon how many micro- or milli-amperes gives a full deflection on the meter which in turn is dependent upon the internal resistance of the meter.

You can always remove whatever is between the + and - poles of the meter. Use a very large say at least 5M Ohms Linear variable resistor in series with the meter and slowly decrease the Ohms until you get a fullscale deflection.
This means the series resistor limits the current tthrough the meter.
To get a 16 Volt full deflection reading you will have to have a regulated variable power supply to use the method I have given you.
To calibrate the meter you will required a variable Zero to say 20 volt regulated power supply. This power supply MUST also have a volt meter. Your calibration is thus also dependent upon the accuracy of the power supply and it's meter you use for the calibration.

This is not infallible so I accept no responsibility if you damage your meter by overloading it. But if you use the variable power supply and set it to zero to start off with you should not have any problems.

Between the two terminals is about an 18-20 ga wire that goes from terminal A then many times around a metal core and then to terminal B...that is it.

It takes 2A to fully deflect from center 0 to the right or left. If I use a 12V reference and the battery is 8 or 16 V, I want it to fully deflect (delta V is +/-4V). This requires 2 ohms total resistance and the ammeter is about 0.3 ohms by itself. No matter what the voltage differential is, 2A must flow thru the ammeter to fully deflect it. Problem with using a higher voltage differential is the resistor will have to be many watts. For 4V delta V, the resistor needs to be 16/2 or 8W as it is. If I make the working range of the ammeter smaller, say 9 to 15V, and R needs to be 1.5 ohms, then the resistor can be 9/1.5 or 6W.

Most of the time, the nominal battery voltage will be 12 - 13V

I need something that can read the voltage difference and produce a variable voltage from 0 to 4V that can source up to 2A and I could switch which terminal it is applied to depending on if 12V > battery voltage or 12V < battery voltage (with hysteresis so it doesn't flip flop around 0) and i've got it. What would be a good way to do this?

[pebe]

You started off this thread with "I have got a shunt ammeter....." and I assumed it was an instrument that used a shunt so that the instrument only measured part of the 2A current flowing in a circuit.

I_Daniel has explained to you, better that I could, the difference between an ammeter and a voltmeter. Note that he said the ammeter is a voltmeter that measures the voltage across a piece of resistance wire. If you have a shunt ammeter then the piece of wire you are describing is the shunt - not the ammeter coil. If you remove the shunt you would then have the basic voltmeter movement and you may be able to make use of this.

But if the 0.3ohms of wire you mention is the basic coil of the ammeter, then it is not a shunt ammeter. You could make the needle deflect, as an experiment, but it would be useless as a measuring device because it would heavily load any circuit you were measuring, and give a low reading.

[blkfrd]

I should not have used the term shunt ammeter. The ammeter has only one path thru it. The ammeter application is where the ammeter itself is shunting a small amount of current. The application is automotive. 2A is nil here. It was common years ago to have an ammeter in parallel with a large gauge wire between the alternator and the battery. They had limited usefullness. They also would go poof if the main line was degraded and too much current passed thru the ammeter or if someone attached jumper cables backward. Just trying to come up with the most efficient way to convert this thing to a voltmeter.

[I_Daniel]

I have a set of meters consisting of a volt meter and an ammeter in the same box - Not home made - the first is the volt meter calibrated at 15 volt and the second is the ammeter calibrated at 15 Amp. Neither of these have any external resistors ! The are nevertheless the exact same models viz Kyoritsu P-25 indicated on the individual faces of the meters.

Just to prove a point I have now opened the meters and found these internall connections.
1. The volt meter had an internal resistor in series with the + line.
2. The ammeter had a piece of resistance wire between the two poles.

Many automotive meters work on a thermistor which as it heats up it passes more current thereby stabilizing the meter reading, (because the thermistor remains hot for a while). It is Usually the petrol gauge. In the case of meters found in power supplies or a VOM the needle would bounce around with the shifting of the car on the road. Some meters even have counter electro-magnets to stabilize the needle.

Unless you know exactly how a specialized automotive meter works and is constructed it takes some doing and expertize to puzzle it out. But the basics remain the same.

That piece of wire around the metal could be your 0.3 Ohm resistance wire.

The "coil" of a meter unless it is a meter operating on a bi-metal principle would be almost monstrous compared to a standard meter as it must be able to withstand 8 Amperes and even more. A bi-metal meter would in any case not give readings of plus or minus. So your meter does not operate on this principle and I must surmise that it operates on the moving coil principle.