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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?

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    \$\begingroup\$ what kind of scope are you using? a model number (or even better, a link) would be informative. \$\endgroup\$ – JustJeff Oct 6 '10 at 22:03
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    \$\begingroup\$ Why not just use a DMM? \$\endgroup\$ – Brad Gilbert Oct 6 '10 at 22:53
  • \$\begingroup\$ I would guess that the current varies with respect to time. \$\endgroup\$ – Kortuk Oct 7 '10 at 4:31
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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.

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    \$\begingroup\$ Third option is to get an actual current probe, but those can be pricey. \$\endgroup\$ – Thomas O Oct 6 '10 at 21:40
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    \$\begingroup\$ good scopes usually have the option of isolating or 'floating' the inputs, which would be exactly what you want in this situation. \$\endgroup\$ – JustJeff Oct 6 '10 at 22:02
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    \$\begingroup\$ 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. \$\endgroup\$ – Kortuk Oct 7 '10 at 0:06
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    \$\begingroup\$ 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. \$\endgroup\$ – Connor Wolf Oct 7 '10 at 10:40
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    \$\begingroup\$ @Jordan S - I looked in the manual for the TDS-200 series, I don't think it has built-in capability to float inputs. \$\endgroup\$ – JustJeff Oct 7 '10 at 22:31
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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.

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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.

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

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  • \$\begingroup\$ Here's a discussion 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. \$\endgroup\$ – Nick Alexeev Jun 14 '14 at 5:43
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    \$\begingroup\$ 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... \$\endgroup\$ – Blup1980 Jun 14 '14 at 16:51

protected by Dave Tweed Jun 14 '14 at 11:26

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