I am repairing an AVO multiminor Mk 4 which was in a sort circuit across a car battery while in the 1 amp measuring position. circuit diagram here
an obvious casualty was the 0.25 ohm resistance R1 which is implemented as a length of resistance wire of approx ( it has been cooked and burnt out ) 22SWG. The length of this wire was approx ( same excuses ) 10 inches overall
Now to my question:
if I want a 10 inch length of resistance wire to be 0.25 ohm , I am looking at a resistivity of 1 ohm / meter. But nichrome wire in 22SWG is 2.82 ohm/meter ( looking at about 7cm total length which would give me quite a fitting problem. Falling back to 18SWG gets the length back into the right ballpark (18swg => 0.924 ohm/meter)
but add to the above that nichrome is a beast to solder do I have any other choice of resistance wire commonly available and easily soldered ?
Best Answer
Resistance wire is sucky to solder, yes. I don't know of any resistance wire that has a low temperature coefficient and is really easy to solder, but Constantan is a lot better than Manganin or Nichrome and still has adequately low tempco for such an application.
But you could also just buy a 1/4-W or better 0.25\$\Omega\$ resistor. For example, a 1/2-W 0.25 ohm 1% 1206 resistor costs less than 50 cents, so you can get a couple to have one for the next time this happens.