[Hallicrafters] Wanted: circuit

[Rich Oliver]

Ed,

Rodney's current source should do the trick.  The key to making an 
accurate, remote measurement of a low resistance is to use four-wire or 
"Kelvin" connections.  Define the terminals of the resistance you need 
to measure then attach *two* pairs of wires in parallel to those two 
points.  You can use a four (or more) conductor cable to do this.  Keep 
the pairs electrically isolated from one another at all other points. 
 Force a known current down one pair then measure the voltage on the 
second pair with a high impedance meter.  This technique effectively 
removes the effects of resistance in the wire pairs and allows you to 
easily calculate the unknown resistance by using Ohm's Law.  With 
Rodney's current source cranking out one Ampere the voltage you read (in 
Volts) will be equal in value to the of the unknown resistance in Ohms 
and the measurement will be independent of cable length.  This is true 
as long as the cable resistance remains low enough that voltage drop is 
within the compliance range of the current source, and low enough that 
is is several orders of magnitude smaller than the input impedance of 
the voltmeter.  Fifty feet should be a piece of cake.  Note that 
capacity effects of the cable might confuse a digital meter so you 
should try attaching a capacitor across the measurement point to see if 
there is a shift.  If so leave the capacitor in place.

73, Rich, KC9GQ

Rodney Bunt wrote:

>Do this, use a three terminal regulator and wire it like a constant current source.
>
>        +-------+
>        | LM317 |
>0-------+       +------------+--------0
>        |       |            |
>        +---+---+            |
>            |                |
>            +---/\/\/\/\/----+
>
>                  1.2 Ohm
>
>This will give you a constant 1amp output. put this into you resistor that you want to measure.
>Using a digital meter, 1mV = 1m Ohm or 1v = 1 Ohm
>
>Rodney
>VK2KTZ
>
>
>
>--- k6uuz at juno.com 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
>>
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>>----
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>>    
>>
>
>
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>----
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>  
>