I have a 50W LED driver and a programmable load. Please suggest a way to measure 120Hz ripple current! I have an oscilloscope and multi-meters.

  • \$\begingroup\$ Set load to constant voltage. Hook up scope. Set scope to AC coupling. Measure the ripple. \$\endgroup\$
    – winny
    Jun 4, 2018 at 19:42
  • \$\begingroup\$ A current shunt is easy but one has to know the voltages in use and currents expected and you have defined neither \$\endgroup\$ Jun 4, 2018 at 19:43
  • \$\begingroup\$ winny defined how to measure ripple voltage which may be what you intended. \$\endgroup\$ Jun 4, 2018 at 19:44
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    \$\begingroup\$ If you did want to measure the current (and not the voltage) do you have a current probe for your scope? \$\endgroup\$
    – John D
    Jun 4, 2018 at 19:50
  • \$\begingroup\$ The best way is with a current probe. Second best way is a shunt with a differential probe. Sometimes you can use a ferrite bead or fuse as a shunt (if you know the resistance of it). You can also maybe use a current transformer, which is pretty similar to a current probe. If you are able to put the Ammeter in series with the current, you can measure with the multi-meter. But you won't be certain you are measuring 120 Hz. You won't be able to see the signal, so you won't really be sure what is going on. \$\endgroup\$
    – user57037
    Jun 4, 2018 at 19:56

1 Answer 1


"Ripple current <2% at 120Hz" means the power supply has decent regulation with input line induced load ripple from line bridge input to DC-DC converter that would not be visible. No need to measure it unless you need to verify the spec.

If you used any reverse biased photo diode (PD) the voltage into a load R will show 2% FS ripple on the current with all other light sources turned off driven by 60Hz or blocked radiance to PD.


simulate this circuit – Schematic created using CircuitLab

If you don't have a PD, use any clear LED.

  • \$\begingroup\$ This is a good idea. \$\endgroup\$
    – user57037
    Jun 5, 2018 at 4:00
  • \$\begingroup\$ @Tony Stewart. Thanks for explaining. I think I found my answer as I was suspecting the same that, we can't really observe 120Hz ripple at the output of DC-DC converter, if it is doing it's job. And yes I am verifying a spec in this case. Thanks for your time in answering my question. Also perhaps there must be an easier measuring technique for output ripple by just observing the Output V using 1x probe with low-noise technique! \$\endgroup\$ Jun 5, 2018 at 20:22

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