Measuring Resistances Less Than 1 Ohm (Lumsdonia Tip 203)

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Measuring Resistances Less Than 1 Ohm (Lumsdonia Tip 203)

Postby TimberSurf » Thu Feb 02, 2017 9:52 pm

It troubled to me, that to check the resistance of silver paint (or circuit works) when used to bridge the insulator on metal wheels, for lighting pickup or just as resistance for block detectors (guards van or tail wagon), my multimeter will barely measure down to a few ohms!
You could make a sophisticated Wheatstone bridge, but the variable resistor would be very expensive!
It occurred to me that a simple voltage divider (two resistors in series) with a little calculation would be sufficient!
A few tests later and a purpose made test lead was made up!
Now I can just attach my test lead to one side and +5V and the other side (black one) to -5V PSU
[a stable supply is not necessary, a 4.5V battery pack would do]

My sophisticated rig below:-
Low ohm test lead text 800.jpg
Low ohm test lead text 800.jpg (48.67 KiB) Viewed 303 times

All you need is a multimeter that measures volts, millivolts, and ohms
100-220 ohm resistor or thereabouts (preferably 1% and 1-3 watt)
5 V regulated power supply (wall wart, bench supply, or 7805 circuit, or even a battery)
Put the high accuracy, high wattage resistor in series with the unknown, across the 5V. Measure the voltage (drop) across each and do the maths

Unknown low resistance in ohms = Unknown (R2) millivolts / 1000 / (R1 volts / R1 resistance)

Full write up can be seen here
Lumsdonia <--- Hit link to go to my website for full story and wiring advice!

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Re: Measuring Resistances Less Than 1 Ohm (Lumsdonia Tip 203)

Postby Bufferstop » Wed Feb 08, 2017 2:26 pm

Reminds me of a formula used in cable balancing, (getting a flat response curve), where it was remembered as making Lloyd George a Roman Catholic (politically incorrect these days). L inductance in Henries X G conductance in Mohs = R resistance in Ohms X C capacitance in Farads. Conductance is the reciprocal of resistance, or 1/R ohms.
It always struck me as a daft way to go about it, as using conductance removed one very small number from the calculation whilst leaving in the capacitance which rarely made it above 0.1F In the days before super capacitors very few people could visualise a one Farad capacitor.
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