Converting an Ammeter

[enjenjo]

Do we have an electronics expert on here? I want to convert an ammeter to a voltmeter. An Ammeter is connected in series with the load, and a Voltmeter is connected in parallel. Both are galvanometers with a similar movement.

In the case of an Ammeter they are wired in parallel with a resistance (shunt) so only a portion of the load flows through the meter itself, the rest goes through the resistor.

In the case of a voltmeter it is wired in series with the load (the resistance), so it reads only voltage to ground, not the load.

So in my case the Ammeter is calibrated to read  40 amps charge or discharge. I have yet to determine the shunt resistance. So my thought is if I can figure out the proper resistance of a resistor wired in series with the meter, I should be able to calibrate it to read 12 volts at the center (zero) and show a charge over zero when being charged, and under zero when discharging.

Am I barking up the wrong tree? It sure would solve some problems.

[moose]

I cannot answer your question about conversion but are you sure about the connections? Most Ammeters I have used and seen were in series to measure the flow in a circuit unless they were induction style, while the volt meters were all parallel.

moose

[kb426]

I am of no help. There is a person who replaces the ammeters with volt meters on Ford pickups so I assumed it wasn't practical to do. Because there is such a large collection of older vehicles in this category, I would have thought it to be done if possible.

[crdnblu]

The October 2020 Hot Rod Magazine print issue, page 70, has an answer for you; though it appears to be a bit more involved than one would expect.  Redline Gauge Works is the company cited; the cost approaches $300 for a converted gauge.

[enjenjo]

I cannot answer your question about conversion but are you sure about the connections? Most Ammeters I have used and seen were in series to measure the flow in a circuit unless they were induction style, while the volt meters were all parallel.

moose

I stated that in the first paragraph. But in most cases the Ammeter is also wired in parallel with a Shunt (resistor) so the whole load does not go through the meter allowing the used of a smaller, and cheaper, meter.

The second alternative I have is using an external shunt to reduce the current through the Ammeter to reduce the deflection by 2/3 so the meter is not damaged by excessive Amperage. But this still requires bringing a potential 100 amps of current into the cab.

A third alternative is bypassing the Ammeter completely and adding a alternator light to monitor the system, but that gives you no clue to the state of charge.

[PeterR]

I want to convert an ammeter to a voltmeter.  I should be able to calibrate it to read 12 volts at the center (zero) and show a charge over zero when being charged, and under zero when discharging.

Am I barking up the wrong tree? It sure would solve some problems.

Well, it can be done, but there is a lot of fiddling.   I have been cheeky enough to slightly reword some of your text to make it a little easier to follow.

In the case of an ammeter the movement is wired in parallel with a resistance (shunt) so only a portion of the load current flows through the meter itself, the rest goes through the resistor.
Ammeter splits the current
Very small portion through the movement (about .0001A)
Most through the parallel shunt

In the case of a voltmeter the movement is wired in series with a resistance, so only a portion of the voltage is across the meter itself, the rest goes across the resistor.
Voltmeter splits the volts
Very small portion across the movement (about .01V), most across the resistor

The first step is to remove the shunt.

If it is the type with a remote shunt as used on some Dodge trucks, then that part of the job is easy.  If it is the integrated type, the shunt may be on the outside of the case across the terminal posts, -not too difficult to remove.  If it is the integrated type with the shunt inside the case, then the case has to be opened.

OK the shunt has been removed, now to convert it to a voltmeter and the fun really begins.  

As a voltmeter, you want the needle to be at the max discharge end of the scale when no activity, and mid point at 12V. But on the donor ammeter the needle sits in the middle when no activity.

There is a way to build a circuit that would force the movement to be at the max discharge end of the scale for say 9V, in the middle for 12V, and at the max charge end of the scale for 15V.  Not too difficult but quite a bit work for a one-off.

The other option is to remove the glass front, pull off the needle and reset it.   Initial instinct is to reset the needle to the max discharge end of the scale when no volts applied and tweak the series resistor so the needle is at mid position for 12V and continues into the charge region when the battery is above 12V.

But remember the movement originally went from zero to max over half of the full scale range. If it is made to go from zero to max over full scale, the movement is being forced through double the designed travel, -- some movements will cope with this, others will not.

So the safer option is to reset the needle to the half discharge reading at no volts then tweak the resistor to give mid point at 12V.  The disadvantage of this is that the sensitivity of the meter is quite low and I doubt it would be of much use.

[PeterR]

The second alternative I have is using an external shunt to reduce the current through the Ammeter to reduce the deflection by 2/3 so the meter is not damaged by excessive Amperage. But this still requires bringing a potential 100 amps of current into the cab.

If you can remove the existing shunt, you can install a new shunt of 1/3 the resistance in the engine bay and just bring in a pair of thin wires to the meter. This avoids running high current cables into the cab.  As additional security you can fit an inline fuse in both meter wires close to the ballast.

[enjenjo]

That last one sounds like the best solution. I didn't get a chance to look at it today, but I will report back when I get further into it. Right now I am working on the power steering.

[Charlie Chops 1940]

I had a Mustang II cluster in my Chevy for a number of years. That gauge said volts on on the face, so I wired it as a volt meter and it burned up. I got another one it did the same so I got a diagram and did it the Ford way and it was fine. I thought it was really an ammeter....

[Pete]

So in my case the Ammeter is calibrated to read  40 amps charge or discharge. I have yet to determine the shunt resistance. So my thought is if I can figure out the proper resistance of a resistor wired in series with the meter, I should be able to calibrate it to read 12 volts at the center (zero) and show a charge over zero when being charged, and under zero when discharging.

Am I barking up the wrong tree? It sure would solve some problems.

Assuming the meter is a D'arsonval movement type, if you remove the shunt and install a resistor of the proper value to get the resolution you want, the meter will read voltage, BUT, only in one direction depending on wire polarity. Reversing the wires will change the reading direction. Without some method to CHANGE POLARITY of the circuit, the meter will always read the same direction.

[enjenjo]

Today I got an ohmmeter that would read milliohms, and come up with .002 milliohms for about 10 seconds, then it dropped to zero. So I am thinking there is no Shunt in it, and was probably in the original wiring to the meter.

[Pete]

Today I got an ohmmeter that would read milliohms, and come up with .002 milliohms for about 10 seconds, then it dropped to zero. So I am thinking there is no Shunt in it, and was probably in the original wiring to the meter.

Some meters have an internal fuse or in lieu of that the coil most likely burned out from too much current from the test meter.
Quality D'arsonval type meters with no multipliers or shunt will sometimes only take 50 microamps for full scale reading.

[PeterR]

Today I got an ohmmeter that would read milliohms, and come up with .002 milliohms for about 10 seconds, then it dropped to zero. So I am thinking there is no Shunt in it, and was probably in the original wiring to the meter.

Apologise for not responding earlier, it has been a crazy last few months.

Although a resistance test would have seemed harmless enough, real care has to be exercised with ammeters out of circuit and I am afraid if it was a remote shunt type, the ammeter may now be defunct.

The coil in the meter movement of an ammeter requires an extremely small current to drive it to full scale.  

Multimeters push a current through the item under test while making a resistance reading.

When applied across the terminals of an ammeter with internal shunt, the current generated by the multimeter passes harmlessly through the shunt and the ammeter needle will not even blink.  But when applied directly across the terminals of an ammeter intended for a remote shunt, the injected current goes straight through the coil and may be many times the maximum current the coil is designed to carry; the ammeter needle swings to full scale and after a very short time the coil burns out.

If an automobile ammeter does not have a heavy copper strip running across the back of the case between the terminals it is likely intended to have remote shunt and must be handled with care until fully wired up.

However before being a complete predictor of gloom, something does seem a little odd with your readings.
When you say the reading was 0.002 milliohms, was that a typo, should that be .002ohms.  And,  it dropped to zero, do you mean the meter showed 0.00 milliohm, or it showed no reading at all indicating an open circuit.

When you ran the test could you see the meter dial or was it lying face down on the bench.  It would be good to know if the needle jumped at all during the test.

[enjenjo]

It was a typo. The reading was .002 ohms. and it dropped to 0.0 ohms.  It would do this repeatedly so I don't think the coil burnt out.

I solved the problem by installing an idiot light and leaving the ammeter out of the circuit. I also installed a 60 amp maxi-fuse in the alternator power wire so if there is a short it will disconnect it, and turn on the light.