Gentlemen, I have just started to plan the restoration of my recently acquired 1962 TR4 and,one of the things I want to do is to fit a 45amp alternator to replace the dynamo. The car's ammeter is a 30-0-30 and I want to change this to a 50-0-50. I understand that this can be achieved by fitting a resistor between the two pins of the ammeter. I have no experience of electronics so I need the help of an electronics expert to tell me what size(?) of resistor I need to fit to extend the range from 30 to 50 amps. I shall not attempt to change the face of the ammeter to show 50 amp, Will someone please help me? Thanks Graham

It's easier to work out for a target 60-0-60 so I'll start there.

Most analogue gauge ammeters and voltmeters are the same basic device - a moving coil meter with a range of much less than an Amp or a Volt. The ones in proper multimeters or bench PSUs (or even HiFi amplifiers sometimes) are typically 250uA but those fitted in cars are probably a bit chunkier. Nonetheless, the gauge itself doesn't measure what you want, so the manufacturer adds a resistor. For a voltmeter, this would be a fairly high resistance in series, so that 12V produces 200uA or whatever is appropriate. For an ammeter, it's a low resistance in parallel, a.k.a. a "shunt" resistor. This passes most of the 30A through the resistor with only the 250uA or whatever passing through the meter.

To calculate the deisred balance, we apply Ohm's law, V=IR. In our ammeter, the voltage across the meter is the same as the voltage across the shunt. We don't need to know what that voltage is but we can calculate it from the current and the resistance. Hence:

Voltage = meter_current * meter_resistance = shunt_current * shunt_resistance

A bit of re-arrangement gives us: shunt_resistance = meter_resistance * meter_current / shunt_current

So if our meter measures 1mA and we want to measure 30A, we get: shunt_resistance = meter_resistance / 30000

Now, this has already been done for the base meter, of course, but the same maths works for adding a second shunt except for one problem. I've made approximations. Specifically, I've assumed you're measuring the shunt current. Actually, you want to measure the total current, but with the meter above, those are pretty close to the same thing. To get the answer for a meter with very similar resistance to the shunt, the maths gets messy, unless we adapt our approach.

We want to turn a 30A meter into a 60A meter. We thus want the same voltage at 60A as we would have had at 30A, or equivalently, twice the current for the same voltage. Thus we need the new shunt to conduct the same current as the meter, which means the same resistance.

So, get a very good resistance meter, measure the resistance of the ammeter, and fit a bypass resistor of the same value. It needs to be a fairly high wattage one as it will be carrying a lot of current. Unfortunately, it's going to be a very low resistance, so your average multimeter probably won't measure it right.

You could have a go with a bit of trial and error - arrange, say, a 10A current to flow then fit different resistors until you hit on one that moves the needle where you want it. If the needle doesn't move enough, try a lower resistance, if it moves too far, try a higher one.

Current fleet 1967 Vitesse 2L conv "Tessa" 1970 Spitfire Mk3 (project) "Toby" 1972 GT6 Mk3 Not enough time or space

Recently sold: 1972 2.5 PI estate "Harry" (gone to russell on here) 1973 Toledo 2-door "Spike" (gone to a club official)

The ammeter is there to see what is going into and out of the battery. Nno matter how big the alternator is the battery will only take what it wants for its charge. Initial charge for a fairly flat battery is 20 or so amps and this will taper off quite quickly after 5 or 10 seconds to 10 or less amps.

The output from the battery could well be greater than 30 amps (on a bad day) but this has nothing to do with the alternator.

Why not leave it as is.? You are not actually gaining anything by up rating it.

Roger

TR4A 1967 daily(ish) driver TR4 1962 having surgery at present

Roger makes a fair point, although I know from experience with Tessa that it's possible to saturate a 50A ammeter at least briefly when the headlights are first turned on. If you're fitting uprated bulbs that's much more reason to upgrade the ammeter than the alternator is.

Current fleet 1967 Vitesse 2L conv "Tessa" 1970 Spitfire Mk3 (project) "Toby" 1972 GT6 Mk3 Not enough time or space

Recently sold: 1972 2.5 PI estate "Harry" (gone to russell on here) 1973 Toledo 2-door "Spike" (gone to a club official)

I seem to remember that the sort of ammeters fitted to our cars are not the low current devices that Rob decribes but carry full current through the meter, maybe a moving iron type movement. I know you can buy higher rated ammetrs to replace your existing one but I suspect you wish to retain the original to keep originality? To double the rating of your existing meter you need a shunt equal in impedance of the meter but measuring that will be difficult without highly sensitive measuring equipment. Alternatively move say the lighting circuit from after the ammeter to before it so that load is not measured. I think the ammeter in that case will still tell you if the system is charging or not?

Alec

Oswestry, Shropshire.

Mk1 1\2 P.I.,Jaguar Mk 2, very long term restoration. Jaguar X Type estate 2.5, Hymer 564 motorhome.

I don't think moving the light circuit is a good idea. The ammeter will then show a large current into the battery when there's actually a current flowing out, because the lights are drawing more current than the alternator is producing but the ammeter is showing the bulk of that current the wrong way. (I'm assuming lights on, with fogs, at idle for the worst case.)

Current fleet 1967 Vitesse 2L conv "Tessa" 1970 Spitfire Mk3 (project) "Toby" 1972 GT6 Mk3 Not enough time or space

Recently sold: 1972 2.5 PI estate "Harry" (gone to russell on here) 1973 Toledo 2-door "Spike" (gone to a club official)

Looking for TR6 Jasmine Yellow - Reg VVC 873 J. Last known to be in Liverpool/North Wales area. Â£50 reward for info leading to the current owner ! Rod 07802 151 786