Square Ammeter Question

[healeynut]

All -

I am planning to convert my Austin A90 to an alternator (already negative ground). Since alternators usually kick out 40+ amps, I am worried this will fry the ammeter on my car.

The original ammeter is a square one, so just buying another Smiths ammeter won't work for me.

I was wondering if fixing the ammeter is simply just a matter of replacing a resistor inside or something easy like that?

Any ideas out there?

Thanks

 

[Bayless]

I doubt that will work. An ammeter is just a coil with current passing through that generates a magnetic field to deflect the needle. Unless it uses some really modern electronics, all the current in the system must pass through that coil. Any resistance would have to limit the output of the alternator. So, inserting a resistance would likely defeat your purpose in converting to an alternator in the first place.

 

[healeynut]

So probably the best thing to do is to purchase a new round ammeter and swap out the guts?

 

[70herald]

Ammeters for anything more than the smallest of currents have a shunt resistor. The shunt resistor is wired across the meter which is just a voltage meter.
The shunt resistor is just a high precision, very low resistance resistor designed to handle high current. You need a LOWER resistance shunt resistor to get a larger full scale current reading.


Ohms law says: Volt=I (current ) *Resistance

Since you want the voltage range on the meter to stay the same, you need a lower value resistor.

 

[healeynut]

Hey thanks for that, so that means I should get a resistor with half the resistance if I want it to track to a generator/alternator that puts out twice the current, right?

 

[tony barnhill]

Just send it to Nissonger for a rebuild to match the alternator!

 

[Bayless]

That shunt resistor is good information. Thanks.

 

[healeynut]

Just send it to Nissonger for a rebuild to match the alternator!

Let's see, $100++ & shipping to Hong Kong - probably a three week wait.

Or I can fix on my table top in about an hour with some solder and a 50 cent resistor...

hmmmmm...

 

[70herald]

Hey thanks for that, so that means I should get a resistor with half the resistance if I want it to track to a generator/alternator that puts out twice the current, right?

The simple answer is yes, but.... do what Tony said. You will need a precision resistor which can handle 40+ amps. It will be much cheaper to have them do the job correctly the first time than getting them to repair the meter if the resistor fries and tries to run the current through the meter instead of the shunt. Of course they may be willing to sell you the correct resistor.

 

[Sarastro]

The "resistor" used as a shunt for a high-current ammeter is usually just a slug of copper. You need a very low-value resistor, and it's hard to make sure you've got the right resistance. In any case, if your meter is within a factor of two of the alternator output, probably you won't hurt it. The alternator is capable of high current, but usually it balances the demand so that the battery current (which is what the ammeter measures, or SHOULD measure) stays close to zero. Mine kicks up to a few amps for a minute or so after I start the car, as it's recharging the battery, and then it settles very close to zero almost all the time. Might flick a little when I turn on the lights, but that's all. Modern alternators have really good regulators.

My ammeter is 0-20 amps, and I haven't pinned it yet. My alternator is 55A.

 

[healeynut]

Ok, that's good to know. I suppose if I go with a 40 - 45 amp alternator, my 30 amp range should be fine.

the only funny thing is since I switched polarity, the guage registers backwards, but I figured that really doesn't matter.

 

[Roger]

Ok, that's good to know. I suppose if I go with a 40 - 45 amp alternator, my 30 amp range should be fine.

the only funny thing is since I switched polarity, the guage registers backwards, but I figured that really doesn't matter.

Just reverse the connections on the ammeter, and I'll bet it's tough enough for the very rare over-current.