I have a circuit that switches a 50W Sunbeam heating pad via a relay (for regulating temperature). My circuit is driven by a 5v AC/DC switching converter. The AC input to the power converter is shared with the heating pad.
When the heating pad is switched off, I notice a lot of noise in the 5v DC output of my power supply. I also notice a lot of noise in the same DC output as I change the heating pad thermal output (from off, to low, to medium, to high, and back). Every "transition" on the heating pad seems to generate a lot of power noise to the microcontroller in my circuit (see o-scope images).
I've tried a simple RC snubber on the AC input to my supply (0.033uF + 120Ohm in series), and also a filter on the 5V DC output (2 electrolytics in parallel... 470uF and 150uF, separated by a 0.47uH choke). Nothing I've tried seems to affect the noise. The AC/DC converter I'm using is the XP Power ECE05
Any ideas? These power spikes are causing my controller to reset.
The switch in the schematic isn't there in the real circuit. The base of the transistor is controlled by the GPIO pin of an Atmel processor, but this illustrates the point.
NOTE: I get identical noise when everything after the power supply filters is replaced with a 50 Ohm 10W resistor. The oscilloscope captures above were all done with just that 50 Ohm resistor as the load so I could eliminate the rest of the circuit as the problem.
- Connecting the heater to a different AC circuit than the power supply and o-scope did make the noise go away.
- Connecting the heater and power supply to the same AC circuit but putting the o-scope on a different circuit did not make the noise go away.
- Adding a 0.1 uF mylar cap in parallel (and after) the existing filter caps added some attenuation to the noise (a volt or two). Other (smaller) ceramic caps I had also did little.
- I noticed that switching the heating pad temp setting a rate of about once a second didn't produce any noise. Once every 1/2 second produced a few noise spikes every couple seconds. Rapid switching caused noise everywhere on the o-scope trace.
I added an EMI filter (EMI103T http://www.bourns.com/data/global/pdfs/emi_t_series.pdf) to the power supply output and I also tried it at the AC input. Neither configuration had the attenuation I would have expected.
Close up of the noise signal. 8-10MHz range. This is why I thought an EMI filter with 15db at 8MHz would have a positive effect... I'm confused why it didn't have more than a 2-4V attenuation effect on the noise on the DC output.