Wanted: circuit

[Gary Fortnum]

>Ed Richards  wrote:

>Hi all;
>
>Does anyone have a circuit for a low reading ohmmeter? I need to
measure
>resistance from 0 to 1/2 Ohm. If I was an engineer instead of a
>technician I could probably design a bridge followed by an op-amp
as a DC
>amplifier. Also, the meter needs to be 50 feet from the point being
>measured. Any help will be appreciated. Thank you.
>
>Ed Richards K6UUZ


Ed,
There are a couple of versions of the same circuit at Maxim
Semiconductors ( www.maxim-ic.com ) that sounds like it will do the
job for you.
 It employs the method of supplying a constant current to a load (Rx).
The voltage measured across Rx can then be read directly as Ohms.
Since it is a current source and voltage is measured at Rx, the
resistance of the test leads and connections do not affect the reading
and the meter can be placed anywhere.

At Maxim's site select :   Appnotes --> Measurement Circuits -->
"Battery Powered Circuit Measures milliOhms and microOhms"
http://pdfserv.maxim-ic.com/arpdf/AppNotes/AppNote106.pdf

Also select :  Appnotes --> Miscellaneous -->
"Dummy Load Maintains Constant Current"
http://pdfserv.maxim-ic.com/arpf/AppNotes/AppNoteA0192.pdf

Note: in AN106, the pwr return (U2-4) should not be connected to the
bottom of R6, but should be connected to com (wiper) of S2.

I have had several occasions to to use something like this to measure
transformers, chokes, coils, shunts, etc where the standard ohmeter
won't reach or an AC device won't work. Unfortunately it is still in the
"Dream On" pile of things to do.
My idea was to incorporate this cct. in a hand held box with one of the
inexpensive 3.5 digit meters that are available now and thus making a
self contained unit. The meter and current source would be brought
out on separate pairs of banana jacks and could be paralleled for a 2
wire connection when there was no significant lead loss, or opened
for a 4 wire connection for a remote reading.

Regards,
Gary.

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