Control of Keenovo heat bed.

[Stevo]

Could I use something like this ? any recommendations on which connections on the duet wifi to make and break the heatbed circuit relay ?

https://www.t-uk.co.uk/product/so942470-single-phase-relay/?gclid=EAIaIQobChMIhfyqqJHb5gIVF-DtCh2BOgYMEAQYASABEgIYsPD_BwE

[Garfield]

Certainly reads OK, depending where you're putting it you may want to request the ip20 covers as a bit of added terminal protection.

Interesting to see a bespoke heatsink and thermal pad are available, also worth considering.

Might even try one of these myself.

If you're using the 750W 25A is more than enough headroom.

[zapta]

IIRC, if you use AC SSR you also need to lower the duet's PWM frequency. Should be covered in the duet's documentation somewhere.

[Garfield]

Pretty sure I read 10Hz - defined using M950

M950

[A Former User]

@zapta said in Control of Keenovo heat bed.:

IIRC, if you use AC SSR you also need to lower the duet's PWM frequency. Should be covered in the duet's documentation somewhere.

Am I imagining that the default main heater was reduced to 10hz as SSRs became popular? Current default (at least for Duet3) is 250Hz. (Open question:)

But yeah, set Q10in M950

[Garfield]

Looking at this I think even 10Hz may be too high - need to keep the signal long enough for the 'zero point' in the AC wave is crossed or it won't switch at all. Depends on the mains frequency where you are - perhaps try 10% of the mains frequency.

You won't hurt the SSR but if it doesn't switch as expected your heating will be all over the place.

[A Former User]

I've only used it on a tiny 50w heater, and at 10Hz and I had no issues. The important thing is that your SSR should have a zero crossing detector and only switch when the voltage is 0 to reduce noise and heat loss (and and and) its harder to find a non zero crossing SSR than not these days; but worth confirming.

[Garfield]

Resistive loads like heaters can be challenging - leave them on too long and the thermal inertia will blow the 'setpoint' easily, leave them off too long and they won't reach it. The optimum would be to have more off than on but even that changes depending upon your setpoint. A heater that is too large can easily run rampant with even the smallest input.

I think the real answer depends upon your set up - try the lowest Hz number that gives stable control - can you even trend the temperatures in the Duet (more to read) ...

But we're digressing far from the OP's question - one hting that I will say is if you go more than 20% of the rated load then a heatsink is mandatory on the SSR or it won't live long. Thermal protection should also be on the mandatory list - SSR's tend to fail 'on'

[jens55]

The default 10 hz works just fine for me on a 1200W heat bed.

[A Former User]

@jens55 said in Control of Keenovo heat bed.:

The default 10 hz

is it? i asked because i tried:

(maybe the experiment is flawed?)

[jens55]

I am pretty sure it is 10hz for the Duet2wifi as I did not explicitly set up frequency. Are you running 2 or 3 ?

[A Former User]

@jens55 said in Control of Keenovo heat bed.:

Are you running 2 or 3 ?

RFF3 on Duet 3, just don't recall reading it changed, or mentioned in the upgrade notes and its potentially a bit of a big deal. Will fire up the Maestro and see if there is a difference between RRF2 and RFF3 when i get a chance.

[Garfield]

Bearer - do you have an M308 line defining the sensor before the M950 line ?

That said the gcode documentation for M950 states

T Temperature sensor number, required only when creating a heater [not yet implemented]. See M308.

The online generation tool adds the sensor to the M950 line - perhaps the Tx serves no purpose but then how do you assign a sensor to the heater ???

[dc42]

RRF2 and RRF3 on Duet 2 assume that heater 0 is the bed heater until you tell it otherwise using M141. RRF3 on Duet 3 doesn't have a default bed heater. The best option when using RRF3 with a SSR is to specify the frequency explicitly in the M950 command that creates that heater.

[A Former User]

@Garfield said in Control of Keenovo heat bed.:

Bearer - do you have an M308 line defining the sensor before the M950 line ?

to be honest i don't have a heather nor a sensor, the board is on a cnc router

[A Former User]

@dc42 said in Control of Keenovo heat bed.:

The best option when using RRF3 with a SSR is to specify the frequency explicitly in the M950 command that creates that heater.

That could probably have been made clearer somewhere, but makes sense as you move away from predefined roles for pins. Thanks for clearing that up!

[Garfield]

@dc42 said in Control of Keenovo heat bed.:

M950 command that creates that heater

So is the gcode documentation in error - is the T statement implemented now ?

[dc42]

Yes, it's been implemented for a long time.

[Garfield]

No worries - needs updating in the documentation.

[dc42]

@droftarts, please fix this, also check whether the RRF3 wiki page has the same error. I'm on a smartphone, otherwise I'd do it myself.