# Voltmeter used with a lamp

Discussion in 'The Projects Forum' started by Lila, Feb 24, 2015.

1. ### Lila Thread Starter New Member

Feb 14, 2015
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hello, I was wondering if someone could help with a project. I make steampunk lamps as a craft and they are pretty basic, as well as my knowledge of circuits, electricity etc. A couple of weeks ago I came across someone who made a contraption that included an Edison light bulb and a couple of LED lights and a working voltmeter ( a round vintage one) and when the on/off switch was turned on everything lit up and the voltmeter needle obviously showed voltage. So I thought this was really cool and now I would like to try it for myself and maybe add a working voltmeter to my design. But again my knowledge is very basic so I was hoping someone could help me out. Recently I posted about using a ampere gauge but since then have learned how complex things like this can get and thought that using a voltmeter would be a lot easier than trying to convert things. I recently purchased a voltmeter but it has not arrived yet however when it does I will post the specs of it. I would appreciate any help.

2. ### MrChips Moderator

Oct 2, 2009
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3,451
Most voltmeters are actually ammeters.

Why is that?

That is because a meter movement is a moving coil that pivots around in a stationary magnetic field.

The coil is simply a number of turns of enameled copper wire. The total resistance of the coil is relatively low. Hence the basic meter movement is sensitive to a very low current of the order of micro-amps.

To make the ammeter measure currents of higher value, resistive shunts are placed in parallel with the meter.
To make the ammeter measure voltages of higher value, resistances are placed in series with the meter.

Hence you can convert your ammeter into a voltmeter by putting a resistor in series with the meter.
If you know the full-scale current reading and the resistance of the meter you can calculate the required series resistance.

You can also do this experimentally by starting with a high value resistance and working your way down to a lower resistance until you get the desired effect.

Johann likes this.
3. ### alfacliff Well-Known Member

Dec 13, 2013
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428
remember, ampmeters are placed in series with the load, volt meters are placed across the load.

4. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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Well. when it arrives... post the specs.. until then...................................................... zzzzzzzz...............zzzzzzzzzzzzzz

of course you should have known the specs BEFORE you ordered it.. Or at least be able to include a link to what you purchased..

5. ### Lila Thread Starter New Member

Feb 14, 2015
12
0
Last edited: Feb 24, 2015
6. ### Lila Thread Starter New Member

Feb 14, 2015
12
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So I received the meter the other day. It's a Simpson model # 56 0-250v ac analog round meter. I made a test lamp using a night light bulb and hooked up the meter to the lamp socket. Sure enough the meter needle moved right over to around 115-120. So I'm assuming that because the meter has a max load of 250v and standard household volts is roughly 120 I don't need a resistor right?

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,784
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Yes you purchased a 250V max AC voltmeter.. It has all the "stuff" needed in the meter already..