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Strange Current Sense Resistor

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Firefox

Sun Dec 28 2008, 06:04AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I need some current sense resistors for a project of mine, and I've found two nice large ones in an old server PSU, but I am unsure of their value. I am guessing they are .008 ohm resistors as they are black-grey-silver-silver-green. However, this layout seems somewhat strange, as I have never seen a resistor with as low a value. What exactly are these resistors?

Dr. Dark Current

Sun Dec 28 2008, 09:05AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You can run e.g. 10amps through them and then measure voltage. Even 1 amp is enough if your DMM is precise enough.

A resistor of such low value can indeed be used for sensing current.

Proud Mary

Sun Dec 28 2008, 05:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

For future reference, a small reel of nichrome or manganin resistance wire is an inexpensive investment that will allow you to make all kinds of very accurate lo-ohm resistors to any value you need.

PS: Don't forget to use the 4-wire Kelvin method when trying to measure any very low resistance, or your results will be meaningless.

3l3ctrici7y

Mon Dec 29 2008, 11:51AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

I've built a number of current sense resistors (usually called a shunt).

The basic gist is you take a known length of a known gauge of wire (which will imply a certain resistance), run your current thru it, and measure the voltage across the wire.

Decide how many amps you expect the load to draw, and what sort of voltage drop and current sensing resolution is appropriate for your application. Go to <http://powerstream.com/Wire_Size.htm>, select a wire gauge, type 1 amp as the load, and adjust the length of wire until it produces the voltage drop (per amp) that you want, or select a different wire gauge if the wire is getting too long or short for ease of construction. Another thing to trade off in the wire gauge is how precisely the voltage tap for measuring the current has to be placed along the length. If the shunt is 30cm long, you have to be much more carful placing the taps than if the shunt were 300cm long. Take that length of wire and double it (since the calculator assumed round trip distance), and measure the voltage across it under load.

If you want to read a meter in real time and read off the amps, then you will need to calibrate the shunt (the position of the taps); otherwise, you can simply measure the voltage drop at a known current, and then in the future when taking readings, just multiply the measured voltage and the known voltage drop of the shunt. A known wattage light bulb connected to the mains is an easy way of getting a known current.

uzzors2k

Mon Dec 29 2008, 02:39PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Just remember that a wire-wound resistor like that will often have considerable inductance. Keeping it straight and only measuring low frequency current will keep the error acceptable, but low inductance is required for high frequency measurement.

Proud Mary

Mon Dec 29 2008, 05:32PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Firefox's 0R008 resistor would surely look like a short piece of wire - the leads and solder junctions of a no-ohm link resistor would surely measure 8milliohm or thereabouts!

3l3ctrici7y

Mon Dec 29 2008, 05:59PM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Uzzors; Ah, this is a good point that I forgot to mention.
The shunt I built that drops 100mV/A is just less than 3m long.
I bent the wire in half, then wrapped it around the former.
Half of the turns are in one direction, and the other half are in the other direction.
While that certainly did/will not remove all inductance, it should certainly help.

What is your definition of high and low frequency?
I'm don't know what Firefox is doing, but I've only measured DC and 60Hz mains.

uzzors2k

Tue Dec 30 2008, 11:50AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I don't know what Firefox is up to either, but with such a small current sense resistor it's probably DC measurement so any minute inductance won't be much of an issue. Any time I use current sense resistor it's in conjunction with a current transformer and signals in the 100kHz range, where inductance is much more important.

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