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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 ?

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    \$\begingroup\$ You could parallel three pieces to get nearly the right resistance. Why does it have to be resistance wire rather than a resistor? \$\endgroup\$ – Nick Johnson May 15 '15 at 16:47
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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.

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Looking at the schematic and the layout of the shunt board, the resistance of the wire should be about 0.15 ohms. The meter movement is 60 uA, 1667 ohms, with a full scale voltage drop of 100 mV. 1 amp across .25 ohms will give a voltage of 0.25 volts. Look closely at the schematic R1 - it is clearly shown as a voltage divider. The construction of the shunt board indicates that the wire section is the 0.15 ohm portion of the divider. As a confirmation, look at the portion of the immediately to the right of the wire. You'll see that the traces make what look like a ladder (actually, 2 ladders). This double-ladder section is the 0.1 ohm part of R1. You should see that some of the "rungs" of the ladders have been cut. This is a standard method for trimming the value of the trace resistance, and was performed after the wire was installed.

Looking at the link you provided, I'd suggest that the length of the wire is more in the neighborhood of 5 inches rather then 10 - 5 loops on a mandrel with a width of about 0.5 inches. Using http://en.wikipedia.org/wiki/Nichrome as a source for cold resistivity suggests that about a 17 or 18 gauge wire is indicated.

Replacing the wire with a standard resistance (of 0.15 ohms) will work, of course, right up until the next time you short out a battery. Then the high current will be directly reflected in the movement current, and you will probably kill it - assuming it survived this time. Making the wire part of a voltage divider allows it to heat up at high currents and protect the movement, until it finally fails like a fuse and protects the delicate winding. For a hand-held analog meter such as this one, I'd suggest that tempco is not an issue.

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  • \$\begingroup\$ that is a nice analysis, interestingly the wire did not go open-circuit but ended up shorting a turn or two. \$\endgroup\$ – aldobranti May 17 '15 at 9:51
  • \$\begingroup\$ this is a nice analysis,thank you. On close inspection of the board, not easily determined from the manual, the PCB ladder section is switched in by the control switch and would seem to implement R2. The tap on R1 is a soldered joint towards one end of the resistance wire nearer the input socket. To get the overall wire length right I am thinking that I should start with about 10 inches of 22SWG Constantan . interestingly the resistance wire did not go open-circuit but ended up with a shorted turn or two, on DCV ranges it ended up reading with about 80% oversensitivity R1 wasn't a safety fuse \$\endgroup\$ – aldobranti May 17 '15 at 10:23

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