Nuts and Volts - August 2010

#2 Check out application note AN543 on the Microchip website. It explains general-purpose tone generation and offers assembler source for the PIC17C42 which I'm sure you can get to run on your 18F part.

READER-TO-READER TECHFORUM

Dan Danknick Santa Ana, CA

[#4102 - April 2010]

Car Autoranging Ammeter

Need a circuit for an autoranging car ammeter. In-line cable shunt output at the battery. Resistor short protection at the battery for a pair of sense wires running to the circuit and meter under the dash. Car battery powered, no microprocessors, low range 0 to 10 amps, high engine start range for starting current, d’Arsonval meter, not digital.

Want to see relatively fast amperage changes. Should this go in the negative or positive side of the battery? Accuracy on 0-10 amp range say 10% or better; 20% or better on the engine start range. KISS.

The simplest solution would be to use two shunts, two sets of short-circuit protectors, and two d'Arsonval meters. The two shunts would be rated something on the order of 20 amperes and 200 amperes full scale, or as appropriate.

I'd use 50 mV shunts (rather than 100 mV shunts) because the alternator output voltage will be regulated with reference to the load side of the low current shunt and this introduces only a 0.4% error in actual battery charging voltage. Both meters would be rated 50 mV full scale, but the low current meter would need to be a zero-center instrument (- 50 mV to + 50 mV).

However, one instrument with the ability to switch between two current ranges was requested. I believe that the scheme illustrated in Figure 1 addresses that challenge. The battery, ignition switch, starter, and starter relay are considered to be pre-existing equipment in the car. A negative-ground system is shown.

The two shunts are as above, delivering 50 mV for full-scale ratings of 20 amperes and 200 amperes, or as appropriate. They are located at or very close to the positive post of the battery. The low current shunt is inserted into the existing lead that connects to all of the non-starter loads in the car, plus the alternator. The high current shunt is inserted into the non-grounded starter cable.

Fuses F1-F4 are 1/4" diameter 0.1 ampere rapid-acting devices (type

Figure 1

AGC or GBB) contained within in-line weatherproof fuseholders. Resistors were specified, but resistors affect the voltage delivered to the indicating instrument, and with only 50 mV to play with full scale, any loss is significant. Fuses are better.

M1 is a 50 mV (full-scale) zero-center instrument of appropriate size and accuracy with its scale hand-calibrated to match the shunts.

K2 is a fast-acting three-pole double-throw insrument relay having gold contacts and a 12 volt DC coil. Gold crossbar contacts would be the best. We're switching millivolt signals and voltage barriers due to dry-contact phenomena cannot be tolerated.

K1 is a sensitive single-pole relay (one normally-open contact is required) having a low current 12 volt DC coil. It exists because the contacts in K2 are small and the starter relay is an unknown device, so K1 exists to switch the load presented by the starter relay. It also adds delay which favors the circuit.

In operation, K2 is normally not energized hence M1 is connected across the low current shunt. When the car engine is to be started, the operator rotates the ignition switch to the START position which energizes relay K2. The uppermost set of contacts on K2 close and energize relay K1; and through K1, the starter relay; and through the starter relay, the starter motor.

But at the same time that K2 causes K1 to be energized, its remaining two sets of contacts have likewise transferred such that meter M1 is now connected across the high current shunt, hence M1 will monitor the starter motor current once it is energized.

When the engine begins to run, the operator releases the ignition switch key, breaking the connection to the START contact.

Since the START contact carries the full load of the starter relay coil — and also the coils of K1 and K2 — the starter motor will be disconnected rapidly and before the contacts on K2 transfer back to their de-energized position, and thus M1 can be connected safely to the low current shunt.