fredrussell
Well-Known Member
Apols! Can't for the life of me remember which pole of isolator switch its wired to - either way its wrong. Boat comes out tomorrow, will have another look then.
Ammeter wiring - can't get my head around it.
- Thread starter fredrussell
- Start date
Apols! Can't for the life of me remember which pole of isolator switch its wired to - either way its wrong. Boat comes out tomorrow, will have another look then.
BabaYaga
Well-Known Member
This might well be correct, I think. Calibration is made when deciding the length of the thick wire.
davidpbo
Well-Known Member
It would be worth getting the meter off. There may be details on the back, a shunt ?, how much current it can carry or the voltage required for Full Scale Deflection, in which case value of the shunt can be calculated which may be similar to the resistance of the thick wire. If that wire is the shunt then the black wire should go to the same terminal on the isolator switch.
So if FSD needs 1 Volt and FSD on the meter is labelled as 10 Amps then the cable resistance needs to be:
from V=IR where V is voltage, R is resistance and I is current (Volts,, Ohms, Amps) , R=V/I
= 1/10 = 0.1 ohms. Difficult to measure accurately without some special gear. (I think but might be wrong).
Out of interest what is the make of the meter? Is the legend on it printed or hand written?
So if FSD needs 1 Volt and FSD on the meter is labelled as 10 Amps then the cable resistance needs to be:
from V=IR where V is voltage, R is resistance and I is current (Volts,, Ohms, Amps) , R=V/I
= 1/10 = 0.1 ohms. Difficult to measure accurately without some special gear. (I think but might be wrong).
Out of interest what is the make of the meter? Is the legend on it printed or hand written?
VicS
Well-Known Member
It is more than likely a direct reading , moving iron meter that does not use a shunt or a perhaps a moving coil meter with an integral shunt. ITYWF that the former will have a non linear scale
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fredrussell
Well-Known Member
Got me there David, on boat tomorrow - will report back.
superheat6k
Well-Known Member
An ammeter without a shunt is limited to ~ 10 amps, unless very heavily constructed. With this all the power passes through the meter.
If you use a shunt, the meter is actually a volt meter measuring the volts drop across the shunt which is directly proportional to the current flow, so the volt reading is representative of the current flow (amps). Shunts are far better for fluctuating DC loads with a range up to ~ 100 amps easily handled, and some will even put up with the occasional load of a starter motor, although this is likely better routed away before the shunt circuit.
This is for instance how Nasa measure current with their BM battery monitor unit.
If you use a shunt, the meter is actually a volt meter measuring the volts drop across the shunt which is directly proportional to the current flow, so the volt reading is representative of the current flow (amps). Shunts are far better for fluctuating DC loads with a range up to ~ 100 amps easily handled, and some will even put up with the occasional load of a starter motor, although this is likely better routed away before the shunt circuit.
This is for instance how Nasa measure current with their BM battery monitor unit.
JumbleDuck
Well-Known Member
It's not quite that simple, because you also have to take account of the current going through the meter. For an electronic meter that's so low that it can be discounted; for a mechanical meter it can matter.
Yes, moving iron meters (which means, for the rest of you, that current flows through a solenoid and pulls a piece of soft iron in against a spring to drive the needle) are generally non-linear, although sometimes the iron is shaped cunningly to give a quasi-linear effect over a subsection of the range. The great advantage is that as far as the mechanism is concerned, fsd is infinite so you don't have the danger of damaging the needle with surges of current.
Although you do sometimes see mechanical ammeters without shunts at that sort of current, it's very uncommon. Most moving coil meters are 1mA or 10mA fsd and then shunted or ballasted as appropriate to turn them into ammeters or voltmeters.
An electronic (analogue to digital converter) meter is a voltmeter and stays a voltmeter even when shunted. A mechanical meter is always[1] an ammeter, however used.
[1] I do have a mechanical electrostatic voltmeter in my collection. It is two feet high, weights 50lb and measures up to 100kV. It's rather rare.
William_H
Well-Known Member
If the Amp meter in the link looks like the OPs amp meter then most likely is a moving coil meter of probably around 1 ma full scale deflection and some hundreds of ohms resistance. First thing for the OP to do is remove the wires from the meter and check the resistance terminal to terminal with an ohm meter. (digital volt meter) There should be a little deflection of the meter. (try reversing the ohm meter leads). If the resistance is very low then it has a built in the case shunt or is a moving iron type meter. If it is a low resistance then it is designed to be wired into the circuit with all current passing through the meter. If it is high resistance then clearly it is intended to have a shunt.
If the meter seems to have some movement it may be wired OK but have dirt or iron fillings caught in the movement or the needle has bent to restrict movement. You can open up the case and gently move the needle to confirm it is free. In doing so you may discover exactly what you have. A coil of fine wire in a square shape moving with the needle is moving coil. It may have a piece of wire (resistance wire) wired directly from inside each temrinal to the other. (a 10A shunt) A big hulking great heavy wire coil fixed with a lump of metal moving in that coil is the moving iron type
Yes as said the main connection wire can be a shunt. But as said the calibration and actual range of the meter will depend on the gauge and length of that wire. More suitable for measuring hundreds of amps. So not likely or practical. Certainly no good if the switch is also in the circuit.
You could buy a shunt but then it needs to match the meter movement so all a bit tricky.
OP needs to check again the wiring and also make sure there is not a shunt in situ somewhere. A shunt looks a bit like a terminal block. Perhaps 50 to 80 mm long with heavy screw terminals one each end and smaller screw terminals which seem to be at the same place as the big ones.
The balck and the red wires on the meter are a bit misleading. They should both be in the positive (or as said sometimes negative) main wires. So red wire is more positive than the black but only by millivolts. Swapping the wires will reverse the pointer movement. good luck olewill
If the meter seems to have some movement it may be wired OK but have dirt or iron fillings caught in the movement or the needle has bent to restrict movement. You can open up the case and gently move the needle to confirm it is free. In doing so you may discover exactly what you have. A coil of fine wire in a square shape moving with the needle is moving coil. It may have a piece of wire (resistance wire) wired directly from inside each temrinal to the other. (a 10A shunt) A big hulking great heavy wire coil fixed with a lump of metal moving in that coil is the moving iron type
Yes as said the main connection wire can be a shunt. But as said the calibration and actual range of the meter will depend on the gauge and length of that wire. More suitable for measuring hundreds of amps. So not likely or practical. Certainly no good if the switch is also in the circuit.
You could buy a shunt but then it needs to match the meter movement so all a bit tricky.
OP needs to check again the wiring and also make sure there is not a shunt in situ somewhere. A shunt looks a bit like a terminal block. Perhaps 50 to 80 mm long with heavy screw terminals one each end and smaller screw terminals which seem to be at the same place as the big ones.
The balck and the red wires on the meter are a bit misleading. They should both be in the positive (or as said sometimes negative) main wires. So red wire is more positive than the black but only by millivolts. Swapping the wires will reverse the pointer movement. good luck olewill
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JohnGC
Well-Known Member
Also, with the shunt just a few mV above 0V; the voltage amplifier used to apply gain to the potential difference across the shunt, only needs a small common-mode range. That used to mean less complex circuits and cheaper components than where required when measuring on the positive side. In recent years, the cost of current shunt amplifiers with large common-mode ranges has fallen drastically. Hall effect parts have also become viable.
Common-mode.
The potential difference across the shunt is amplified by a circuit with a positive and a negative input.
The circuit will usually operate correctly when both inputs are close to 0V.
The circuit will also operate correctly as the voltage at both inputs moves away from 0V. This is common-mode voltage.
But for cheap circuits (basic op-amps for example) the common-mode range too small for this application.
Moving iron meters also measure RMS rather than average values.
JumbleDuck
Well-Known Member
JohnGC
Well-Known Member
lw395
Well-Known Member
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
The only reason to wire it like that would be to measure a charging current when the isolator is off.
Or to measure small loads, the isolator is switched on for the big load, i.e. starting.
It's a 0-10 A meter not a 10-0-10?
It's not the least logical bit of boat wiring I've ever seen, but I don't approve!
JumbleDuck
Well-Known Member
Just me lagging behind - never was much good at history
Well, that's me phased.
JohnGC
Well-Known Member
So you're a rotating vector now?
Dave_Seager
Well-Known Member
An ammeter would normally be connected to all current flowing to or from the battery except for the starter motor. The separate feed to the starter may explain the confusing wiring.
