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I have an LED driver (LM3410) that adjusts it's voltage based upon how many LEDs you place in series to it and I'm curious to see what voltage it is actually outputting. To test it I connected my oscilloscope probe on the v+ output and ground and after getting a little smoke I'm a little worried about trying again. I'm pretty sure I put the thing into over-drive as it tried to "power" my oscilloscope. My over-voltage protection seems to have saved it since it still seems to work (miraculously).

So my question is: what is the proper way to measure voltage output from a DC-DC boost converter that changes it's voltage based on load?

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  • \$\begingroup\$ What kind of oscilloscope are you using? \$\endgroup\$
    – Dave Tweed
    Commented Jan 29, 2013 at 21:24
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    \$\begingroup\$ You should use your oscilloscope in high impedance mode, rather than 50 ohm. Using a high impedance probe (if it is a hook/grabber it probably is) the amount of current going in to the probe is negligible. Where did the smoke come from? \$\endgroup\$ Commented Jan 29, 2013 at 22:10
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    \$\begingroup\$ Do you get smoke only when the probe and the scope ground are connected, or is just connecting the scope ground to your circuit sufficient to get smoke? \$\endgroup\$
    – Phil Frost
    Commented Jan 29, 2013 at 22:10
  • \$\begingroup\$ Adding a scope probe to the output of a DC/DC converter should not cause it go to into overvoltage protection (not by any means I know of, anyway) - if the feedback got shorted, maybe, but the output? \$\endgroup\$ Commented Jan 29, 2013 at 22:20
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    \$\begingroup\$ What scope, what impedance probe/input, and where are you connecting your ground clip? A diagram might help. \$\endgroup\$
    – Oli Glaser
    Commented Jan 29, 2013 at 23:52

2 Answers 2

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From the problem description, two possible hypotheses come to mind:

  1. There is a ground level mismatch: The DC-DC boost output has a ground that is different from your oscilloscope's ground
  2. The probing was done with no LED load, so the boost circuit is attempting to generate the highest voltage it can. How high this can be depends on the circuit design, of course.

With some additional detail on the boost conversion design, additional hypotheses, or a definitive answer, might be possible.

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The proper way to measure the out voltage of a current driver is by probing in parallel with the load. Make sure that your oscilloscope is set to "High Impedance" or "High Z". If you want be take further precautions, you can use a 100x probe.

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