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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?

PICyourBrain
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4 Answers4

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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,

  1. Put the resistor in the return path so that one side of it is ground. That way the ground probe does not hurt it.
  2. 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.

Kortuk
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    Third option is to get an actual current probe, but those can be pricey. – Thomas O Oct 06 '10 at 21:40
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    good scopes usually have the option of isolating or 'floating' the inputs, which would be exactly what you want in this situation. – JustJeff Oct 06 '10 at 22:02
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    You have nice scopes than I have used. It normally causes very large noise isolation problems by leaving that connection floating, and it needs to be very low inductance. – Kortuk Oct 07 '10 at 00:06
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    The not-so-nice scopes can also generally do this as well. Just break the ground lead of the power cable off (or use one of those three-prong to two-prong adapters). Note: doing so can be dangerous, because a failure in the scope will no longer short to ground, and trip a circuit-breaker. I've never had any problems, but don't leave it like that all the time. – Connor Wolf Oct 07 '10 at 10:40
  • It's a Tektronix TDS 210. A getting few years on it now but it's quality. – PICyourBrain Oct 07 '10 at 13:14
  • @JustJeff: Where would I find the setting to change the input isolation? – PICyourBrain Oct 07 '10 at 13:16
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    @Jordan S - I looked in the manual for the TDS-200 series, I don't think it has built-in capability to float inputs. – JustJeff Oct 07 '10 at 22:31
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    @FakeName: One way I've seen used to isolate a scope is to use an isolation transformer on the mains to the scope. Same cautions about hot chassis apply though, so be careful! – Jesse Oct 10 '10 at 20:02
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    I would prefer do that with the power supply because I only rarely touch my power supply. Does not sound like a great idea with either though. – Kortuk Oct 11 '10 at 02:37
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    Do not break the ground connection of the scope's mains plug. Also, better don't use an isolation transformer to supply the scope. Instead, float the supply of the circuit under test; then, the protective earth of the scope can remain in place and do its job of providing user safety. – zebonaut Jan 08 '11 at 14:07
  • @zebonaut, I think nothing needs to be floated here, these are just a few peoples opinions on ways around it. In reality there are 18 different ways to get around this problem. – Kortuk Jan 10 '11 at 00:27
  • Most wall warts and nearly every single lab supply is isolated and floating. Must be an odd supply. – Cybergibbons May 08 '12 at 11:57
  • @Cybergibbons, that is something I would not test my luck with. I use floating scopes at work to get around things like this on the units I would work. – Kortuk May 08 '12 at 22:47
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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.

endolith
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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.

Hans
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Use a B&K (or comparable) isolation transformer to'float' the scope.

mb-texas
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  • [Here's a discussion](http://electronics.stackexchange.com/a/73994/7036) on why floating an oscilloscope is not a good idea. On a different note, I'm not sure that isolation transformer at the oscilloscope nor at the DUT would reliably addresses the original question. Finally, your post is a bit thin for EE.SE . You should substantiate (or at least, corroborate) your suggestion. – Nick Alexeev Jun 14 '14 at 05:43
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    In order to give the info to people that doesn't want to read the discussion: Floating a scope could be deadly to the user. which is somehow *bad*... – Blup1980 Jun 14 '14 at 16:51