Analog ammeter (how to use it).

[dgeneral02]

I have an analog ammeter with which I want to measure amps from a dc motor into a 12v battery. The ammeter has a circuit board behind it with two resistors and four diodes (1N4007) soldered on it. How do I use this meter.

[Flux]

Probably with difficulty unless you can give more details of the basic meter movement.

The rectifier implies that it has been used for ac measurement or reversing dc. The diodes are nominally 1A. I suspect it is a milliammeter and has been adapted to measure volts or milliamps.

Unless you can determine the basic movement I can do little to help. Have a look on the scale or label on the back and see if you can find the basic movement details ( micro amps, milliamps or similar and ideally the movement resistance). Failing that measure the resistors and determined whether they are connecetd in series with the movement, across it or a combination of both. Unless you understand meters it may be more trouble than it is worth.

Flux

[dgeneral02]

The meter is a moving coil ammeter. It has a scale of 0-50 amps. I also drew a schematic of the circuit behind the ammeter.

[Flux]

OK  to use it on dc you don't need any of the circuit.

The scale doesn't show us the movement fsd so you will need to measure it or calibrate by trial and error.

You need a shunt suitable for the amps you expect out of your motor and with a millivolt drop to suit the meter.

Not sure what facilities you have but ideally you need to supply the movement from a small battery ( 1.5 to 3 v will do). You need a variable resistance in series to limit the current and you decrease the resistance until the meter reads fsd. With a digital millivoltmeter you measure the volt drop across the meter terminals ( FSD) . If it comes out to something in the region of 50 to 100mv you can use a standard shunt. If it is an irrational value then you can add some series resistance to get the mv drop to something rational. You then regard the meter movement as one of the coil connections and the other as the end of the series resistance remote from the other coil wire.

If you want to keep it at 50A fsd then use the existing scale, there may be a bit of non linearity from the diodes but it will be near enough. Otherwise you will need to re-scale it or use a multiplying factor.

If you want a simpler solution and have another meter to compare it with just shunt the meter movement with a thick piece of copper wire and adjust its length until it agrees with your standard meter, that way you can't use commercial shunts but the actual fsd doesn't really matter.

Flux

[xxzxxz]

See the Flux post.
Analog ammeter - not problem.
For me i`ts better choice, but you must calibrate it.

[tecker]

That's an AC meter in the 300 to 500 mil range .You have to calculate the circuit . You would definitely be hit or miss on the shunt . I think you use these with a current transformer . Find a CT in that range and you would be good in for AC .DC I don't know.

[dgeneral02]

Ok thanks fo your replies. I have decided to use two other moving iron ammeters i found but they give different readings when i use them to measure amps from my solar panel. Moving iron ammeters IMO are not reliable.

[Flux]

High quality moving iron meters are fairly good for ac, on dc they tend to suffer from residual magnetism, if you take a reading, reverse the leads and read again you may find a difference, if so the average will be close to correct.

Moving iron meters at the low cost end of the range can be pretty poor and it depends on what they were intended for.

Moving iron measures rms, so it may not agree with a mean reading meter on dc if there is ripple, there should be no ripple with solar unless you have a pwm charge controller operating.

The other catch is that many cheap moving iron meters are intended for battery chargers and they are scaled to the strange form factor of the peaky charging waveform.These things when used on smooth dc can be in error by up to 8% .

Flux

[tanner0441]

Hi

Tecker Current transformers only work on AC. Clip on meters to read DC normally have a hall effect device to measure the magnetic field round the cable. Moving iron meters are not known for their accuracy or ability to read low values, but if all you need is an indication of charge then it might do the job.

Brian.

[dgeneral02]

You are on point Brian. I have observed over time how one of the meters rated 0-150 amps reacts at different levels of insolation. The maximum i have seen it go is slightly over 75 amps which i know is far from reality. However, the meter gives me a rough idea of how much power the solar panel could be putting out.The panel heats up in a very short time on sunny days so with the meter in place i can see how  efficiency drops gradually.

[tanner0441]

Hi

I would still go for the moving coil meter, if you don't have access to the correct shunt you could try a short length of thin steel strip.  you will have to calibrate it, start with things you do have access to, car headlamp bulbs are a good start. You will not have precision but it will repeat fairly accurately which your moving iron meters will not. Your ammeter is actually measuring the voltage across a resistor which will be proportional to the current flowing through it.

I built a home plating set up many years ago just after leaving school and used steel strip as resistors to control the current, we didn't have transistors that could handle those sort of currents, well none a young teenager could afford.

Good luck with it.

Brian