Electronics > Beginners

measure current without disconnecting 'device'

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mohf:
Hi guys,

I've been thinking and wondering, why is it not possible to measure DC current without disconnecting your circuit/device/...? Like: having an ammeter which measures amps by connecting it parallel instead of series.

I don't know if something like this exists (can't find anything). But after thinking, I thought about doing something like this:

The "new" ammeter first measures the voltage, then charges an internal capacitor to 0.1 volts lower. After this voltage is reached, the device connects the internal capacitor (with maybe a resistor in series) to the parallel pins of the meter, measures the current flowing inside this capacitor. By doing some RC calculations you should be able to calculate the resistance & current.

I don't know if this is possible, maybe I am saying just BS and it's just not possible. I am NOT an expert at electronics and have NOT studied electronics engineer (yet!), I am just a hobbyist. Was just wondering and though, why not just ask.

Sure such a circuit would be much more complex, less accurate, slow, inefficient and has a lot of limitations (like taking note of the circuit's capacitance)  than just plugging the meter in series. But in some cases it's not possible to do so, and having an approximate measurement is OK too.

HLA-27b:
Read about clamp meters in wikipedia, the wheel has been invented already.

http://en.wikipedia.org/wiki/Current_clamp

http://en.wikipedia.org/wiki/Hall_effect


bilko:
Or put a small resistor in series with the load and measure the voltage across the resistor. Use ohms law to calculate the current.

Rufus:

--- Quote from: yachtronics on April 30, 2012, 07:54:55 pm ---Or put a small resistor in series with the load and measure the voltage across the resistor. Use ohms law to calculate the current.

--- End quote ---

An ammeter is a small resistor placed in series with the load.

The OP is suggesting using a bit of the existing circuit as a shunt with some fuzzy thinking about shunting the shunt with the meter and a capacitor to determine what the value of the (existing) shunt is.

It could sort of work not very well. A better idea would be for the meter to inject its own current and measure the resulting voltage across the shunt.

Just tried it, stuck an unknown current through about 4 inches of 22 swg tinned copper wire and measured the voltage across it, got 3.849mV. Stuck an extra amp through the wire and got 7.325mV.

That makes the shunt 3.476 m ohms and the unknown current 1.107A. The unknown was actually 1.113A.

Another approach would be to have the meter generate an opposing current in the shunt and servo it for zero volts across the shunt. The generated current being what you measure.

Need kelvin connections and have small signals with possible thermal emf problems either way.

bilko:

--- Quote from: Rufus on April 30, 2012, 08:40:25 pm ---
Another approach would be to have the meter generate an opposing current in the shunt and servo it for zero volts across the shunt. The generated current being what you measure.

Need kelvin connections and have small signals with possible thermal emf problems either way.

--- End quote ---
I think that you might upset the circuit a little bit. The op is talking about a powered circuit, he has not defined which part of the circuit or what circuit he wants to measure. It could be hf, it could be base current to a transistor, it could be part of a feedback loop, it could be anything. Your suggestion will only work in very limited cases

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