Re: [CAD_CAM_EDM_DRO] Re: Driving unmarked steppers

You can measure very low resistances using the four wire
kelvin method and one meter.

Two wires are hooked to a good approximation of a current source. The
constant current will be forced through the unknown device even with
variations in the resistance of connections and wires to the source.

A separate two wires are hooked to the unknown device
to a high impedance voltmeter. Almost no current flows due
to that high impedance so again it will measure accurately
despite contact and wire resistance to it.

So you force a current and measure a voltage. Resistance
is simply V/I.

One might ask "what is a good current source" ? Well a good
lab supply can do that but so can a norton equivalent... a battery and a
resistor.

You can easily measure sub milliohms with such a setup.
I used this in production with a/d cards in a computer for
a polymer thick film car side mirror heater I developed.
I measured the resistance of silver conductive ink.

Tell me what you want to measure and if you have a meter
and I can be more specific about how to make the current source
approximation.

All you need is junk parts really.

Les
Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: "Hugh Currin" <currinh@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, February 15, 2002 6:01 PM
Subject: [CAD_CAM_EDM_DRO] Re: Driving unmarked steppers


> Paul, et al:
>
> I know little of this so maybe Mariss or others with some electrical
> background can straighten me out. But not to keep my foot out of my
mouth, I
> think you can get better information on low resistances by using two
meters,
> one mearsuring voltage and another measuring current. With these two data
> points ohms law will give the resistance. It is my undertanding that ohm
> meters are not too good at low resistances, at least quantitatively.
>
> FWIW. :-)
>
> Hugh Currin
> Klamath Falls, OR
>
> On Friday 15 February 2002 02:04 pm, you wrote:
> >
> > This is a really useful bit of info -And yes, I do have some other
unknown
> > motors of the newer square design. 4 wire size 23, about 75mm (3") long.
> > Comparing the numbers with some data sheets, the wattages tally pretty
> > closely. All I need to do now is to find a good milliohm meter.....
> > A visit to the local store and ask for a demo on real world components
is
> > in order.....
> >