# Dimmer switches and analogue voltmeters

I want to make a steampunk type table lamp. I want to use a dimmer switch and an old fashioned analogue AC voltmeter to display the "voltage" as a feature/gimmick. This displayed voltage doesn't need to be accurate. So my question is:

Would the needle display a varying voltage as the dimmer switch is varied?

## 2 Answers

Most analog meters actually measure DC current. In order to have the display change with the the dimmed voltage, you would need to rectify the AC.

Connect the meter through a series resistor to limit the current through the meter to the full scale value at maximum voltage (R = V(max) / I(meter full scale)). Here is a good reference about AC volt meters.

If you happen to already have an analog meter that covers the voltage range that you want, then it has a built in resistor, so you don't need to worry about adding a series resistor. • Ouch! looks like a good way to wrap the "needle" around its stop-peg. At least add a series resistor somewhere in your circuit diagram, as a proper voltmeter does. – glen_geek Mar 13 '18 at 18:03
• The picture is from the linked article. I did mention a series resistor in the text. – crj11 Mar 13 '18 at 18:04

When you dim the light, the dimmer circuit reduces the effective voltage. Any analog voltmeter can be reasonably easily adapted to show the voltage change. It would be best to start with a voltmeter that can display the normal voltage without dimming the light. Then it will show a reasonable approximation of the voltage when the light is dimmed without any modification to the meter. If you have a meter designed for a lower voltage, it can be adapted using a series resistor. If it is designed for a higher voltage, you could use it without modification, but the reading would be a lower point on the scale. If the meter is designed for a much higher voltage, you might want to modify it or possibility use a small transformer. For assistance with that you would need to show details of the meter.

The above assumes starting with an AC voltmeter. If you have a DC meter, refer to the answer by crj11 for converting it to read DC.

• Thank you for your answers. Just to clarify: The voltmeter is an analogue AC voltmeter with a FSD of 300v. So it should be OK. As long as the needle goes up and down as the brightness goes up and down, then the displayed voltage is immaterial. – DrJase Mar 14 '18 at 19:05
• Yes that should be fine. – Charles Cowie Mar 14 '18 at 21:02