[Noisebridge-discuss] non-invasive DC current sensor question

[Gopiballava Flaherty]

On Nov 16, 2012, at 5:26 PM, Andy Isaacson <adi at hexapodia.org> wrote:

> You have to be able to separate the DC+ from DC- for the effect to work.

To clarify, each of the two wires generate opposite magnetic fields. If the wires are next to each other, the fields cancel out and you measure zero current. You need to put your probe around just one of the wires. (Also true for AC)

Some probes are able to be split so you can put a wire through them and then close them up around it. Others are not splitable, so you have to put the wire through or possibly wrap it around a few times. More wraps == more sensitivity.

(As an aside, if you decide to measure current using a current sensing resistor, make sure you pay attention to the way your chip is supposed to be used. If it's a "high side current sense amplifier", it really does expect to be used in the right part of the circuit. You may think that you're measuring a small voltage across a resistor, but there's also the difference between the terminals of the resistor and ground. That difference is gonna bite you. If you're using straight op-amps, you need to pay attention to the common mode rejection ratio - the voltage range relative to ground that you can put into the amp. Even if the voltage you're measuring from your current sense resistor is less than a volt, if the voltage relative to ground is high then your op-amp will fail. I like the Linear LT1990, which can have a +/-250 volt input range…)

Thanks,

gopi.


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