Measuring very low resistances / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Measuring very low resistances

1 2 3 next

Move Thread

LAN_403

FastMHz

Fri Feb 17 2006, 06:21PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Hi all...I need some suggestions for ways to measure very low resistances, such as those of small coils. My multimeters only go down to tenths and display all zeros...I need hundreds and thousandths.

Has anyone used the 4 wire method that could explain more about how to perform it?

All ideas greatly appreciated!

Eric

Fri Feb 17 2006, 07:26PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

The four wire method simply means that you seperate the current source and the voltage measurement circuits so that any voltage drops in the current source circuit don't effect the voltage measurement accross the test peice. That way you can measure only the voltage drop (and thus resistance) across the test peice.

You could do this very simply without building anything if you have a current regulated power supply. For example, if you hook up a 10A current regulated source to your coil and then measure the voltage across the coil with a standard multimeter, you'll have a quick and dirty four wire scheme. In that instance, each millivolt on the meter equals .1 milliohms of resistance in the coil.

FastMHz

Fri Feb 17 2006, 07:33PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Interesting...I'll have to see if I have such a supply...if I don't, would something like shown in the schematic work?

The reason for such a low power source as a AA is because my ammeter only measures 10amps max.

Eric

Fri Feb 17 2006, 07:48PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

An AA will put out around 10 amps, short circuit (which is what this is) but only for a little while and at some risk of explosion. They aren't made for that and the current won't be constant. Any constant current supply will do.

In your schematic the voltmeter should be connected across R, assuming that's the coil you are testing.

FastMHz

Fri Feb 17 2006, 07:54PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

10 amps, eh? Maybe a AAA would be better? I don't have a constant current supply...just an old radioshack 3amp regulated supply but it's certainly not accurate enough for this. The battery would be shorted for just a second or so, just long enough to read both meters. It shouldn't even get too warm I wouldn't think. I see the error in my schematic - thanks for pointing that out...if I did this test, would it theoretically work?

Eric

Fri Feb 17 2006, 08:29PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

Sure, in theory. But I have a feeling that the battery will change in current significantly during the test, it droops off significantly in the seconds after you first hook it up. If you can get a stable current measurement AND voltage measurement at the same time then you are set.

Does that radio shack supply have current limiting? If so, use that instead. (post a pic if you're not sure)

HV Enthusiast

Fri Feb 17 2006, 08:36PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Just curious, why the need to measure such low resistances? What is your application?

FastMHz

Fri Feb 17 2006, 08:36PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I doubt it does...here's a pic:

I want to know the resistance of my ring launcher and coilgun coils so I can estimate the surge current through my SCRs...or am I making a simple thing hard?

Eric

Fri Feb 17 2006, 08:39PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

No, you're right it doesn't... You could always hook a low value resistor in series with that supply (like 1 ohm / 10Watt) to get a reasonably stable current source.

FastMHz

Fri Feb 17 2006, 08:40PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

The resistor in series with the power source if a great idea...that should help a bit and eliminate the need for batteries

UPDATE: ok, I did some real testing and ended up using AAs instead of the power supply because I was really afraid of blowing my ammeter. The results sound about what I would have expected too. I took two measurements and averaged them:

1st - 1.111a, 41.8mv
2nd - 1.07a, 39.7mv

Average - 1.0905a, 40.75mv

Resistance of coil - 37.37 milliohms

So...if I feed it a pulse of 700v the max that would go through the SCRs, assuming the caps can deliver, is 18731.6 amps.

My SCRs are 3 in parallel and accourding to datasheet can handle 30ka in 10ms pulses. Assuming this test was somewhere near accurate, my SCRs are not at risk.

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob