Keep it safe..!

[Chriske]

Hi all,

I always build my own printers. One of a 'standard' part in all my printers is a SSR, like this one. I've also built a few of my printers for friends.
Anyway : Suddenly that SSR got stuck. Silicone heater bed kept warming.  I entered my workshop and a burned electronics odour immediately set me in 'alarm' modus.
Bed temp at that moment was 265°C...!  Glass(bed) all over the place..!

Lucky me I was at home...!😳
I'm absolutely sure many of us (printer builders) use these SSR's....!
You've been warned..!
 Main question now : were do we buy trustworthy SSR's...??

[Dr_Ju_ju]

You'll pay a lot more than the cheap Fotek ones, but reliable component suppliers do stock them e.g. https://uk.rs-online.com/web/p/solid-state-relays/7150846/  or  https://uk.farnell.com/opto-22/380d25/relay-solid-state/dp/1653772

[Gina]

Yikes!!!    I use exactly the same make and model!  This is frightening.  Seems we need some sort of safety device to cut power if the bed gets too hot.  Obviously all the error catching in the world won't stop this as things are because this is the only bed heater control. 

I guess it needs some sort of thermal fuse attached to the bed that will break the circuit if the bed gets above say 150°C.  A bimetal strip and microswitch might be possible but we don't want it resetting when the bed cools down again.

[Gina]

4 minutes ago, Dr_Ju_ju said:

You'll pay a lot more than the cheap Fotek ones, but reliable component suppliers do stock them e.g. https://uk.rs-online.com/web/p/solid-state-relays/7150846/  or  https://uk.farnell.com/opto-22/380d25/relay-solid-state/dp/1653772

The RS-online ones are affordable, compared with the cost of everything else in a high end printer.

[tekkydave]

5 minutes ago, Gina said:

Yikes!!!    I use exactly the same make and model! 

Same here.

[Chriske]

I'll replace them all asap
Problem is you need to run a company/business to buy from them.

[Dr_Ju_ju]

You can get non-resetting thermal fuses (https://uk.rs-online.com/web/p/thermal-fuses/7976055/), but you'd then have more mains potential wires floating around, which could have other safety issues.... 

[Dr_Ju_ju]

Nooo, you can buy from RS\Farnell directly, I do it all the time....

[Chriske]

Not were I live, dam...

1 minute ago, Dr_Ju_ju said:

Nooo, you can buy from RS\Farnell directly, I do it all the time....

 

[Chriske]

Good thinking maybe I could buy from RS in UK

[Chriske]

Frankly, I had no idea these (made in RC) rubbish parts were this bad.
I'd think a safety margin or a fuse was built in...NOT...!
Problem these days : All things we buy here in European countries are also marked 'Made in RC'

[Dr_Ju_ju]

Everything is made in PRC\Vietnam\etc. , we've effectively given up basic component manufacture, unless you are supplying top end \ dollar components, when it may well then be economically viable, but even name brand parts are also sub-contracted out to the far east..... 

[Chriske]

Planning to install a second Arduino, or a Nano, to do a extra temp-check on both heaterblock and bed, parallel with the primary thermistors.
When something like this happens again, the secondary safety circuit will cut power completely.

[Chriske]

1 minute ago, Dr_Ju_ju said:

...but even name brand parts are also sub-contracted out to the far east..... 

Sadly enough...!!!

[Gina]

I guess even the higher quality SSRs could fail in the same way so maybe a thermal fuse would be safer.  Though a lot less convenient!

[Dr_Ju_ju]

KISS -- Keep it simple stupid, the more technical\complicated you make it, the more that can, and probably will, go wrong......  

[Gina]

9 minutes ago, Chriske said:

Planning to install a second Arduino, or a Nano, to do a extra temp-check on both heaterblock and bed, parallel with the primary thermistors.
When something like this happens again, the secondary safety circuit will cut power completely.

Not a bad idea!  A Nano could certainly do that but you still need some sort of mains relay.

EDIT...  Cross posted with Julian.  He has a good point and that brings me back to the thermal fuse idea.  The Duet control boards have a fault sensing routine that checks that the heating is turning on and off and I can't see the power MOSFET control failing into the ON state though I suppose it's possible.

Edited March 13, 2020 by Gina

[Dr_Ju_ju]

You could probably use a bimetalic thermostat (e.g. https://uk.rs-online.com/web/p/thermostatic-switches/2282614/) that would be used to drive an external double pole power relay, wired in dead-man's mode, such that when it 'opens' it kills the supply keeping the relay powered....  

[Chriske]

A bimetalic thermostat, would not last very long. The beds are all PMW powered.

[pete_l]

52 minutes ago, Chriske said:

 Main question now : were do we buy trustworthy SSR's...??

Answer: RS components. But pay £20 each instead of £3 from Aliexpress.

[Dr_Ju_ju]

It would just be measuring the temperature of the bed, and wired in series with the incoming mains power \ relay coils, and not any part of the 'normal' control system(s) , so would not be affected by any PWM signals etc....

[Chriske]

Did some 'dismantling', I have not been gentle...🤗
Emotional people should skip this post...
You have been warned...!

Here we go..
Tools needed to do this job : Hammer , screwdriver, pliers, lots of force..!

Before

 

A few seconds later

 another 20 sec later...

[Gina]

That is well burnt out!!!  😮

[Chriske]

Dooh...!
NOW I understand...! Yes that's it...!!!

The moment the SSR is completely molten, then the bed stops warming automatically.
Silly me, not trusting the manufactures of this nice looking JUNK... !..😡

[Dr_Ju_ju]

If your bed driver is PWM based, and not a simple on\off switch, what frequency were you driving it at ? as it may well have started internally arcing, as these devices are designed to be 'simple' switches and not expecting fast on\off cycles, which may well coincide with the local mains frequency (50Hz) thereby creating far higher transient current spikes, which would have the potential to 'punch through' any insulation....