# How to measure 120 Hz ripple current at the output of an LED driver?

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.

• Set load to constant voltage. Hook up scope. Set scope to AC coupling. Measure the ripple. Jun 4, 2018 at 19:42
• A current shunt is easy but one has to know the voltages in use and currents expected and you have defined neither Jun 4, 2018 at 19:43
• winny defined how to measure ripple voltage which may be what you intended. Jun 4, 2018 at 19:44
• If you did want to measure the current (and not the voltage) do you have a current probe for your scope? Jun 4, 2018 at 19:50
• 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. Jun 4, 2018 at 19:56

## 1 Answer

"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.

• This is a good idea. Jun 5, 2018 at 4:00
• @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! Jun 5, 2018 at 20:22