I wanted an anlog panel meter which would read 50 amps. Standard MU45 type meters only go to 20A. And I didn't want to custom build my own shunt. Automotive type meters are available, but they're centre-zero (not what I wanted), and typically not very accurate, with poor resolution.
Jaycar have a nice range of shunts, designed for use with 200mV digital panel meters.
QP-5412, QP-5414, QP-5416.
They're 1 milliohm. At 50 amps this will give 50mV. My 1 mA MU45 meter needs 200mV.
The simple solution is to use an op-amp, to multiply the 50mV to 200mV. This is simple if one has dual (positive & negative) supplies for the op-amps. But to run it off a single 12 volt supply is not so easy. Fortunately, I had some micropower rail to rail input & output op-amps (LMC7111) on hand from another project.
So, here's the circuit, in pdf.
meter circuit
I slapped it together on a printed circuit board.
I used the "join the dots with a texta" method. The board is circular, and fits through the mounting hole for the meter.
In the photo, I haven't yet fitted the 5.1 volt zener.
The red wire is +12. Black is zero volts. Brown is the sense input. NOTE: the black & brown wires must connect directly to the shunt.
I scanned the original meter face, and photo-shopped it to produce this.
Calibration is easy. Replace the shunt with a fixed resistor. I used 2K2. Hang a pot from pin 3 back to +12 volts (I used 1 Megohm). Put a meter across the 2K2, and adjust the pot for exactly 50mV. Now adjust P1 so it reads 50amps. Short out the 2K2, and check that it reads zero.
It can be changed to read any value. The op-amp gain is (R2 + R1)/R1. The gain needs to be slightly higher than Vout/Vin, to allow adjustment with P1.
There are plenty of other op-amps which can be substituted for the LMC7111. Main criteria is rail-to-rail input & output. Otherwise it will never zero.
Current drain is 14 microamps when reading zero, and 1.028 mA when reading 50 amps.
Amanda
