Controling 120V 750W heatbed with SSR

SamKudarauskas

I am currently trying to wire my heated bed and I am not sure I have all of the correct components and I'm not sure how to wire it. I have a Keenovo heating pad 120V 750W, a meanswell SSR model number RS-25-24, and a meanswell LRS 350-24 (for my duet 3 mb6hc). Are these the correct components to power my heated bed? If yes that how exactly do I wire it? This guide: https://docs.duet3d.com/User_manual/Connecting_hardware/Heaters_bed doesn't have a wiring diagram for me to follow on it and I don't want to break anything.

Do I need any additional electronics to use this heated bed?

I would appreciate some wiring guidance along with any advice on what additional parts (if any) that I need. I apologize also if I missed an article that explains how to do this more clearly.

Sam

gtrider

@SamKudarauskas Honestly, do not take this wrong but from what I am reading it does not sound like you should be the one doing this. But along with the SSR I would have an 8 amp fuse based on allowing the fuse to be 125% larger than the load. And a thermal fuse mounted to the underside of the bed. Pick one that will open at a higher temperature than you plan on running the bed at. For example if you may run it up to 115C get one that opens at 125c. That is to protect in case the SSR fails closed As well as making sure all parts are properly grounded. When you search look for Line Powered 3d Printer Bed and you might have better luck. Like this one. https://www.youtube.com/watch?v=1VyFejiKkSQ

gtrider

@SamKudarauskas I base that advice of having a more experience person doing it on that voltage can kill if you make contact. And chaffed wiring can cause fires as well as energize frames and such. Done right it does make a reliable and quick heating method

Edit, if you look at Teaching Tech site look at the reply from the person who discuses picking the thermal fuse size. I made that mistake with my first one and picked a fuse I thought was correct and it failed the first time I printed with the bed set to 110C.

dc42

@SamKudarauskas your heating pad will draw about 6.35A @ 120V so as @gtrider says you should fuse the circuit a little above this. If the bed is capable of exceeding about 150C (which depends mostly on the bed size and) then you must also fit a thermal cutout.

According to the product page at https://www.digikey.co.uk/en/products/detail/mean-well-usa-inc/RS-25-24/7706177, Meanwell part number RS-25-24 is a power supply, not an SSR. I am not aware that Meanwell sells SSRs.

Pick a branded DC-AC SSR from one of the usual electronic component distributors, not one of the cheap ones on eBay because they are often fakes (typically the triacs in them are rated for a much lower current that the supposed rated current of the SSR). To avoid needing to fit a large heatsink to it, choose at least a 25A SSR, preferably 40A. For example, Auber Instruments SRDA40.

SamKudarauskas

@dc42 @gtrider Thank you both for your help. I will heed your advice and pick a different bed heating option.

Would something like the 24V 400W Keenovo heating pad powered by my current LRS-350-24 power supply and controlled with a DC-DC SSR be a safer option? Or should I try to find a smaller bed that can be directly powered by my duet board such as the one by prusa that is 255 by 215? (I was originally looking for a build volume of closer to 280 by 280)

Sam

dc42

@SamKudarauskas a power density of 0.4W per square cm gives a reasonable (but not fast) heating speed, so for a 28x28cm bed you should be looking at 300W or more. That's a lot for a DC bed heater; so personally I would use an AC mains powered bed heater as you were intending to do. If you have no experience with AC mains wiring then I suggest you seek advice for someone local who has.

If you want to use a DC bed heater, then the Duet 6HC can drive up to about 400W at 24V (for more than 300W I suggest you replace the 15A bed fuse by 20A). Your PSU will need to handle at least the bed heater power + extruder heater power + fans + stepper motors; so your 350W PSU is unlikely to be sufficient for a 300W heater. See https://docs.duet3d.com/en/User_manual/Connecting_hardware/Power_choosing#total-power-needed for how to calculate the power needed.

SamKudarauskas

@dc42 thank you for your help. Would this prusa heat bed work by being directly powered by out0 of my duet board? https://www.prusa3d.com/product/magnetic-heatbed-mk52-24v-assembly/

I'm fine with losing a few mm of build volume so long as it means simplifying my electronics setup.

Sam

droftarts

@SamKudarauskas If this is accurate for the Prusa bed, it should be fine:

Voltage: 24V
Current: 6.25A

That would give a 150W bed, and with a bed size of 264.4 x 254 mm, about 0.47W per square centimetre.

Ian

SamKudarauskas

@droftarts perfect, thank you for all of your help.

Sam