Measuring resistors

[globalweb]

Anyone have a link to how to measure a resistor ( below 1 Ohm) is much appreciated. I know there are a few here but I can not find it.

Thanks

[pinkmouse]

Google Wheatstone Bridge.

[AndrewT]

Hi,
string ten to twenty in series.
measure the string.
then apply a voltage to the string and measure the individual voltage drops.
A 200.0mV reading DVM can achieve reasonable accuracy.

[Conrad Hoffman]

The best way is to put a known current through it and measure the voltage drop. The method above works, but you still have to rely on the accuracy of the string of resistors. What you really need is some low value resistor with an accurately known value. If you have a 10 ohm 1% metal film or something similar, put that in series with your unknown resistor. Apply a voltage across the string that won't overheat either of them. Measure the voltage across the 10. Now you can calculate the current (I=V/R). Measure the voltage across the unknown and calculate the resistance (R=V/I). The accuracy of this method is limited only by the accuracy of your reference resistor, and your meter. There is no good way to develop your own resistance standard. You can make the resistor yourself (or buy a good film or wire wound), but you have to have it calibrated by somebody with a traceable standard to really know the value. I have a few old Leeds & Northrup resistor that I send out for cal every few years (they don't change much), and an old HP 6 1/2 digit meter. With that pair, I can measure any reasonable value to .01% or better. If all you want to do is match resistors, IMO much more important for audio, just use the same technique and match the voltage drops for a given current. You can go to a few ppm if you have a sensitive enough meter or op-amp.

[djk]

Build a current source with an LM317 and measure the voltage drop across the resistor.

[Conrad Hoffman]

The current source is, of course, the easy way, but you still have to know the current accurately, so one is trading the issue of reference resistor accuracy for current meter accuracy. Best bet is to look around the shop and decide if one has a nice 1% or better low value resistor, or if one has a nice calibrated meter with current capability. Note that if ones DVM has good resolution, but isn't calibrated, it will still likely be linear. That means if you compare against a known resistor, your measurement of the unknown will still be accurate, regardless of the accuracy of the meter. Probably too much history, but back in the old days, meter accuracy was terrible, but zero could be determined to microvolts. Voltage supplies were unstable and not known to great accuracy (other than standard cells). Thus the popularity of bridges over direct measurements- all you needed was some crude voltage, and an uncalibrated meter with a zero line, aka the galvanometer or null meter.