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How do you make a analog ac line voltage monitor 120 volt (RMS) scale? Answered

I would like to build a analog 120 volt RMS ac power line voltage monitor. That plugs into the wall to monitor my AC line voltage. Using a analog meter for display. How does one go about constructing such a device?  



Best Answer 1 year ago

Here is a complete circuit, using a modified Wheatstone Bridge. The zener diodes are used to "start" the meter reading at 90 VAC input. The resistor values were chosen to give a mid-scale reading with 115 VAC input, using a 1mA DC panel meter that has 100-ohm internal resistance. If your panel meter is different, then you'll have to change R1 and R3 values.


First find your meter......

Classically a "moving iron" meter is used, because they can measure AC directly. These days, you'll be OK with a bridge rectifier and a more common moving coil meter.

Once you have your meter, and you know what its impedance is, you calculate a suitable series resistor.

Here are some moving iron meters


Sorry it has taken me so long to get back to you. How ever the US Navy had first call on my body. I live in the USA and I'm looking for something modern. And a nice big meter would be nice. Something like the size of the old Simpson 260 series VOM.

Could you draw me a schematic? Does the meter manufacturer provide you with a data sheet listing the impedance of its meter? If not how does one go about finding that information? And thank you steveastroukfor the help.

Thank you very much! I'll start looking for a meter come Monday. What would the ratting on the bridge rectifer need to be? Would 1/2 amp be OK?

The scale on the front bears very little relationship to the actual meter mechanism at the back.

Let me know where you are in the world,. and I can make some recommendations.

Are you looking for some kind of steam-punk retro vision, or something modern and utilitarian ?

If you get a moving iron meter, you can buy it off the shelf for a 110V supply or whatever.

If you are buying a moving coil meter, just take a look at the spec - or send me a link - before you buy it, and I'll tell you what to use.


Bloke sdnsg may be interested to know that analog meters were / are small electric motor rotors with a spring that attempts to return the pointer movement back to zero.

Thanks for that info iceng. I had no idea that they were a small electric motor rotor.

Makes sense when you think of it..........

Way back when Tesla was sparking for a living with GE
Somebody noticed the torque on a motor increased with voltage.

It's only easy to see the design process. 
  • Taking a small motor add a pointer
  • Fix a spring to the shaft
  • Prevent a full rotation
  • Discover brushes are not needed
  • Simplify the armature ( rotor )
  • Use permanent magnets for the field
  • Sell a useful analog product  :)

1 year ago

Here is a complete circuit. It uses a modified Wheatstone Bridge. The resistors R1 and R3 will have to be adjusted to suit your panel meter. The values shown are for a 1mA DC 100-ohm meter.