Control the heatbed with a relay

Hello, I bought a 30A relay with six slot (DC+/ DC-/ IN/ NO/ COM/ NC). So I schemed out the circuit diagram as some friend said at another website.
It had two wiring diagrams.

As the picture (A). I wired the "+ wire" from the 12V power to the NO slot on the relay, the "+ wire" from the COM slot on the relay to the "+ site" of the heatbed, the "- wire" from the 12V power to the "- site" of heatbed, the "+ wire" from the D8+ slot of RAMPS to the DC+ and IN slot on the relay and the "- wire" from the D8- slot of RAMPS to the DC- slot on the relay.

As the picture (. I wired the "+ wire" from the 12V power to the DC+ and the COM slot on the relay, the "+ wire" from the NO slot on the relay to the "+ site" of the heatbed, the "- wire" from the 12V power to the DC- and the "- site" of heatbed and the "- wire" from the D8- slot of RAMPS to the IN slot on the relay.


I don't know which one is right and will work fine. Could somebody help me please? Thanks a lot!!



Edited 2 time(s). Last edit at 09/13/2017 04:39AM by jacksletter2002.

If you use PID Bed heating the relay will fail in short order as they were not designed for fast switching.
If you are using bang bang bed heating either wiring should work, still I wonder about the relay life span.

Circuit A is the lease complicated for wiring.
Circuit B is the best layout design for the controller.

They do sell a Heated Bed High Power 3D Printer MOS Module Power Expansion Module.

OR Build your own Schematic Ver 1.9 in PDF (Open source hardware).

[www.digital-sqrt.com]

Edited 7 time(s). Last edit at 09/13/2017 06:21PM by Roberts_Clif.

In addition to those comments, using bang-bang temperature control on the bed may lead to print quality problems. With bang-bang control, the bed temperature will bounce up and down, and the bed will warp slightly with each cycle, creating variations in layer thickness in the Z axis that look like Z wobble.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

OK, thank a lot. If I used marlin's firmware with PID control setting. The best choose for me will be the solid state relay (SSR) for fast switching....I think. Is that correct?

Edited 1 time(s). Last edit at 09/13/2017 10:31AM by jacksletter2002.

Yes that is correct.

Be careful how you choose the SSR, in particular avoid SSR-40DD and other types with high voltage drop.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Mechanical relay works but isn't the best option, I ran one for ages, no problems. You need reasonably thick aluminium heat spreader to "buffer" the heat changes that digital dentist mentioned.

External mosfet is the way to go, cheap, easy and some of them are able to handle really big loads. Essentially it works the same as a dc ssr but it's designed for 3d printing.

This type of thing [www.google.co.uk]

Edited 1 time(s). Last edit at 09/13/2017 01:51PM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

If you would prefer not to use a Chinese board and are running 24VDC you might want to consider the SSR6M12-DC-200D.

The only issue is it has a minumum 3.5V control voltage which is above the 3.3V gpio voltage of some controllers. The datasheet worst-case voltage drop is 2.8V at 40A, but it is a true MOSFET and presents a resistive load so in practice very little heat is generated.

I wouldn't bother with a DC SSR for 12V though, its just too much amperage to deal with in a cost effective way.

Thank you, everyone! I have successfully wired the relay to the heatbed, and worked fine. But, like you guy said, mechanical relay works but isn't the best option. It worked frequently because the PID control setting. I will change the mechanical relay to the MOSFET module.