Controlling the power to a 3D printer via Octoprint - inrush

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Some people have commented on Octoprint relay contacts getting stuck ON when using a relay to switch the mains power to their 3D printer. This is because the relay contacts are being welded together by something called "inrush" current. Many switched mode power supplies demand a very high initial current spike when they are first switched on - this inrush current can be 50A or more for a very short time and it happens just as the large capacitors charge inside the PSU. I have created an active inrush eliminator to make it safe to control the printer PSU via Octoprint. See: https://www.thingiverse.com/thing:4413002
I hope this is helpful and solves some of the issues caused by low cost switched mode power supplies in our 3D printers.

Octoprint power control inrush eliminator and relay contact protector in v-slot box

I was thinking of adding a power control relay to my 3d printer during my electronics revamp, it makes a lot of sense that there would be inrush current as evidence by the arc when you plug the power supply into the receptacle. I was thinking of just switching the 24v input with the relay, but it won't be powering anything but the board and extruder as the bed is on a separate DC-DC solid state relay. Do you think it's still too much current for the 10a relay?

Hi. If you are using the common 10A 30VDC / 10A 250VAC relays then I'd definitely switch the mains side and not the 24v side. I'm assuming you are in the UK on 240 volts which means the following current drain:

IDLE - 200mA mains, 48 watts, 2A on the 24v side
PRINTING - 1.2A mains, 288 watts, 12A on the 24v side

That means your 24v line will be handling 12A all the time you are printing so this exceeds the maximum current for the relay. I would not risk this.

I'm not sure about your specific setup but you mention having another relay on the bed (which is the biggest drain). Where is this relay connected to and how would the power be switched into this solid state relay. Can you post a diagram?

My advice would be to power the raspberry pi from a separate PSU to the printer. This way you can switch the whole printer PSU with a mains side relay. This way you can do things like wire in a smoke detector and have it completely remove power if it detects smoke. If you only switch parts of the 24v line then power might still be connected to the printer when you expect it all to be off.

I have the pi on its own power supply. I'm in the US so 120 V AC. The bed is controlled using a solid state relay which has 24v coming directly from the PSU. The way the Taz brings 24v to the control box is two pairs, one for bed and one for everything else. So the relay won't touch the bed aside from supplying the control voltage for the 100A DC-DC SSR. So the only substantial load on the relay side will be the 40 watt heater when the extruder is heating.

Ah you have a Taz, nice - I have a Taz 6 and Ender 3 on loan. I have added OctoPrint servers to both printers and they are switched from the mains side. Interestingly the Ender 3 needed the inrush circuit because it kept welding my relays on but my Taz 6 has been on the same relay for two years without the inrush relay circuit and has never welded the contacts. I guess the Taz has a better quality power supply with some type of inrush moderation already included. What are you trying to achieve with the electrical re-work?

Nice! A fellow taz'er! It's a Taz 5 and the original Rambo board is showing its age, giving me some issues. Overheating X stepper motor cable, overheating stepper drivers, strange random motor issues. I'm changing out the board to an SKR 1.4 turbo and redoing much of the wiring. Didn't trust the build in MOSFET for the bed on the SKR so I went with a DC SSR so it would make it easier to later upgrade to a mains voltage bed heater later on. Already designed and printed a huge electronics enclosure for it, it's just currently on hold due to other projects taking up my time (building an RS-CNC32)

Taz 5 SKR 1.4 / TFT35 Electronics Enclosure w/ LM2596 and Relay Mounting

Nice project and designs. Good luck keeping it moving forward. See my Taz 6 Octoprint setup. I put two mains sockets on the back of the Octoprint server box and then switched the power sockets. Its effective and means I don't have to cut any cables on the printer. One is for printer power and the other for the lights. It has a RPi4 in the box with a fan. I used the twin relay board which works well and uses the same relay as the single board you have in your enclosure I think.

thanks! Wow, that looks really clean! Love the membrane keypad, are you using it to jog the machine? Did something similar with mine interfacing the keypad through octoprint to control the printer

3x4 and 4x4 Keypad Controller For Octoprint or CNC

Ok confession time :-) Currently the membrane keypad is not connected. I ended up using the Arduino on a keypad door lock for my son as a fun learning project for him. The original idea was to use the Arduino to scan the keypad and then waggle GPIO pins (one per button). Your solution is better integrated.

ahh, that's pretty cool! good to get the young ones interested in electronics, and not in a playing games on the ipad all day kinda way ;)