This is one of the parts of the blog instructions that I found confusing. In his blog post, Scott talks about making sure your PID controller has a Solid State Relay (SSR) output, but in his parts list he has you buy a regular mechanical relay. The difference is that an SSR uses a transistor to turn on and off current through your contacts, while a mechanical relay uses an inductor and a magnet to essentially flip a tiny switch inside the relay to complete the circuit.
Typically an SSR will last longer than a mechanically relay as there are no moving parts. Also, using a 1500 watt heating element you’re drawing roughly 13 amps through the circuit. This means that your relay needs to support a 13 amp load and depending on your PID, the control voltage to operate the relay needs to be around 5 volts (the first PID controller I got had 6 volts across the outputs). It’s hard, but not impossible, to find a mechanically relay that lives up to these specs. On the other hand, it’s relatively easy to find an SSR that does. Another nice thing about going with the SSR is that it eliminates the need for any soldering in this project.
The downside of SSRs is that unlike a mechanical relay, they generate heat when in use. The more current on the load, the more heat they generate. The good news is that the site I used to buy my SSR also sells a matching heat sink that can be attached using thermal grease (I used some arctic silver I had left over from mounting a processor heat sink) to the back of the SSR. The bad thing about the combined heat sink SSR combo is that together they barely fit into the acrylic enclosure. What I ended up doing was mounting them last. Once they’re in, it’s really impossible to work inside the case without removing it.
To mount the SSR heat sink combo, I drilled two small holes in the side of the enclosure as far towards the back as was possible with the SSR and heat sink still inside the enclosure. I used small nylon bolts and nuts with a nylon washer between the sink and the enclosure to avoid transferring heat from the hot sink to the plastic of the case. To let the heat from the sink dissipate, I cut three small slits in the top of the enclosure directly above the heat sink.