Ammeter wiring - can't get my head around it.

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fredrussell

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My boat has an analogue ammeter on control panel. Its a small square job, 0 to 10 amps on scale if memory serves. It currently doesn't work and I would like it to.

The ammeter has a red wire and a black wire exiting rear. Currently the black wire goes to one of the poles of the main battery isolator switch, and the red wire goes straight to battery positive terminal.

Is this the correct way to wire such an ammeter? When I measure amps with my multimeter you put the meter 'between' power source and device being measured (in series?), or am I being dim?
 
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All the current passes through the meter, If your meter has been connected with one lead to positive and the other to negative it will have been wrecked
 
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You are not being dim and I have no idea what this instrument is doing wired that way. On all the boats that I have owned (that have had an electrical system) the main isolator switch is on the negative cable (which I always find counter-intuitive, but hey-ho.) Connecting battery positive via an ammeter to the main switch would lead to a heavy discharge through the meter, burning plastic smells and lots of excitement when the switch was closed.
 
Is the ammeter wires the only one going between the battery and the isolator switch?
What size are the wires?

If they are the only wires surely they in series with the load?

If they are not the and there is a another cable in parallel then the meter might be indicating the current by measuring the voltage drop across the cable with that cable acting as a "shunt". Not quite sure how you would calibrate it but it may be possible.
 
Bodach na mara said:
You are not being dim and I have no idea what this instrument is doing wired that way. On all the boats that I have owned (that have had an electrical system) the main isolator switch is on the negative cable (which I always find counter-intuitive, but hey-ho.) Connecting battery positive via an ammeter to the main switch would lead to a heavy discharge through the meter, burning plastic smells and lots of excitement when the switch was closed.
Click to expand...

But some ammeters were rated to cope with high currents allbeit probably via an internal high current shunt. I used a 40A one on the input to a mains board many years ago and I am sure it was in series with the live.
 
fredrussell said:
My boat has an analogue ammeter on control panel. Its a small square job, 0 to 10 amps on scale if memory serves. It currently doesn't work and I would like it to.

The ammeter has a red wire and a black wire exiting rear. Currently the black wire goes to one of the poles of the main battery isolator switch, and the red wire goes straight to battery positive terminal.

Is this the correct way to wire such an ammeter? When I measure amps with my multimeter you put the meter 'between' power source and device being measured (in series?), or am I being dim?
Click to expand...

Does the ammeter have a separate shunt or is the shunt inside the ammeter? If it has an external shunt, is the ammeter wired across the ends, i.e. in parallel, with it?

Richard
 
Bodach na mara said:
Only if there is a load as per Vic's diagram. Connecting the ammeter black wire to the positive busbar and the red to battery positive would do the job.
Click to expand...

The OP has a switch between the battery and the busbar, as do I. So what I wrote is correct. Obviously if there is no load the ammeter will not read anything.

The convention now is that ammeters are situated on the return to the negative, rather than the positive. I don't see why it should make any difference but guess it is a way not to miss anything.
 
Here's a pic of how it is at present:



The upper wire in diagram is a whopper. Lower wire, to and from ammeter to isolator switch is much smaller, 10A max I'd guess
 
fredrussell said:
Here's a pic of how it is at present:



The upper wire in diagram is a whopper. Lower wire, to and from ammeter to isolator switch is much smaller, 10A max I'd guess
Click to expand...

That aint going to work.... at least not until you open ( switch off ) the isolator then it will. Unless its kaput.

moreover wired like that means that the ammeter will be bypassing the isolator making that ineffective
 
fredrussell said:
Here's a pic of how it is at present:

Click to expand...

Well, that's silly.

When the switch is turned on, all the current will go via the top wire and the ammeter will measure nothing useful. When you turn the switch off, instead of isolating the supply it will instead power everything through the ammeter - which will then measure the draw if it's up to it, might catch fire if it isn't, and either way won't have the intended effect of turning everything off.

It was probably added by an idiot, lots of them about.

Pete
 
fredrussell said:
I didn't wire it, honest! Its on a new to me boat.

To my way of thinking, ammeter should be wired in series on upper (in my pic) main feed to isolator switch from battery. Correct?

The only thing is the wires coming from back of ammeter are way narrower gauge than main feed wire.
Click to expand...

Which suggests that it perhaps needs an external shunt because it won't handle the current .... in which case it, or a fuse inside it, is already blown.

Richard
 
RichardS said:
Which suggests that it perhaps needs an external shunt because it won't handle the current .... in which case it, or a fuse inside it, is already blown.

Richard
Click to expand...

But has it been doing anything up until now? All current from battery has been going via the larger gauge wire surely?
 
fredrussell said:
But has it been doing anything up until now? All current from battery has been going via the larger gauge wire surely?
Click to expand...

But when you close i.e. switch off, the isolator switch any current flowing would go through the ammeter. If that was more than the ammeter could handle then it, or its fuse, would blow. You could isolate it and test it with an multimeter on Ohms. You will probably find that it is open circuit.

Richard
 
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Not very helpful but I can't remember now which pole of the isolator switch the ammeter is connected to. I do know that with the isolator switch 'off' nothing electrical works on boat so if ammeter is currently connected to busbar pole on isolator switch nothing is getting through ammeter current wise.

I tried switching all lights on and needle moved a tiny fraction, so my hunch is that ammeter is working but only showing a tiny current as the bulk of current is travelling through larger main feed wire.
 
vyv_cox said:
The convention now is that ammeters are situated on the return to the negative, rather than the positive. I don't see why it should make any difference but guess it is a way not to miss anything.
Click to expand...

I think it's so that the wires from the shunt to the meter are at earth potential or just a few mV higher, reducing the chance of them doing nasty stuff in a short circuit to earth. Probably doesn't matter on a GRP boat, though.
 
fredrussell said:
I didn't wire it, honest! Its on a new to me boat.

To my way of thinking, ammeter should be wired in series on upper (in my pic) main feed to isolator switch from battery. Correct?

The only thing is the wires coming from back of ammeter are way narrower gauge than main feed wire.

Ammeter is of this type (but scale is 0 - 10 amps on mine): http://www.ebay.com/itm/0-1A-DC-Ammeter-Amp-Current-Panel-Meter-Analogue-Analog-NEW-/180669653687
Click to expand...

I'd simply remove it. It's probably ruined anyway. An ammeter isn't going to tell you anything useful, and a 10A ammeter is borderline (depending on what equipment you're powering).

Edit: While I was writing this, you posted that the ammeter may not actually be wired the way you said it was. I give up!
 
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