Ammeter driver.....

P

PeterZRH

Well-Known Member
Here's an esoteric question if ever there was one - has anyone thought of driving the ammeter with a closed loop current detector? For the non-electronically minded this is what a current measuring clamp meter uses i.e. you stick the battery cable through it and it can measure current to/from the battery non-invasively. I say this because when I fitted the round instruments to my car I deliberately chose NOT to fit the shunt as it looked like an open invitation to cause a fire with the entire output of the battery or alternator going through here. I reckon all that's needed is an appropriate module, a DC regulator/converter, likely an op-amp as driver for the meter and a few resistors (likely variable for calibration/zero point setting) for about £30 with no cable chopping required. Anyone interested in such a thing if I ever find time to prototype it?

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Closed Loop AC/DC Hall Current Sensor CYHCS-D5S-100A, Output: 50mA-100mA, Power Supply: ±12V ~ ±18V, Window: 20mm
 
So would I.

I've just had BOP's alternator changed to a more modern & efficient one (not by me - I'm not confident at all with the high current side of things). The downside is that the ammeter no longer works as apparently there isn't a suitable connection....:(
 
jp928 said:
I would just as soon replace it with a voltmeter.
Click to expand...

Indeed but unless you can get a semi-circular gubbins to fit then it'll involve hackery on the rather lovely P6 dashboard pod. The gauge isn't even suitable for conversion as it is center zero and obviously not marked for voltage. It could in principle be driven with a few resistors to read voltage but volt meters tend not to be linear in nature because only the range 10-15volts is interesting, not 0-15v. Perhaps some zener diode jiggery-pokery might fix that.

The idea being if something is in my car, I want it to work - I don't like looking at something "broken".

Potentially offering it as a "plug and plug" safer replacement for the shunt - particularly if like me you have uprated charging circuits or alternators. It could even be cheaper than the original part.
 
What exactly does the shunt do? I've noticed in my 2000 TC S2 that the ammeter needle doesn't show a charge, even when the charge must be considerable, but it does show a discharge correctly. Is that something to do with the shunt or is something wrong? Charging system seems to work fine in that the battery is always full of energy. I recently replaced the alternator (bearings noisy and brushes worn out in the old one) but the odd ammeter behaviour was unchanged.
 
If you switch the car off and turn on the lights you will show a draw. If not, you either don't have a shunt and all the brown wires coverage at a common terminal or the ammeter is broken. The change of alternator won't change this behaviour.

The shunt is a very, very low value, high power resistor. When current flows in either direction there is either a tiny positive or negative voltage across it which is what the ammeter shows. Now this only works because the ENTIRE electrical system bar I think the starter runs through it. Now what's bad about that then causes heating in the resistor which increases with the square of the current so an 80 amp draw is 6400 times the power dissipation of a 1 amp draw as well as introducing a voltage drop in the circuit and a single point of failure if they burn out. You could call it in the extreme a fuse for the entire battery.... The ammeter is also "live" and unfused all the time.
 
Phil Robson said:
So would I.

I've just had BOP's alternator changed to a more modern & efficient one (not by me - I'm not confident at all with the high current side of things). The downside is that the ammeter no longer works as apparently there isn't a suitable connection....:(
Click to expand...

Sounds like the alternator is wired directly to the battery. The rest of the electrical system will still hang off the other side of the shunt unless the installer changed the shunt for a common terminal.
 
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