I want to measure how much power my PIC is using. At the output of my 3.3V regulator I put a 1 ohm resistor in series with the rest of the circuit. I was going to measure the voltage drop across that resistor to get the current and so on. With the 1 ohm resistor installed the circuit works just fine but when I put the probe of my o'scope across it it my 3.3V goes to zero. It seems as though the scope is creating a short to ground or something. Does anyone know why this happens or how to make it stop?
Answer
The ground clip on your o-scope is actually tied to ground. it is a hard short and rather low resistance. This means that you are shorting the 3.3 rail to ground with your ground probe.
To fix this there are two options,
- Put the resistor in the return path so that one side of it is ground. That way the ground probe does not hurt it.
- Use two probes, one for each side of the resistor and use the math function on your O-Scope.
Let me know if this is not clear. I can add more information.
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