Quick diagram I whipped up showing how to connect an SSR to wall-outlet power. It was useful in keeping someone from electrocuting themselves/burning their house down, so I figured it might be useful to anyone else that’s interested in using mains power for their heated bed. This is the setup that I use, and it doesn’t break a sweat. Do not let that thermistor ever fall out of place though. It’s more dangerous, but certainly worth it if you don’t like waiting 10 minutes for your bed to heat up.
Should consider putting something like this on there as a backup. http://www.pepiusa.com/modjs.html
It will open the circuit if it overheats, and stay open until power is removed and it cools down.
Choose one with a trip point above normal max operating temperature for the bed.
For safety reasons I prefer not to power my bed with AC. I’ve read that even running a 24V 400W alirubber bed directly from the power supply and controlled by an SSR will significantly improve heating times. Is this correct?
I’ve speced out the following SSR: Fotek SSR-40 DD DC-DC 40A 3-32VDC Input 5-60VDC Output
I’m not very familiar with how an SSR works but i’m assuming in my case, input is 0-24V (from D08) and output is 0-24V as well. Does it matter that the above SSR has a different input and output range?
@Eugene_Lee , you look pretty good on your selection. The reason I run mine on AC power is lower amperage. 12v @ 400W = 33A! Crazy high!; 24v is still 16.6A! That’s why those PCB heatbeds take so long to warm up; they’re nowhere near 400W. At 110v AC (USA) I’m only looking at around 3A; which is much more manageable from a component perspective. Granted, once everything is heated up - you’re only going to be pulsing that power at 20%ish duty cycle, but I’m concerned about cold-start.
I wish there were more thermocouple inputs, so that they can be used to check temps redundantly. There are other ways to cut power should thermo or SSR fail, none seem quite as elegant.
If you do wire AC directly into your printer make sure you are putting the ‘line’ side to the SSR and ‘neutral’ to the heat bed. In north america Neutral and Ground should have no potential[voltage] between them.
@Daniel_Joyce , do you have a specific resettable fuse that would disconnect at a temperature that you recommend? I’ve been doing a little bit of research and triacs do indeed tend to fail shorted (vs open) so now I need to add that circuit to be careful.
Just realized you might want to use a straight blow out fuse and not resettable, since if the triac fails, with a resettable fuse it will cycle on and off as the fuse cools.
Just figure out what the expected max draw current and get a fuse close ot that
@Daniel_Joyce , you’re going to be drawing the max amperage as the bed heats up from cool though. You’d want a temperature-based resettable fuse that could be thrown on there with the thermistor.
The other option is to add a small arduino as a watchdog. Have it use a thermocouple to monitor the heat bed, and a mechanical ‘normally off’ relay. It energizes the relay keeping the mains power going to the SSR only so long as the heat bed doesn’t stay hot for too long, or get too hot.
