emcrelaysolid-state-relay
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.
SSRs use triacs as switching elements, and those are for AC, not DC. A triac is like two transistors so arranged that, once ignited, they keep each other switched on. The only way to switch it off is to reduce the current to below the hold current. AC devices do this 100 or 120 times per second, at zero crossings of the mains, but at DC it will not switch off.
Instead of interrupting the circuit you can also briefly short-circuit the triac's terminals to switch it off.
The Solid State Relay linked in the question will not work for loads which could operate in either direction. For such purposes, search for SSRs specified for bidirectional DC or AC/DC loads (as distinct from just DC, AC or AC/Zero-crossing types).
While narrowing down to a specific such part will involve price, availability, voltage, package, size and perhaps other considerations, here are a few examples to illustrate that such relays do exist. Note how they are specified, to help extend the search for the ideal part:
Unfortunately Digikey does not seem to carry any bidirectional load SSRs specified for greater than 6.5 Amperes, so the search needs to continue elsewhere.
Alternatively, consider limiting the current across each SSR to below the maximum load current rating of the part.
Best Answer
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.