Finally figured out how to get rid of the interference of my desk LED lamp

[bayjelly]

So I've had this "PHIVE" desk LED lamp for a while. I like it, but the problem that I always had with it was that it generates so strong interference that I often had to turn it off when using the scope, which is extremely annoying obviously.

Seriously bad, I'm talking ~500mV p-to-p spikes when the ground lead is connected to the spoke tip, enough for false triggers even from low impedance sources. Setting the scope to 20MHz bandwidth limiting would help, but of course that's no way to live.

So I thought, maybe not the lamp itself but the power supply is the problem, and connected the lamp to my (linear) lab supply instead. With good news and bad news.

The bad news is that the interference is exactly the same with the lab supply, so it's generated in the lamp itself (hence ferrite cores also did nothing). The good news is... it goes away completely, and I mean entirely, once the voltage fed is below ~9.5V. The power supply it came with is 12V. But above around 9.4V, the interference suddenly kicks in in full strength. It's binary: Either quiet at low voltage or fully kicking at higher voltage.

I know nothing about strong LED lighting, my guess is that below 9.5V the current to the LED is constant, and above it it's switching? Anyway, I ordered a 9.3V power supply from eBay and it looks like I can now keep the lights on when using my scope.

Maybe that helps someone else with similar problems.

[tooki]

My guess is that it uses a buck converter with an output of around 9.5V (3 LEDs in series are right around that voltage), and that below that, the converter simply doesn't switch at all.