SSR provides direct AC power to the bed rather than relying on the Duet to provide power. For the US version of E3D high temp bed: "These are wall powered (120V) heated beds to be used for printing high temperature plastics or where fast heat-up times are required. They must be used with an SSR to switch and control, due to using main voltage (wall outlet power)." If you used a different bed that maxed out at 90-100c and did not run off 120V you could skip the SSR.
I bought their HB and it's for US 120v so I guess I use it. So rather then the power for the HB coming from the Duet board it comes from the PS via this SSR which is controlled by the board? Wait... ! What? I admit I was NOT paying attention!! t The HBP is 120V?! Wow! That thing is gonna get up to heat FAST! Anybody out there using this yet???
You are piggy backing on the power supply's incoming mains. The PS isn't providing any power to the bed. This allows you to have 1 switch for the whole printer. Your Duet is telling the SSR when to switch on/off. I am using the 120V bed. I've been calibrating with PLA so I've really only put it up to 60c, but it gets there extremely fast.
Yep. It's crazy fast to heat up. 80C is around 2 mins and I think 105C is under 5 mins. Once the bed reaches heat you'll see the red light flashing power on and off to the bed. As @blarbles says, not actually power to the bed, but power to the SSR. The SSR has a little yellow light on it that you'll see also flashing. That's the gate to mains power. In the UK, this means we're pushing 240V into the bed.
