How to measure resistors below 1R

[bigpanda]

Hi

It might seem nuts. I never measure the resistors below 1R. It seems that the DVM or moving coil meter (that I had ) are unable to measure such. Do I need any special meter for this?

 

[masfe]

Look at this: http://www.comunidadelectronicos.com/proyectos/med-res.htm

In spanish.

good bye

José Ramón

 

[SY]

The best way to measure them is either to use a four-point system (where a known constant current is passed through the resistor, and the voltage drop is measured) or to put a bunch of them in a series string, run a voltage through the string, then measure the voltage drop across each resistor. The latter method is most suitable for matching, rather than getting an absolute value.

 

[jackinnj]

you can also use a Wheatstone bridge which you'll find in most high school physics labs --

since EBay is now taken up more by dealers than garage sales it is getting trickier to score a bargain on an HP3468 or HP3478 -- but good 5.5 digit meters from Fluke, Racal Dana, Keithley still abound. RLC bridges tend to be "clunky" but can be very, very acurate and comparatively inexpensive.

 

[cocolino]

originally posted by SY
The best way to measure them is either to use a four-point system (where a known constant current is passed through the resistor, and the voltage drop is measured)...........

Commercial 4-point resistance measuring equipment can be quite expensive.
You can DIY such a thing relatively cheap.


The following photo is taken from my DIY constant-current 4-point measuring adapter. It is used together with a conventional DVM in the 200mV DC measuring range.

3 precision resistors (the 2 lengthy blue things and the smaller black one aside), 1 FET, 2 batteries, 1 switch, 1 OP-Amp and that is it almost.

It is made according (with modified PCB) an ELEKTOR puplication.
Works fine!
If You`re interested I can look up which ELEKTOR publication it was.

 

[cocolino]

another pic from the front plate

 

[cpemma]

I've used a Kelvin Bridge (a Tinsley, but much older model than the one shown) for very low resistance measurement (microhms or less). Ran off a small lead-acid battery, so I guess current was fairly high compared to modern ones. 😉