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I want to make a shunt for an ammeter using a manganin wire. I don't know how to chose the wire diameter for the desired current. For example, my shunt will have 0.05 ohms and it must withstand 5A for a continuous usage and 10A for short periods of time. What diameter of wire should I choose ?

Autistic
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Marus Gradinaru
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  • You have to also give the accuracy you require. \$I^2R = 5~W\$ of heating and this is substantial to dissipate. You can Google [Shunts, Low Ohm: Low-Power](https://www.google.co.za/search?q=Shunts%2C+Low+Ohm%3A+Low-Power) to find 1 % components that are available. – skvery Mar 18 '17 at 12:05
  • I know that the shunt will dissipate 5W, but I don't know which diameter of wire can dissipate that power without heat up more than 60*C. I don't want to buy resistors, I want to make them myself from manganin wire. – Marus Gradinaru Mar 18 '17 at 12:13
  • It's not (just) about the wire diameter. It's about the whole system that carries its heat away (wires, circulating air, encasing, etc.). Like @skvery says: "this [heat] is substantial to dissipate". You will need to determine how you transport this heat away into the universe. With a better heat-transport system, your wire may be thinner. If your system is bad, the wire heats up and could end up de-soldering itself. – peter Mar 18 '17 at 12:39
  • @peter Longer wire means more surface area for cooling, all other things being equal. – skvery Mar 19 '17 at 15:51

1 Answers1

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Start with the maximum power dissipation, which is 5 W.

The first thing you need to decide is how hot you are willing to let the wire get, which you haven't told us.

Then you find the length of wire that can dissipate 5 W while staying within your maximum temperature rise spec. This has nothing to do with the electrical properties of the wire. It's mostly just a geometry issue. You can probably look this up.

Once you have the minimum length of wire you need, find the diameter that gives you the desired 50 mΩ resistance over that length. Round up to the nearest available diameter, and make the wire longer to compensate.

Olin Lathrop
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