I have a microcontroller based circuit that sleeps most of the time and works in short bursts of less than 1 milisecond and I don't have access to a digital osciloscope so I could record this. I only have a simple one channel analog scope.

So, how did they do that in old times? Is there a way to set up some external circuitry to measure current that is drawn in short bursts?

What I'm thinking is a small low side resistor with a capacitor in parallel, but I'm not sure...

  • \$\begingroup\$ One possibility was to have an oscilloscope camera - set the scope for single trigger and open the shutter :) \$\endgroup\$ – W5VO Dec 1 '12 at 20:15
  • \$\begingroup\$ Just do it repeatedly and look on the analog scope. \$\endgroup\$ – starblue Dec 3 '12 at 21:27

An indirect way to measure the magnitude of a current pulse would be to power the circuit from a capacitor. During each burst of activity by the processor, the capacitor voltage will drop by a value that's proportional to the charge (current × time) consumed during the burst.

ΔV = ΔQ / C = I × Δt / C

As long as you know what C and Δt are, you can determine the value of I:

I = C × ΔV / Δt

Obviously, the capacitor voltage will also "droop" over time because of internal leakage current and any quiescent current drawn by the load, so be sure to take this into account.

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How often do your short bursts occur? Can you tweak the system so they occur fairly often? Then just turn your trace intensity up.

Anyways, the way measurements on things like that were taken in the old-days was by using an Analog Storage Oscilloscope , which uses either specialized phosphors or a specialized grid arrangement, together with additional electron guns to maintain an image of the analog waveform after only one sweep.

One easy way to kind of "fake" a storage scope is to simply use a camera which can take long exposures.
Point the camera at the scope screen and focus it, turn off the room lights, and open the shutter. Assuming you have the scope set up to trigger properly, you should get a nice image of the single sweep.

If you just want to measure average current draw, your simple low-side resistor, preferably with an op-amp and subsequent R-C filter would work.

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  • \$\begingroup\$ Temporarily making rare events happen much more frequently can indeed be an excellent technique when investigating a microcontroller circuit with a plain analog scope - you combine what the scope can do (give you a picture of repeating events) with what the micro can do (change behavior to support debugging). \$\endgroup\$ – Chris Stratton Dec 2 '12 at 4:41

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