# how to determine the resistance of a running circuit

1. ## how to determine the resistance of a running circuit

sometimes you want to know the resistance of a running circuit, some components have different resistance in off or on state, like MOSFETs or bjt, how can I accomplish it without ruining the circuit?
it will be useful to calculate the dissipated energy of a continuous circuit and its efficiency though.

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2. ## Re: how to determine the resistance of a running circuit

If you put a small resistor in series with the device, you can measure the voltage across it to get current, then measuring the voltage across the device you can calculate the resistance.

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3. ## Re: how to determine the resistance of a running circuit

Originally Posted by barry
If you put a small resistor in series with the device, you can measure the voltage across it to get current, then measuring the voltage across the device you can calculate the resistance.
Why to bother and put resistor In series when I can measure current and voltage of whole device? Any particular reason?

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4. ## Re: how to determine the resistance of a running circuit

Your original post said you don't want to 'ruin the circuit'. How do you propose measuring the current without breaking a path an inserting an instrument (e.g., meter)??

5. ## Re: how to determine the resistance of a running circuit

assuming your device has a power input and return, you can measure the voltage across those inputs
to measure the current, you need to disconnect the return and put in a resistor and measure the voltage across the resistor as barry said
OR, if you have a clamp on current probe, either stand alone type or connect to scope type, use that, which is essentially the same as barry said

that will tell you the input power.
if you also measure the output power, you should be able to calculate dissipate energy and efficiency.

circuits with lots of I/O can be a bit more complicated, as each I/O moves energy around

maybe a photograph.

better information provided yields a better answer

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6. ## Re: how to determine the resistance of a running circuit

First, we can easily get the voltage across the device.
Second, we can get the current through the device.
The method depends on the meaning of "not ruining" in your sentence.
If it means that we can insert an instrument in the circuit while not changing the structure of the circuit, we can directly insert an amperemeter here to measure the current.
If it means that we cannot interrupt the circuit, we can measure the voltage and the resistance of another device to calculate the current.
Third, we can calculate the resistance of the device with its voltage and current.

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