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Reliably soldering Alloy 90 wire for current shunts

A discussion started in 1995 and continuing through 2000.
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Q. Hi,

Learn More: What is a shunt and how does it work?

Suppose you want to measure a high current…

Large currents need large conductors. You can't put large currents through the small wires of a reasonably sized meter without frying it, any more than you can put Niagara Falls through a garden hose to measure the flow rate.

So, instead, you pass the current through a good-sized copper block. But this copper block is of a very precise size and very precise composition (for example, Alloy 90), so that the small voltage drop across it will be very exact. We call this copper block a shunt.

Say the maximum current the circuit will ever have is 1000 amps and we choose to use a copper block / shunt with a resistance of 0.000005 ohms. Ohm's Law ("Volts = Current x Resistance") tells us that the voltage drop across this resistance will be 50 millivolts --

V = I * R = 1000 amps * 0.000005 ohms = 0.0050 volts = 50 millivolts

Since the resistance of this copper block / shunt is fixed, the voltage drop across it will be proportional to the current. So if we measure 25 millivolts across the shunt, we know the current passing through it is 500 Amps.

So, what a shunt does for us is it allows us to use a millivoltmeter, label the meter readings in Amps instead of millivolts, and use it to measure (and possibly control) large amperages. -- Editor

Our company has had serious problems with solderability of small (0.5") alloy wires (10 AWG to 24 AWG) we use as current shunts for sensing overload and short circuit conditions in our switching power supplies. A temporary solution has been barrel plating a copper strike followed by .0003 -.0005 tin plate. This works until you get to the thin wire gauges. Can you suggest any other solution? The alloy is called Alloy 90. It does not solder well in a no-clean process.

Thank You,

P. Lopes
power systems


A. Do you need to use a no-clean flux for your process? This is one of the more difficult to use processes, and consequently will require a very solderable surface. But to the meat, is Alloy 90 a 90/10 Nickel/Iron alloy, as the name would imply? If so, keeping this gem solderable could be a challenge. Probably some sort of plating is mandatory. But it is difficult to plate very small wires.

Would you consider immersion plating? If so, we have recently put together an immersion plating bath for Palladium that seems to work well on pure Nickel. This might work well for you.

This finish is inherently not the easiest to solder to in the world, some no-cleans work well, and the good news is, that if it works well fresh, it will not change solder-ability characteristics over the next year.

Rudy Sedlak
Mountainview, California


A. Alloy 90 is a copper nickel alloy 88 Cu/12 Ni. The 90 comes from the resistivity of the material (90 ohm-cmil/ft).


Todd D [last name deleted for privacy by Editor]
fuse mfgr. - Des Plaines, Illinois

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