[HomeBrew] Wanted: circuit

[Dick Foster]

Ed,

I suggest you simulate a "4 wire" ohmmeter.  I'm going to assume that you
know that:
(a) The resistance is one ohm or less.
(b) The resistor under test can handle 100 mA.
(c) You're looking for "amateur" precision, say +or- 10%.

Take a 10 volt power source.  Put it in series with a 100 ohm, 2 watt, 5%
resistor and your unknown resistance.

Put your DVM on the 200 mV scale.  Place your DVM and probes as close in to
the unknown as possible.

The voltage source and resistor provides essentially a constant current of
0.1 A (provided the resistance of the unknown is small, say 1 ohm or less).
Of course you can use essentially any voltage and resistance combination
that will give you a current small enough not to overheat the resistor under
test and large enough to provide a measureable voltage drop for the
resistance range of interest.

The resistance of the unknown is the determined by dividing the measured
voltage by the current.  Say you read 0.1 volt (100mV).  You know the
current is 0.1A (100mA).  So, the "unknown" resistance is 1 ohm.

This technique is built into laboratory grade low resistance meters.  It's
usually referred to as the "four wire" or Kelvin method.

Very 73,
Dick W5TA

----- Original Message ----- 
From: <k6uuz@juno.com>
To: <homebrew@mailman.qth.net>
Sent: Sunday, June 22, 2003 6:14 PM
Subject: [HomeBrew] Wanted: circuit


> ** Please do NOT cross-post messages to multiple mailing lists on the
"To:" or "CC:" line of the e-mail message. **
>
> 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
>
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