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Measuring low current in a low resistance coil using a 6514

Posted: July 13th, 2012, 10:44 am
by glor12
We are trying to replicate the Tolman-Stewart experiment. In this experiment, you rotate a coil and brake it suddenly. When you brake the coil, the electrons continue to move through the coil. You can measure the charge that passes by a point in the coil or the current and determine the charge on an electron.

What is the best way to do this? When we use a 6514 electrometer in charge or current mode and connect it to our coil (125 ohms), the charge just increases. In current mode, the current just increases. The same thing happens if we just short the inputs. I can't find anything in the manual that says we can't measure the current in a very low resistance circuit (nearly a short). Any suggestions would be appreciated.

Thanks,
--matt.

Re: Measuring low current in a low resistance coil using a 6

Posted: July 13th, 2012, 11:40 am
by Mark W
Can you post more details so we can help you. Signal levels? how are you connecting to the DUT?

Mark

Re: Measuring low current in a low resistance coil using a 6

Posted: July 13th, 2012, 1:29 pm
by glor12
I am auto-range mode, and the signals increase linearly from zero to the max. I forget what the max is for current, but for charge it's 20 uC. So that's the levels I see.

I am connected to the DUT (a coil of wire -- that's it, no active elements or even passive elements) using the supplied cable. It's triax, which ends in three alligator clips. The ground is clipped to the metal enclosure that surrounds the DUT, and the red and green are clipped to the ends of the coil. But we see this linear increase even when the alligator clips are shorted to each other.

Thanks,
--Matt.

Re: Measuring low current in a low resistance coil using a 6

Posted: July 16th, 2012, 5:24 am
by Andrea C
Hi Matt,

For Amps and Coulombs functions, the 6514 uses Unguarded Input Configuration. This means the measurement would be between the Red and the Black alligator clips on the traix to alligator cable (see figure 5-1 in the 6514 Reference Manual).

Also, if you want to observe the "zero offset" of a current or charge measure function, it should be done with the leads in an open circuit not shorted together (for volts use short circuit). You can then use the REL feature to zero out the instrument.

Andrea

Re: Measuring low current in a low resistance coil using a 6

Posted: July 16th, 2012, 5:41 am
by glor12
Hi Andrea,

We are using the red and black alligator clips on the triax alligator cable. We connect the red and black clips to either end of our coil.

I don't want to observe the zero offset. Again, what we are trying to do is measure the charge in a low-impedance system. When we connect the leads to our coil the charge continuously increases, even when there should be no charge in the system. Similarly when we try and measure in current mode, the current continuously increases with time, even when there should be no current in the system. As a check, we shorted the leads together, and the same thing happens when the red and black clips are shorted together.

Any explanation of why this happens would be great. Alternatively, how do we measure current or charge in a low impedance system?

Re: Measuring low current in a low resistance coil using a 6

Posted: July 19th, 2012, 4:29 am
by Andrea C
Hi Matt,

The electrometer uses a op amp based feedback ammeter circuit.

The source resistance of the DUT will affect the noise performance since the gain of the op amp will be increased as the source resistance decreases.

Check Out 'Amps measurement considerations' section of the 6514 Instruction Manual on Nose and Source resistance.

What current levels do you expect? If too small, then shunt type ammeter wont be practical either.

Looks like original Tolman-Stewart experiment used Galvanometer.