how to wire an ammeter?

[cryptokid]

hey guys, i bought a guage kit today, naturally its for american cars so the wiring is all different.
the way it looks to me is that i take the main positive cable from the alternator to the battery and cut it in half. then i wire each end to the appropriate terminal on the ammeter.
is this correct?
and if it is correct, this means i need to wire a pair of positive cables all the way up to the dash?

 

[TheLoneRanger]

Crypto, aftermarket ammeters are wired in as you describe. Sometimes they connect between the positive battery cable and all of the accessory wires on the starter solenoid. You don't need to use cables to the ammeter, but you do need to use very heavy wire, because all of the car's electrical power goes through the ammeter.

Because of this, I am not a fan of ammeters. They constitute a fire hazard, because of the high current draw through them, and the fact you always have a live 12v wire inside the passenger compartment (even with the key off).

I much prefer voltmeters. They are easier to install, and give a better indication of charging system and battery condition. And they are much safer.

 

[Shannow]

Yep, there's been quite a few cars (usually early british types) burned out by faulty ammeters.

 

[MolaKule]

I prefer voltmeters as well. You can have a ammeter system by having all the current go through a high power, low resistance value resistor, a shunt resistor.

The voltage "V" across a resisor is equal to the current "i" going through the resistor times the value of resistance (V = iR), or i = V/R.

For example, you would feed the red wire from the alternator through a high power resistor (mounted to the firewall) and the other side of the resistor would go to the car's electrical circuit.

You would then meaure the voltage drop across the resistor by a voltmeter.

For example, if the voltage measured across a 0.05 ohm resistor was 2.5 V, then a current of 50 Amps was flowing through it.

What this shows is that a high power resistor of 0.005 or less ohms is needed to measure current, so that the resistor doesn't drop too much voltage across it. With a 0.005 ohm resistor, you would measure 0.25 Volts across it with a current of 50 Amps.

Let's further assume that you need to measure up to 150 Amps.

The power dissipated in the resistor would be:
P = i^2*R = 2,500*0.005 = 12.5 Watts.

So a resistor of 0.005 ohms with a power rating of 20 Watts would be necessary to measure currents in a car's system.

Edit: Some amp measuring systems or kits come with a shunt resistor and a voltmeter calibrated in Amps. So on the firewall yould have four wires coming from the shunt resistor. A heavy 10 or 12 guage fuse wire going to the shunt resistor, another heavy wire spliced to the cars electrical system, and two 18 guage or less wires from across the resistor going to the voltmeter, again calibrated in Amps.

[ March 29, 2003, 05:46 PM: Message edited by: MolaKule ]

 

[Ken2]

Some ammeters I've seen are only good for a maximum of 60 amp alternator and require #10 wire. If your alternator is rated higher than the rating of that ammeter, send it back and use a voltmeter.

The voltmeter directly measures the result of charging-system performance. With normal alternator/voltage-regulator function, battery voltage is maintained at 14.0 to 14.5 volts-and this is reported directly by the voltmeter. In the event of alternator-system failure, voltage will be low and continue to drop as the battery discharges. In the event of an "overcharge" condition, the voltmeter will climb above its normal zone.


Ken

[ March 29, 2003, 05:43 PM: Message edited by: Ken2 ]

 

[cryptokid]

i guess i could use a volt meter. but i already have the ampmeter, and am installing it like theguy said, using a shunt. this eliminates any risk or having heavyguage wires running in the passenger area, etc..

thnaks guys.