I had been running for over a year with my 24V supply hooked directly to RAMBo and then to my Onyx. When I rewired my Rostock a couple of weeks ago top permanently use my Azteeg X3 Pro I decided to "do the right thing" and use an SSR to control the Onyx. It had been working for a couple of weeks and just now, I noticed that distinctive "magic smoke" smell. Then I heard sparks so I immediately unplugged everything. When I opened the bay the smell was bad and the Fotek SSR-25 DD was extremely hot - way too hot to touch. The wires in and out were cool but the SSR was melting down. I've not experienced this with an SSR before and I got the 25A version as I thought that was plenty, maybe not.It turns out that simply looking at the peak amperage the SSR can handle is not sufficient. The issue is many inexpensive DC-DC SSRS have a high voltage drop which results in them getting hot, VERY hot. Even a heat sink with a cooling fan is not always sufficient. So be safe and look for a DC-DC SSR with a low voltage drop. Luckily, a good, affordable option exists - the Auber 100A #MGR-1DD80D100. Here is the product description:
I use these exclusively on all of my 3D printers and they have been performing very well. I do have them mounted on a heat sink (no fan) and they are not even warm to the touch.