Keep it safe..!

[Chriske]

What you are saying JuJu, all these SSR should not be used in PMW systems...??

[Gina]

I don't use PWM for my bed - just simple ON/OFF.  It cycles with ON and OFF periods of about a second or so.  You can't run SSRs off PWM, they don't work that way.

[Gina]

SSRs depend on the sinewave mains current and switch at the zero crossing point thereby reducing RFI.  So with 50Hz mains they only switch every 10ms.  If you switch the DC control on/off frequently I can't say what will happen.

Edited March 13, 2020 by Gina

[Dr_Ju_ju]

Correct, as Gina has said, these devices are not deigned for fast switching of mains voltages (or even DC for that matter). You must consider them as simple switches i.e. on or off switches and not the 'normal' bed driven system, switching a purely resistive  load, which is what a bed heater panel is.....

[Chriske]

Wonder if that silicone bed is still OK...

[Dr_Ju_ju]

Another thought, how is the printer earthed ?? as if the ssr was bolted to the frame, with the frame itself bonded to the incoming earth (note that anodised aluminium is a very good insulator), that should then have blown any trips RCD's etc.... 

[Carbon Brush]

If you buy an SSR with a reputable name, the probability of failure is quite low.

You will also help yourself buy buying devices rated for significantly higher voltage and current than the actual load.

I won't start on overvoltage or transient protection devices (incorrectly called surge limiters) as this involves more mains wiring.

Over temperature detector closing on a 12V supply to start wailing siren avoids mains wiring on the print bed and brings you running.

Over temperature detector opening when hot can be used to turn off a low voltage relay coil. The contacts being mains rated cut SSR power.

Deliberately avoiding specifics in the explanation. Just giving the ideas.
If you need to ask a lot of questions, you probably don't know enough about wiring low voltage relay coils switching mains to keep yourself safe.

HTH, David.

 

[Chriske]

The 240V bed is earthed...of course..!  Safety first..!
I never use anodised alu.

[Dr_Ju_ju]

But what was the SSR bolted to ??  as the back metal plate of the SSR should have been connected to earth ...... 

[Chriske]

Oops...did not know...!

Dam..

[Chriske]

Thanks guys for all the help...
The thing is, two months ago my best pal Marc died. We were a close team. I did the designing/drawing of telescopes and other stuff and he did all the electronics.
I have a basic knowledge electronics, but mostly I'm in the dark.
There is another guy helping me from time to time, but his free time (due to his work) is very limited. And don't dare to ask him to often.

So again, THANKS very much to you all.

[Gina]

Must check mine.  I think the case is connected to mains earth but not totally sure.  Printing ATM but I'll check continuity from SSR case to mains earth when finished.

[Chriske]

Bed 'seems' ok
80 Ohm(the old one) and I had a spare and measured 82 Ohm.
The built in thermistor and a new one both 125K.

Fingers crossed...?

[Chriske]

53 minutes ago, Dr_Ju_ju said:

But what was the SSR bolted to ??  as the back metal plate of the SSR should have been connected to earth ...... 

Why is that ...?

[Dr_Ju_ju]

It would then be doing its 'job', i.e. dissipating any heat build up (if bolted to something substantial), but also as a general safety precaution, in the case where a 'live' voltage gets connected to the plate, forcing any safety device e..g. fuse\trip, to cut the power.

From my days, when I used to build systems for the BBC, where safety was fanatically pursued, ALL metalwork would be earth bonded, so that even now, I ensure earth continuity through all the metalwork, additionally, I also tie down all the 0v lines to earth as well.... 

[Icosahedron]

This is more likely the problem.

[Chriske]

7 minutes ago, Icosahedron said:

This is more likely the problem.

Thanks..!!!

All SSR's I used in all printers will be replaced asap

[Chriske]

Meanwhile all parts except for the heatbed are replaced and re-connected.

[Gina]

Looks like I may have a fake but I'm only running at 3A and the unit doesn't get warm so seems alright.

[Gina]

@Chriske I suggest you set the bed temperature control to bang-bang ON-OFF mode rather than variable (PWM).  I forget what it's called in Marlin.

[Gina]

Can't find exact information with Google (other search engines... ) but you want BANG BANG control rather than PID (which uses PWM).  PID is good for the hotend as it holds a steady temperature but SSRs don't like it,  HTH.

[tekkydave]

Mine is running in PMW mode and has been for 3 yrs. This is the first I've heard about not using SSR in PMW mode!

[tekkydave]

According to the Duet docs it uses a 10Hz switch frequency for bed in PMW mode so should be fine.

https://duet3d.dozuki.com/Wiki/Choosing_a_bed_heater#Bed_heater_driven_using_a_Solid_State_Relay

 

[Dr_Ju_ju]

It will depend on where in the frequency cycle the switching occurs, if you are switching at the mid point of frequency cycle, you'll be ok, as you'll effectively be switching 0v, but if you are switching at the peaks, then you will be switching at full voltage, which then could have high voltage\current switching transients, very much dependent on load type, i.e. resistance\inductance etc. 

For it to always to be switching at the mid point, would require some form of frequency synchronisation, which isn't available in these 'simple' systems, although I have used it in broadcast equipment for certain type of equipment.

[Gina]

Surely the SSR will switch at the next zero crossing point after the change of DC input.  Or aren't these SSRs that clever?

1 hour ago, tekkydave said:

According to the Duet docs it uses a 10Hz switch frequency for bed in PMW mode so should be fine.

https://duet3d.dozuki.com/Wiki/Choosing_a_bed_heater#Bed_heater_driven_using_a_Solid_State_Relay

I think this may have been updated since I read it several years ago - I don't remember reading about the PID being at 10Hz.  I feel sure the instructions back then were to use bang bang mode but I could be wrong.