My printrbot's motherboard (printrboard, as it is called) has a mosfet for controlling a heater for the print bed. Early on with the printer I noticed that this component tends to get a little toasty, so I used its 12V output to control a relay which handled the load better.

Except the relay goes CLACK ... CLACK ... CLACK as it turns the heater on and off to maintain a consistent temperature while the printer is running.

Yesterday I replaced the relay with a solid state relay (SSR), and it's certainly a relief to avoid the periodic clicking sound! I'm going to run some hour-long tests on it today to make sure everything works well, and then put it to use!