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?
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.
Does the circuit need to be grounded? If you can supply it from a battery or double-insulated power brick, you should be able to connect any single point to the scope probe's Earth ground without affecting the functionality.
Maybe another channel is already tied up to the real ground of the system, of the scope is sharing the same real ground in another way. It's not like the normal probes on your scope are + and - , they are actually Voltage and Ground (single-ended).
If you really would like to measure + and - , you would need a differential probe. But I guess you haven't got those lying around, so using 2 probes on both sides, or moving your resistor to the low-side of your device could work more practical.
Use a B&K (or comparable) isolation transformer to'float' the scope.
protected by Dave Tweed♦ Jun 14 '14 at 11:26
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