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Date:         Sat, 25 Feb 95 00:04:20 EST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         cs@sypro.com (Cetin Seren)
Subject:      ammeter

I've not kept up with my e-mail for some time, so this may be a little stale subject, but here's a tidbit about ammeters that might be a better solution than pulling big wires through your bus and into the dash where the ammeter is:

Internally, a Voltmeter is a very sensitive ammeter in series with a big resistor:

:-----/\/\/\/\/\----------AMMETER---------:

again, internally, a high-amperage ammeter is a sensitive low-current ammeter in parallel with a high-power rated but low-resistance 'shunt':

--------AMMETER-------- | | | | :----------| |----------: | | | | ----/\/\/\/\/\/\/\-----

Here's the way the ammeter works:

if the ammeter maxes out at 1 milliamps, and the shunt resistance is 1/999th the value of the *internal* resistance of the ammeter, then this combination will cause the ammeter to max out when the TOTAL current is: (1+999 = 1000 milliamps = 1 amp ) 1 amp. In other words, the resistance carries 999/1000 of the total current, and the ammeter the other 1/1000.

One can use this principle to calibrate any simple ammeter (a good, sensitive handheld unit is needed for calibration with the following configuration):

shunt ----/\/\/\/\/\/\-----:+BATTERY-:--------- | | | | | | wires to the dash | | | | -dash-ammeter-

or, even better:

The battery ground strap -----------:+BATTERY-:--------/\/\/\/\/\/\/\/\/\/\/\/\-----GROUND | | | | | --dash-ammeter-----------------GROUND

The shunt has to be quite thick, and short, should not have resistance more than .01 Ohms ( power dissipated = I**2 * R ) so that even while cranking it will not heat up too dangerously (assuming a 200-amp surge: 200*200*.01 = 40 Watts -- it will melt solder at prolonged exposure, but hopefully one will not have to crank the engine longer than 30 seconds at a time).

I would advise against pulling big wires through your bus to get to the dashboard; even the battery being inside (as in my '87) has the same problem: The additonal wire lengths limit the current the battery can deliver to the starter (no matter how big the wires are, they still have a caharacteristic resistance, measured in Ohms/meter, so the longer the wires, the bigger the internal resistance). In the '87, this is counteracted by the fact that the battery stays quite warm in comparison to being outside, so it delivers more juice. Another thing: When you pull wires yourself, you have to watch out for the contacts -- they are the primary points for failure, they rust easily and cause even bigger resistance, and, worse, when the contacts get rusty, and you still pump heavy current through them, they will heat up, melt, and start fires (no kidding, it happens a lot more often than people think).

I'd say, the best thing to do is to ;

a) find somebody who owns a GOOD multitester, that is capable of measuring resistances down to .01 Ohms or less.

b) bribe them so you can use it for a weekend.

c) measure the resistance of your ground strap -- see if it is anywhere near to being measurable (it may be too low to register any resistance at all on your multitester). If not, try to see whether you get anything measurable when you put two or three ground straps end-to-end.

d) measure the resistance of your overall circuit:the length of the total wires you will have to use (now they do not need to be heavy gauge) + the interanl resistance of the ammeter you are installing. The resistance of the body stretch is negligible, assuming good contacts.

e) the ratio of the resistances of the 'shunt' (the ground strap, maybe?) to that of the wires+ammeter will give you the factor you have going through your battery. Conversely, if your battery is dishing out 60 amps, and the ratio of resistances is 1000, then only 60 milliamps will pass through your ammeter, which is quite a typical rating for a common ammeter.

Notice that, with this approach, and using the ground strap, you could actually measure the currents being drawn while cranking.. Of course, if that makes your scale too big for normal operation, you might try the first option I've drawn above...

Good Luck, Cetin cs@sypro.com


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