Can I use a DC Voltmeter to read AC current?

[FurioTheMad]

I am building a lamp, I would like to have the current run through a dimmer switch then through a voltmeter so you can see the amount dimmed on the gauge.

I am finding more DC gauges that fit the style I like, like this one;

**broken link removed**

Will this work?

 

[Reloadron]

Nice photo of a classic analog DC voltmeter. However to answer your question it would be no, that meter will not measure AC current. However, even that is not quite true. The tiny print below the 300 volt full scale, what does it say? All DC voltmeters like this are actually a current meter. The scale is setup to indicate volts but in reality the meter movement has a full scale current. You may see something to the effect of F/S = 100 mA where the 100 mA is just an example. However the full scale current would be the DC current required to drive the meter movement to a full scale value. Somewhere on the meter the full scale current value is usually marked.

Therefore yes, with proper conditioning from an AC current sensor such a meter movement could be used but it is not very practical or a simple conditioning process. You would do much better to find an analog AC amp meter (AC current meter) to measure the current to your lamp. So could it be used? Likely yes but would it be easy or practical? Nope.

Ron

 

[FurioTheMad]

Thanks for the fast and informative reply.
I have seen a few AC i liked, but most are rectangle. The DC meters are much more common, seem to follow the style I'm after, and are a lot cheaper ($8 for that one) so I was hoping I could use one but no big loss.

This one is great execpt for the $33 price tag (shipped).

**broken link removed**

So my hunt continues....

I imagine I will have several other electric questions along the way, since my experience level is basically an 8th grade electronics class 20 years ago where I made a strobe light.

 

[BrownOut]

Sure you can. The idea would be: Current-to-voltage-convertor -> DC volt meter. The AC current can be sensed by a small value resistor, AC coupled to a precision convertor and rectifier, scaled and then connected ot the voltmeter. What is the value of the current you want to measure? Converting to 300V for your meter isn't the best idea. Do you have another meter, say 0-3V???

 

[FurioTheMad]

You would do much better to find an analog AC amp meter (AC current meter) to measure the current to your lamp.

Please forgive my inexperience, I don't fully understand. I figured since my household electricity is 120 volts I would need a voltmeter.

I believe my house is 100 amps, on my main breaker there is a 100 sticker. So with an amp meter it should run at 100 amps when fully on, and 50 amp when dimmed half way? Would a volt meter respond the same way?

I like the amp meter idea because they commonly come in 0-100 amp range. Where as the volt meter is usually 0-250 or 300. I would like to see the entire sweep used.

This is purely a cosmetic feature, but I want it to function properly, so I have no bias towards amps or volts maybe gigawatts.

Something like this maybe, but not this exact one it doesn't have that feel.
**broken link removed**

Thanks

 

[BrownOut]

Dude, you said this is for a lamp, right? No lamp will use 100A. You're service may provide that amount of current, but each appliance will only use whatever current is required. For example, for a 50-watt lamp:

I = P/V = 50/120 = .416A or 416mA.

 

[Reloadron]

Please forgive my inexperience, I don't fully understand. I figured since my household electricity is 120 volts I would need a voltmeter.

I believe my house is 100 amps, on my main breaker there is a 100 sticker. So with an amp meter it should run at 100 amps when fully on, and 50 amp when dimmed half way? Would a volt meter respond the same way?

I like the amp meter idea because they commonly come in 0-100 amp range. Where as the volt meter is usually 0-250 or 300. I would like to see the entire sweep used.

This is purely a cosmetic feature, but I want it to function properly, so I have no bias towards amps or volts maybe gigawatts.

Something like this maybe, but not this exact one it doesn't have that feel.
**broken link removed**

Thanks

OK, let me give you an example of what I was getting at. Attached is an image of a 100 Amp AC Amp Meter. This is a typical panel and switchboard type analog meter. Remember how I mentioned full scale current? If you look closely at the meter face you will see what I mentioned where we see F/S = 5 AAC or full scale meter movement deflection is in reality 5 amps AC. The meter relies on an external shunt. Thus the scale is calibrated to read 0 to 100 amps but there is no way that little meter could actually read 100 amps of AC current as it sits.

As BrownOut points out, even a 100 watt lamp at 120 VAC would not draw quite 1 amp of current.

Additionally amp meters unlike voltmeters are placed in series with the load where a voltmeter is in parallel with the load. Now before you really start messing with mains voltages (like 120 VAC) I suggest very strongly you get familiar with how these meters are used, how they are placed in a circuit, and common safety practices. If you place a current meter like the images across a load really bad and ugly things will happen.

<EDIT> The pictured meter in this post is available here at a cost of about $11 USD. </EDIT>

Ron

 

[MrAl]

Hello there,

The old AC voltmeters used to use two germanium diodes i think to rectify AC into DC. It's been a while now though.

In any case, if you want to measure AC current such as that for a light bulb, then you should get an AC current meter that goes from say 0 to 10 amps. Depending on your light bulb however you may want one even less than that, 0 to 2 amps perhaps or 0 to 5 amps. They sell analog AC current meters for about $15 on the web, made for panel installation. If you need a link i'll have to find one as i cant remember offhand.
Oh yes, AllElectronics has a 1 amp AC panel meter for $12 USD. That should handle a light bulb up to 100 watts. There are other places too.

 

[FurioTheMad]

I think I follow now, somewhat.

This is I think will work for me;
**broken link removed**

I planned to run the circuit like this basically, I still have a few undecided design factors. Possibly a visible fuse, and a two light set up with a switch for one or both lights.

**broken link removed**

Thanks for all the info.

 

[BrownOut]

Remember how I mentioned full scale current? If you look closely at the meter face you will see what I mentioned where we see F/S = 5 AAC or full scale meter movement deflection is in reality 5 amps AC. The meter relies on an external shunt. Thus the scale is calibrated to read 0 to 100 amps but there is no way that little meter could actually read 100 amps of AC current as it sits.

Ron

I didn't know that. I'll have to check the meters I bought from the antique shop

 

[Reloadron]

I didn't know that. I'll have to check the meters I bought from the antique shop

Yeah, generally somewhere in there you will see the full scale current for the meter movement. I'll tell you what I learned in this thread. I was surprised at the prices some of the really well made analog meters are going for. Not analog panel meters like many we see today made of plastic but those metal housing well built meters. I wish I had a pile of them.

@ FurioTheMad Your circuit should work fine for bulbs up to about 100 watts.

Ron

 

[4pyros]

I should remind every one that an AC Dimmer (triac type) is a wave chooper and will not change the Peak to Peak voltage of a AC lamp. So the reading on a test metter will not change much. The average current will change more. Andy

 

[4pyros]

Ron; We posted at the same time see above. Andy

Ok never mind now I see he is using a current meter.

Sorry originally I thought he was going the try and measure a voltage change.