Triac SSR, a G3NE series AC type, causes noise in active load.

Thread Starter


Joined Jan 18, 2017
I have a device that controls a silicone rubber heater, which is an active load and whose resistance does not change a lot when heating.
250W heater for 110V has about 50 Ohm cold resistance, which is about 2A which is about what it is at 120V at full power.

I control it with a G3NE-205T-US DC5 SSR, which is a 240V AC capable 5A AC triac switch with opto input.
The SSR is control from a digital circuit that puts 5V across the input, a MOSFET Low Side switch (pull to GND).
The SSR is zero-crossing, and is beings switched for at least 10 seconds ON and then 30 seconds OFF. So, no switching noise due to firing angle control, no such nonsense.

I have conducted emissions when the relay is ON. When it is off I am way down under the CLASS B FCC. When it is on, I am way over, about 5dB or so. Or 5dB under CLASS A.

So why I think it is the SSR? The last test I did was, while the SSR was energized and the board was thinking it was controlling it by applying the ON voltage, I shorted the relay's output. And the noise immediately went away, with the load being connected to power and the input being controlled as usual.

I do not get it what could be causing this. Yes I could put am RC snubber and all other stuff, but I would like to understand, where the noise is from?
(Oh......the noise is ... 150kHz. Which is nothing I have in mhy controls, and the DCDC power supply that makes 5V is 100kHz.)

Ideas, anyone?