Tag Archive for calculated values

Voltmeter Scaling

As a follow-up to my last post, I ran a bunch of figures on how to set the proper scale ranges for the voltmeter and ammeter for the power supply project. The numbers end up being really close to the forum post I stumbled upon a few days ago which is great. Using 100mV instead of 1V for my reference voltage, I was able to recalibrate it by a factor of 10 and solved my former problem where I would have had to use 1MΩ as my divider resistor and it equalling the input resistor. This way, I end up using more sane values and I’m pretty sure when I retrofit my voltmeter they will work out ok.

CircuitLab mock-ups for voltmeter/ammeter scale divider resistors

CircuitLab mock-ups for voltmeter/ammeter scale divider resistors

On the back of the envelope, it would make sense to use round numbered divider resistors to get even factors of 10, but as it turns out in analysis, those numbers end up being slightly off for some reason probably involving calculus.

Not wanting to waste my sunday morning trying to remember math I last used in high-school, I consulted CircuitLab (invaluable resource and well worth the subscription price) and mocked up some quicky input divider circuits for my voltmeter and ammeter. The results I will display here along with the schematic.

Voltmeter Divider

Scale Resistor Ideal Value Resistor Calculated Value
2V (0-1.999V) 1kΩ 1.001kΩ
20V (0-19.99V) 10kΩ 10.1kΩ
200V 100kΩ 111.1kΩ

Ammeter Shunt Resistors

Scale Shunt Resistor Value
2A (0-1.999A) 0.1Ω
20A (0-19.99A) 0.01Ω

So that solves that. All that is left is to build it up (may breadboard it first to avoid unnecessary soldering) and see if she floats. Obviously, I’m going to have a hell of a time finding the Voltmeter divider resistors in those odd formats so I will just play with series combinations until I get as close as possible to those values. It will save me hunting for expensive 1% resistors when I can just compensate with 5% resistors of which I have a ton.

For the ammeter, you will note the numbers are a bit off, this is due to the 4 significant digits in CircuitLab’s calculation which is absolutely fine and I can trim any error with the reference voltage.

For the ammeter (as well as the current sense for the limiter) I have some interesting ideas which may warrant it’s own post, coming soon.