Heated Bed Power control

[Richard Holder]

Hi, I may seem to be slightly obsessive about the temperature control on the M2 heated bed but I come from a background of high performance controllers and electronics systems.
Anyway, I thought the temperature control was PID but having tested my Rambo board it is a straightforward on/off controller with a huge hysteresis - I was going to use the Rambo output to control an SSR but on/off control is a non-starter - for me.
I have looked at the Arduino code but can't see anything to turn on 'proper' PID temperature control - maybe I've missed something so any help would be appreciated.  But then again there are a far few  posts on various sites which mention altering the PID variables !

Regards
Richard

[Dean Piper]

Richard,

While it may not have PID based control it will still work. Again I've used this method and have had no issues. You need to get a Solid State Relay that will activate in the range of say 6-24 volts, I'm using 220V AC relays so it can be used worldwide. Then just hook the SSR DC control side to your heated bed outputs making sure positive matches positive. I then took a standard household plug and cut it hooking the load up to the AC input side of the relay, then hook the load side of the silicone pad up to the AC output of the relay and connect the neutral from the wall to the neutral on the silicone pad. It isn't super safe without a ground so you could also tie the ground into the neutral wire if you wish, not sure if there will be current leaking there though. If it heats too fast then you will need to use lower input voltage or limit it in some other fashion, i.e. a light switch dimmer, the temperature will always level out if you give it a good 5 minutes though.

That's it and it works great! Good luck!

-Dean

Richard

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[Dean Piper]

On this page: http://www.futurlec.com/RelSS.shtml

Model: SSR10A

Should work just fine for you, and it's less than $10. The heat pad I've bought from Mc-Master Carr and they are about $50. I think trinity labs sells them too, possibly for cheaper.

-Dean

[Richard Holder]

Thanks Dean,

I think unless I can change the code on the Rambo board to give true PID that I will simply use a dedicated PID controller, thermocouple and SSR - these things don't cost much and I am a stickler for tight temperature control - I think its in the genes :-)

Richard

[Dean Piper]

Richard,

I'm not entirely sure but the latest version of the Marlin firmware may support PID heat control for the bed. If it's not in the regular config.h it could be in the advanced config file. 

It would definitely be worth checking. Or you could set your outputs to 2 extruders and hook your bed to the second extruder and set to the proper temp during printing. Then you would be using the PID control of the heated nozzle for your heated bed. Kind of a hack but it would work.

-Dean

Richard

[Richard Holder]

Dean, you are right and the details are here:
https://groups.google.com/forum/?fromgroups#!topic/mendelmax/XQAtmN1BMWc

You have saved me a lot of work and if you were in the UK I'd either buy you a pint or give you some 1.75mm ABS as a thankyou :-)

Richard

[Dean Piper]

Richard,

So glad I could help out! Best of luck getting your firmware updated. It shouldn't be too hard if you have a config to compare against from an older firmware.

You're very welcome too. I just want to help where I can, and always pay it forward!

-Dean

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