School project help.. Lm3914 chip

[hibre flyers]

Hello we are currently competing in an electric car project. We need a way of showing the amps being used by the motor so that the driver knows if he/she is the right gear.

We have found a circuit that uses a LM3914 chip we have connected this up and it is working as a voltmeter with the max volt showing is 8v.. The car is run off two 12v car batteries.

Would be grateful is anyone could advise us or send me a circuit diagram.

Thanks

 

[MikeMl]

You need an "ammeter" circuit. This is usually done by placing a calibrated very-low-Ohm resistor in series with one of the motor leads("shunt"); either "high side", or "low side"(ground side). The differential voltage across the shunt is amplified, and made "ground referenced", so it can be presented to an A/D converter or your LM3914. A typical shunt maybe rated 1mV/A, i.e. 1mΩ, so at 50A of motor current, the voltage drop across the shunt would be 50mV.

Look up the following terms: "Current shunt", "Differential amplifier", "Instrumentation amplifier", "High-side current monitor", "Low-side current monitor". The last two of these are available as IC chips. Look up a data sheet for my favorite, a ZXCT1009