Ammeter question.

[Gordonc]

Hi all,

I have a 3 amp AC ammeter which I would like to use alongside an AC 0-250V voltmeter, however, when I inserted the ammeter in series with the rest of the circuit the indicator needle flew across to the meters maximum reading, hard over to the right of the dial. The maximum load the circuit is drawing should read less than 1 amp at around 210-230 volts.

The ammeter is Class 2.5 and has TEST 1000V written on the meter face.

Do I need to insert a shunt, diode bridge or resistor to get the ammeter to operate correctly?

Gordon.

[Nickthedentist]

It does sound like it's got its own rectifier but no shunt.

Try shunting it with a very low resistance initially (e.g. a length of 5A fuse wire or similar) and gradually replace it with higher resistances until you get the required behaviour.

N.

[Doug_h]

I imagine that a 3-amp meter would have fairly hefty screw terminals on the back. Some of these meters, as sold, had the necessary shunt resistor across these terminals - is it possible that your meter once had this but it's been lost ?

[David G4EBT]

It will almost certainly have had an external shunt across the meter terminals.

If you look closely at the bottom of the meter dial tucked away from view, you may see the actual sensitivity of the meter movement stated in small writing, or it might be stated on the back n a little label. You'd then need to make a shunt to pass most of the current. If for example the basic movement is 100mA, to read 3 Amps FSD, 2.9 Amps would need to flow through the shunt.

You may have to experiment in making a shunt from a length of say 22SWG or thereabouts copper wire, with a digital mutlimeter in series with your analogue meter so that you get the same current reading on each meter to show that the shunt has the right resistance for the meter to read 3A FSD. You can then form the shunt wire into a coil and fix it between the meter terminals. The thinner gauge the wire, the shorter will be the length needed, but it will have to be capable of passing 3 Amps without heating up unduly.

Hope that helps a bit.

[kalee20]

I must say, for any unknown ammeter ,I'd do a sanity check first by winding up the power from zero. It's quite usual for shunts to be external, and it would be a shame to bend the pointer or even burn out the coil, by pushing a few amps through a 1mA movement.

As this is an AC ammeter, it could be designed to be used with a current transformer rather than a shunt - which would vastly reduce the effects of non-linearities of any internal rectifiers.

[pmmunro]

I agree with Kalee 20 about your ammeter probably needing a current transformer.

It would be useful to know the type of movement which should be shown by words or symbols on the scale plate. The scale shape will be significant too and it may be possible to see the core of the movement, depending on the design.

If it is designed to be used with a current transformer, you need to be aware that these should never be run with primary current flowing and the secondary open circuit. This could lead to a dangerously high secondary voltage. If no load is connected, the secondary of a current transformer should always be short-circuited.

PMM

[Ed_Dinning]

Hi Gordon, bring it over and I'll check it to see what it actually is and how it needs to be modified to read as you require it.
As the other posters have said, it is quite normal for these instruments to have an external shunt.

Ed

[Gordonc]

Hi All,

I have attached some photos of the meter in question. I'm afraid it doesn't show much. This meter was bought from Hong Kong via eBay. Some of the meters did say that they should be operated in conjunction with a CT, however this particular one did not, so I assumed (no doubt naively) that it could be used as a direct connection.

Regards

Gordon.

[Herald1360]

With a scale like that it could be moving iron!

A careful poke at it with a DMM and a power supply might be instructive- does it seem to be purely resistive or is there a kink in the volts across the meter vs current through it curve?

[pmmunro]

If the scaleplate is to be believed, it is a moving iron instrument as indicated by the standard symbol for this type of movement. (Sumarised here - http://www.iskraft.is/Uploads/docume...lar%202009.pdf) .

The non-linear scale also indicates that it is a moving iron movement. This type is "universal" , I.e. It will read AC or DC without modification.

Connect it in series with a fixed resistor of 10k ohms and a similar resistance pot. and apply 0 - 12volts from a variable supply and see what deflection it gives. If the deflection is minimal, reduce the resistor to 1k and retest. This will tell you if a shunt or CT is needed to indicate 3A fsd.

PMM

[Gordonc]

Hi all,

Herald1360 and pmmunro are correct. My thanks go to Ed Dinning, who confirmed that the ammeter is indeed a moving iron type. Ed also proved that the meter is operating as it should on both AC and DC by applying a 1amp load which the ammeter displayed correctly.

The initial reason for the needle moving hard over to the right was down to my error in connection, however when I then realised this and correctly wired it, I had not realised that the total current draw of the radio I was testing was only 46.5mA and that this would barely register on the scale of a 3 amp ammeter.

One lives and one learns. Many thanks to all respondents and my apologies if I have wasted your time.

Moderators - you can now close this thread.

Gordon.