Measuring current on a loop without a conventional source

[Legion Zero]

Greetings,

I have a problem that's been bugging me for a few days now. I've asked electrical engineers, students and graduates, and I can't seem to find a clear-cut answer to these questions.

THE SCENARIO

One piece of copper wire has one end connceted to a terminal from a multimeter and the other end connected to the other terminal of the same multimeter. That's it. Nothing else. It forms a closed loop. I induce a changing magnetic field by running a magnet through the loop.

MY QUESTIONS

1) Is the multimeter properly set up to measure voltage in A/C? I think the general consensus on this one is that it is properly set up. But I want to be sure.

2) Is the multimeter properly set up to measure current? If not, how would I measure current under this scenario? In other words, what do I change (besides the obvious switch on the multi)?

3) If the scenario above is not properly set up to measure current, could you explain (to a dummy) why not :P?

I'm not an EE student, and my circuit knowledge is very limited.

I thank you in advance 07.

 

[Studiot]

You should have your meter set to DC, probably one of the more sensitive ranges.

Since the current you generate is going to be a brief pulse with a pronounced peak if you have a sophisticated meter you may be able to use what is called a sample and hold meter. - These are like the peak programme meters on audio recorders.

 

[berkeman]

Greetings,

I have a problem that's been bugging me for a few days now. I've asked electrical engineers, students and graduates, and I can't seem to find a clear-cut answer to these questions.

THE SCENARIO

One piece of copper wire has one end connceted to a terminal from a multimeter and the other end connected to the other terminal of the same multimeter. That's it. Nothing else. It forms a closed loop. I induce a changing magnetic field by running a magnet through the loop.

MY QUESTIONS

1) Is the multimeter properly set up to measure voltage in A/C? I think the general consensus on this one is that it is properly set up. But I want to be sure.

2) Is the multimeter properly set up to measure current? If not, how would I measure current under this scenario? In other words, what do I change (besides the obvious switch on the multi)?

3) If the scenario above is not properly set up to measure current, could you explain (to a dummy) why not :P?

I'm not an EE student, and my circuit knowledge is very limited.

I thank you in advance 07.

It sounds like it is basically set up correctly. Whether you set the meter on DC or AC, the changes will be quick enough that it will be hard to get reasonable numbers, unless you have a peak hold feature as Studiot mentions.

Some meters have a bar graph feature at the bottom of the display, and this kind of experiment is one very good use of the bargraph. The bargraph's response is fast enough so that you could see what the voltage and current waveforms basically look like.

[URL]http://www.sensorsportal.com/HTML/E-SHOP/PRODUCTS_PHOTOS_10/Fluke_87_C.jpg[/URL]

 

[vk6kro]

Yes, you can do that, but the result will depend on which range you use on the multimeter switch.

If you switch to DC volts, very little current will flow in the loop because of the high resistance of the meter as a voltmeter.

If you switch to DC current, you may be able to see something, although, with a single turn of wire, the current produced will be quite small.

The resistance of the meter will be large compared with the resistance of just wire, so the current produced will be less than if the loop was completed without the multimeter.

In both modes, the voltage or current will be one polarity and then the other as the two poles of the magnet pass through the loop.

 

[Averagesupernova]

What you have is not really a conventional circuit. When you have the leads of the meter hooked as you describe, running the magnet past while the meter is set to volts will show the voltage that your tiny little generator is putting out without any current in the circuit. When a meter is set to volts virtually no current flows. Similar to a 12 volt battery has 12 volts on its terminals even though nothing is hooked to it. If you do the same thing while the meter is set to current, you will measure the short circuit current of your tiny little generator. This generally NOT how current is measured. Current is generally measured while a load such as a light bulb is in circuit. So imagine you have a tiny little bulb that faintly glows when your magnet passes the loop of wire. To measure current in this circuit you would cut the wire and insert the meter in series with the bulb. If you remember that current is a rate of electrons per second it is probably easier to remember how to measure current.

 

[KingNothing]

To measure current in this circuit you would cut the wire and insert the meter in series with the bulb.

I think you may have misunderstood. He's connecting the two ends of a wire directly to the multimeter. In this regard, it is both in series and parallel.

 

[Legion Zero]

Thanks for all the responses! Just to give you a little background on what it is I'm trying to accomplish, I'll expand a bit.

The copper wire I described in my scenario is actually a solenoid of about 500 turns, 1 ft in length, and about 5 inches in diameter. Yes, I know it changes things a bit :). However, I think the principle is still the same and it's the reason why I simplified my scenario above.

As an ME student, I recently built an electricity-generating buoy (works with the waves pushing a magnet through the solenoid). When I presented my project to the engineering committee I was heavily criticized for not measuring the current correctly (ie, connecting the leads the way I did in the above scenario).

I haven't been able to sleep trying to figure out how I could have connected the multimeter differently in order to get "correct" current readings. I feel like an idiot because it seems like something I should just know.

This will probably generate some questions as to the success of the buoy. I was able to generate a max of 12 V A/C and a pitiful max of about 1 mA. The ocean wave frequency was also pretty bad (about 0.3 Hz). But obviously that's out of my control. Waves will be waves.

Anyway, I appreciate the responses. From what I can gather, it seems like I had it set up as properly as it's going to get and still produce inaccurate current readings, correct? I know this is not a conventional circuit. It really was never meant to be.

 

[vk6kro]

Was your magnet entirely inside the solenoid ... or in and out of it depending on the waves.

 

[Legion Zero]

Was your magnet entirely inside the solenoid ... or in and out of it depending on the waves.

The magnet is 4 x 4" (length and diameter). The magnet's vertical motion mostly remains inside the solenoid, although it very frequently reaches the edges of either end of the solenoid. I suppose building it so that it completely leaves the solenoid would produce a greater change in magnetic flux. It definitely can use a lot of improvements, no doubt.

I was just happy it produced measurable results... That is, until I was scolded for not measuring the current properly.

 

[KingNothing]

There are a ton of students who try to measure current by putting a multimeter in parallel with some electrical component. They may seen that and assumed you were doing it wrong.

 

[vk6kro]

The magnet is 4 x 4" (length and diameter). The magnet's vertical motion mostly remains inside the solenoid, although it very frequently reaches the edges of either end of the solenoid. I suppose building it so that it completely leaves the solenoid would produce a greater change in magnetic flux. It definitely can use a lot of improvements, no doubt.

I was just happy it produced measurable results... That is, until I was scolded for not measuring the current properly.

The two poles of the magnet would produce roughly equal and opposite voltages in the solenoid if the magnet was entirely inside the solenoid. So, you would get very little output, as you found.
If you get a chance to revisit it, you should be able to produce much better results than you got before with a better choice of coil and magnet positioning. The coil should be short and multilayer and the magnet should completely leave the coil for part of its cycle.

Measuring the short circuit current is a valid measurement, but for a practical project, you could probably measure the voltage across a load resistor.