Ammeter Gauge Connections

Pretty simple, as vehicle wiring goes.

You want to splice it into the big thick alternator output cable.

BIG.

If you want the ammeter to read current flow in and out of the battery rather than just current flow into the battery then putting it into the altenator wirte wont do. The ammeter must be in the main feed from/to the battery so that all currents EXCEPT the starter motor flow thru it . steve the grease

It may not be a direct reading ammeter, some are designed to be used with a shunt (large heavy duty low impedence resistor) and if you attempt to use this type direct you will get a big bang and a space where the guage used to be!

Hmm ........ I thought that only heavy current usage devices would need heavy cable. A ammeter gauge will be measuring + or - amp output from the Genny, and whilst the ACTUAL amperage readings can/will be high (or not) the gauge itself will not be using heavy current ...........

A bit like my digital voltage/amps/Ohm meter, that has not got huge cables, but can still measure massive voltage and amperage currents.

Or am I talking total crap and must do as I am told??

Simple ammeters (rather than the shunt type RL Driver mentions) actually measure the current flowing through the meter. To measure the output from your alternator, you have to divert this output through the meter. So the meter wiring needs to be the same gauge as the alternator output wiring.

Unless there's something fairly odd about your digital multimeter, it will only measure 10 amps. Under these conditions it's expected that you won't be measuring for very long.

I wouldn't want to pass comment :-)

Bloody hell why is life not simple ....... it really could be !!

Well actually, my meter measures up to 20 amps, na na na na na ........ iand t cost £1,000,000

Nice pass QrizB

OK as usual, more info follows ........

The Ammeter is a CAV jobbie, with as far as MY GLASSES can tell it ranges from 100 amp charge/discharge ... and it is made in England :-)

It also says (bottle bottom glasses with double extentions) ' for use with shunt Z813/30 '.......... but don't quote me as the writing is very tiny and I am very very old .........

Still like to get it working though !!

ps

Digital VOM for sale £500,00.00 ono ....... maybe exchange for very thick cable ??

I am sure life really could be simpler ...........

Ok, in that case you need shunt number Z813/30 (you probably guessed that by now didnt you!!) to be able to use that guage.

There is no such thing as an ammeter, it is actually a voltmeter, with a large resistor(called a shunt) built in. This uses ohms law which states that in a curcuit the Voltage=current x resistance, therefore if you have a fixed resistance, as the current increases, so does the voltage. The shunt is the fixed resistance of a known value, the voltage (and hence the current) is measured accross the ends of the shunt. This is all very simple, until someone realised that if you moved the shunt out of the case of the 'ammeter' and put it close to the current being measured, that you would not need to have large wires running up to the meter itself, as the actual voltmeter only takes a tiny current to operate. This is all very well until (as has happened here) the shunt and the meter get separated, when unless you know the exact value of the shunt's resistance, your ammeter is useless.

Or in simple terms, unless you can find a matching shunt, bin the meter!

This inscription shows that it is definitely a shunt type that needs an external shunt. This may (possibly) be still on the vehicle somewhere near the alternator - or where extra batteries were. It will probably consist of two brass blocks joined by a brass strip, the whole lot mounted on an insulating base, and probably about two inches long. Each brass block will have a heavy terminal (for the alternator wire) and a small terminal (for the meter wire).

Note that for a direct reading ammeter the wiring needs to be heavier than the ordinary alternator wiring, as the distance to and from the instrument panel is usually much greater than from the alternator to the battery. This is why ammeters disappeared about the same time as alternators appeared. The difference from dynamos is not because of any inherent difference, but simply because a typical dynamo puts out 15 - 20 amps, whereas a typical alternator puts out 30 - 50 amps or more. JD

wanted to measure current flow in and out of the battery rather than the flow in you should put the ammeter in the battery supply lead rather than the altenator lead, after all whats the use of knowing what is flowing in if you dont know what is flowing out! usually you would want to measure the net flow .... engine off, lights on, net flow out of the battery . With the engine running you would normally expect to see a very small flow into the battery. Steve the grease

Science Lesson:

there are such things as ammeters!! In fact the majority of car gauges are in fact ammeters. They work by having a small loop of wire through which the current flows, this causes a magnetic field which deflects the needle. They have the advantage over the voltmeter method in that a shunt resistor is not needed with the associated voltage drop and power dissipation problems. However, in general they are not as accurate (fine for car use).

It is east to tell which type if you only have the meter, a voltmeter without the shunt will be high resistance across its terminals, a true ammeter will be very low. (you could be fooled if the shunt is internal, but this is not likely on a meter for measuring 50 or 100 amps.)

Regards Jeff

In message , Jeff writes

All moving coil meters are basically activated by the current flowing through them. To measure voltage a large value resistor is added in series with the coil. This reduces the current flowing through the meter and also reduces the electrical load on the voltage supply being measured thus avoiding a false reading. The choice of resistor calibrates the meter. To measure large currents a low resistance (called a shunt) is put in parallel across the moving coil of the meter - often externally to the case, so that most current passes through the shunt and not through the coil. Again the meter is calibrated by the choice of value of the shunt wrt the impedance of the coil. So by choosing the resistance and wiring in series or parallel you can make your basic instrument read almost anything.

HTH My O level physics is getting on a bit.

Good explanation. But many automotive ammeters are not moving coil, but rather moving magnet or moving iron. Of course, the same principal applies - they could still be used as a voltmeter, although current draw would be rather high. Most practical meters are basically current measuring devices, but not all (electrostatic voltmeter for example). Even a digital voltmeter/multimeter probably ultimately uses an FET as its measuring device, and this, although voltage operated, does draw some current, but not directly proportional to the voltage. Then there are bridges - but we won't go there. JD