January 28, 2009
about 4 years ago
These are pretty simple to adapt as a voltmeter or ammeter over a wide range. The meter movement needs about 230mV for full scale deflection (fsd). The 5V version has a 5k series resistor to give fsd at about 1mA. You can adapt them for any fsd voltage above ¼ Volt by changing the internal series resistor. A multi turn trim-pot is the easiest way to get it right. 10k will give you up to about 10V, 20k for 20V etc.
For an ammeter it’s a bit harder as you need a shunt resistor across the terminals to carry most of the current. Calculate the resistance to give 250mV across it at the current you want as fsd then add a low series resistor inside the meter to bring the fsd back to the correct point.
I made a replacement 5A meter for my battery charger with a 0.05 Ohm shunt (two 0.1Ohm power resistors in parallel) and needed a 10 Ohm series resistor to get the correct reading.
Works a treat.
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