voltmeter questions

[antispatula]

I don't really get these things. So if I'm measuring the current out of like an amp or something, I don't get any reading. Why? Is it because I need a meter that reads in smaller units? Oh, and how would I measure the current off of an unbalanced cord? If there's only a positive and a ground, where the heck would I put the black lead?

 

[mshilarious]

I don't really get these things. So if I'm measuring the current out of like an amp or something, I don't get any reading. Why? Is it because I need a meter that reads in smaller units? Oh, and how would I measure the current off of an unbalanced cord? If there's only a positive and a ground, where the heck would I put the black lead?

When you are measuring current, you place the leads across whatever circuit you are measuring. If you are measuring voltage, you place the leads on whatever voltage points you are comparing--often one lead is to ground, but not always.

Now, as for measuring current off an unbalanced cord, well, if it isn't plugged into anyway, for example a speaker, there is potential voltage, but no current, because there is no completed circuit. So you could measure voltage from tip to sleeve, but current won't read anything.

If you plug the cord into your cabinet, you can measure current in the circuit if you can access the tip and sleeve when plugged in, or perhaps the speaker leads.

 

[Robert D]

So, assuming we're talking about a multi-meter, and not strictly a voltmeter, there's two things you need to know. First, voltage and resistance are measured across the circuit, but current is measured in series with the circuit. There should be a seperate lead jack for amps, and maybe one for milliamps. You take the red lead out of the volts/ohms jack and plug it into one of those. If you have no idea how much the current is going to be, start on amps. The milliamps input has better resolution, but if you pass too much current through it you can fry the meter.
Second thing is, are you measuring AC amps, or DC amps? There should be a selector knob or buttons to choose.
In all cases, amps are measured by breaking the signal path and putting one lead of the meter on one side of the break, and the other lead on the other side, so that the current has to flow through the meter to get back to the point where you broke the circuit. Current is like water flowing through a pipe. Think of it as cutting the pipe and putting a coupler to connect it again, only the coupler is also a flowmeter.

 

[antispatula]

aahaha my mistake, I meant voltage, not current Actaully, what I REALLY need to read is db.

 

[mshilarious]

aahaha my mistake, I meant voltage, not current Actaully, what I REALLY need to read is db.

dBV = 20 * log (volts)

dBu = 20 * log (volts/.775)

 

[punkin]

To measure current, you'll need to interrupt the circuit...then when you place you're meter across the open circuit, your meter will complete the path. Make sure you start on a high scale otherwise, you'll blow the internal fuse in the meter.

What are you trying to figure out?

dB is a measurement of reference...dBm is another matter.

You might be chasing your tail...tell us more about your planned adventure.

 

[c7sus]

I don't really get these things. So if I'm measuring the current out of like an amp or something, I don't get any reading. Why? Is it because I need a meter that reads in smaller units? Oh, and how would I measure the current off of an unbalanced cord? If there's only a positive and a ground, where the heck would I put the black lead?

Amprobe makes current transformer-based devices that can give you the current draw of a circuit without actually touching the circuit. Fluke makes them too, but the "trade name" for devices of that type is Amprobe. it's a set of jaws that open and close around the wire in question. Power up your amp and it can tell you how much current the circuit is passing anywhere, AC or DC if you get the right model.

Handy device. But you can't read an AC power cord with it because the hot and neutral cancel out each others' magnetic field, which is what the current transformer takes its' readings from.

But you can read the unbalanced load on a neutral with an Amprobe. That can come in real handy sometimes.

 

[FALKEN]

if you got the same one from radio shack that I did; you need to hit the "select" button to switch it from DC to AC, in order to get any kind of reading.