Pro/Forums - View Single Post

Since87 · 05-25-2003, 10:44 PM

I've been thinking more about how to measure the current through the switching inductors.

It's turning out to be an uglier problem than I thought at first. The big problem is the resistors to go in series with the inductors.

A length of copper would be great, except for one problem. Copper has a Temperature Coefficient of Resistance (TCR) of 0.4%/C. In other words, unless the copper is kept at a stable temperature, the resistance won't be known and it will cause error in the current measurement. The most practical replacement for copper is probably Nichrome with a TCR of 0.017%/C. However Nichrome has 67 times the resistivity of copper, which means that the shunt resistor would need to be physically large.

Looking through the Digi-Key catalog I found some 0.005 Ohm Nichrome resistors that would do the job. (Ohmite 620HR005) It would take four of these total. Two resistors paralled, in series with each inductor to provide a voltage proportional to the current flow to feed into the rest of the circuit.

The resistors are 1.675" long, and it might be a cramped fit to get them installed, but once the resistors were in place the rest of the circuit could be put in a more convenient location.

So are you up for doing a hack job on your motherboard?

05-25-2003, 10:44 PM	#84
Since87 Pro/Guru - Uber Mod Join Date: Sep 2002 Location: Indiana Posts: 834	I've been thinking more about how to measure the current through the switching inductors. It's turning out to be an uglier problem than I thought at first. The big problem is the resistors to go in series with the inductors. A length of copper would be great, except for one problem. Copper has a Temperature Coefficient of Resistance (TCR) of 0.4%/C. In other words, unless the copper is kept at a stable temperature, the resistance won't be known and it will cause error in the current measurement. The most practical replacement for copper is probably Nichrome with a TCR of 0.017%/C. However Nichrome has 67 times the resistivity of copper, which means that the shunt resistor would need to be physically large. Looking through the Digi-Key catalog I found some 0.005 Ohm Nichrome resistors that would do the job. (Ohmite 620HR005) It would take four of these total. Two resistors paralled, in series with each inductor to provide a voltage proportional to the current flow to feed into the rest of the circuit. The resistors are 1.675" long, and it might be a cramped fit to get them installed, but once the resistors were in place the rest of the circuit could be put in a more convenient location. So are you up for doing a hack job on your motherboard?