Circuit for converting ammeter to voltmeter.

[blkfrd]

I have a shunt ammeter (ammeter is used as a low current path shunt, no shunt internally) 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.

 

[dougy83]

Is there a shunt resistance in the device? e.g. a 0.3ohm resistor or lump of metal or wire across the coil? Or did you just describe that the coil is the shunt element as well as the deflection coil?

 

[blkfrd]

Is there a shunt resistance in the device? e.g. a 0.3ohm resistor or lump of metal or wire across the coil? Or did you just describe that the coil is the shunt element as well as the deflection coil?

The .3 ohms is the resistance of the wire when measured between the 2 ammeter terminals. There is no discrete resistor in the ammeter

 

[dougy83]

The .3 ohms is the resistance of the wire when measured between the 2 ammeter terminals. There is no discrete resistor in the ammeter

That's unfortunate; the usual way to convert an ammeter to a voltmeter is to remove the shunt and insert a series resistance.

You could use a buffered output opamp (or 'power opamp' as you stated). You can use a virtual ground created with another power opamp and offset the input voltage to allow single rail operation. Using a low supply voltage is a good idea, as you only need a 1.2Vpp swing, but at 2A.

 

[dougy83]

I just realised you mentioned being a bit rusty with opamps, so a circuit is attached. It's just a differential amp on the left and the 'virtual ground' circuit on the right. The meter coil is between the two.

 

[blkfrd]

Thanks for the ideas. When I called it a shunt ammeter, the ammeter itself was the shunt where it was the low current path in parallel with a main current path between a battery and charging unit. These old ammeters relied on the resistance of smaller wire size to shunt only a small amount of current through it...not a very robust design that was prone to problems or worse yet...too much current and smoke.

 

[ChrisP58]

Thanks for the ideas. When I called it a shunt ammeter, the ammeter itself was the shunt where it was the low current path in parallel with a main current path between a battery and charging unit. These old ammeters relied on the resistance of smaller wire size to shunt only a small amount of current through it...not a very robust design that was prone to problems or worse yet...too much current and smoke.

So, does it take 2 amps through this meter to get full scale deflection? Or did most of the 2 amps take a different path, with only a few milliamps through the meter?

 

[dr pepper]

If there isnt a shunt inside or on the back of the meter then it might be a moving iron ammeter, these take the full current of the load through the actual meter, if its one of these then its no use to you as a voltmeter.
A moving coil is fairly easy to recognize, follow the pointer down to the centre, there will be a frame that moves around a circular magnet held by 2 hair springs, if this has very fine wire windings on then its a moving coil.
I have a few meters that have been converted on a couple of projects.

 

[blkfrd]

2A through the meter to fully deflect from center 0 in either direction

 

[blkfrd]

Its not a moving iron ammeter. The coil is fixed and there are no fine wires in the meter.

 

[blkfrd]

Some additional info:

The ammeter is not going to be modified. It takes 2A to deflect it fully from center 0 in either direction depending on the direction of current flow. All 2 amps of current flows through the ammeter and there is only one path thru the ammeter. I'd say its 18 to 20 ga wire that goes from terminal A then many turns around a metal core and then to terminal B...thats it.

I've already been able to make this work in the lab with lab power supplies. It is possible to make it work with a 4 switch synchronous buck/boost converter that can both source and sink current, but the cost is an issue so i'm investigating simpler approaches.

 

[dr pepper]

Sounds like an automotive ammeter.
Its a little unusual to drive a meter with 2 amps, but I spose it'll work.
Have you thought about pwm from a constant voltage through a resistor.