5
\$\begingroup\$

I have a question regarding SSRs. I am working on redesigning a PCB for a medical device that needs to pass class B radiated emissions, therefore radiated EMI must be very low.

We have a CPU fan running off 12 V (~890 mW) and I was going to try using a SSR to control its operation, due to the cost savings of an SSR vs. a standard mechanical relay. Do you think the greater "on" resistance of the SSR (1-2 ohms vs. milliohms) could lead to increased EMI? The fan does have a dedicated hub driver.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Only when switching, which won't be often if what you are replacing is an electromechanical relay. \$\endgroup\$
    – DKNguyen
    Commented Sep 4, 2020 at 14:01
  • 1
    \$\begingroup\$ Do you think the greater on resistance of the SSR (1-2ohms vs milliohms) could lead to increased EMI? I think that the resistance has almost no relation to EMI emissions. What does relate to EMI emissions is fast transitions like voltage and current spikes. Those spikes can occur when you switch on and off. What helps a lot is making sure the current loops where those spikes occur are short so I would put a small capacitor (1 nF) across the SSR (on the switching side). Maybe I would also add a capacitor (again 1 nF) in parallel with the fan. That by itself should be enough. \$\endgroup\$ Commented Sep 4, 2020 at 14:21
  • 1
    \$\begingroup\$ Many solid state relays have controlled turn on/off times. You need to look at the data sheet for the candidate parts for this feature. \$\endgroup\$
    – SteveSh
    Commented Sep 4, 2020 at 18:23

1 Answer 1

5
\$\begingroup\$

The SSR will almost certainly produce less EMI. A physical relay can suffer from bounce and repeated transients due to the rapid current and voltage fluctuations this can induce. Breaking the circuit while current is flowing can also create noisy sparking. SSRs do not have these problems.

Resistors are not strong sources of EMI, don't worry about that.

The key to reducing EMI from any switch is to perform the switching as slowly as is practicable. This reduces both the peak of the switching transient and also the presence of the higher frequencies which cause most of the trouble. Best to avoid operating the SSR - or any switch - with a fast rise time. If you have to and it causes problems, then slugging the input with a low-pass RC filter is something you might like to investigate. But don't switch too slowly or you might dissipate too much power in the SSR and damage something: data sheets are your friend here.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.