Heat bed SSR - destroyed

Hi,
I have a aluminatus A1 from the first batch. My mosfet died and stayed on, so I tried to swap to other mosfets and ended up killing pin 8 on mega 2560.
Reassigned bed to pin 9 and connected SSR dc to dc 25amp double checked polarity on input and output before turning on bed.
This worked for a bit 20 seconds then magic smoke came out of input end of SSR.

Am I meant to change the heat bed pie settings? Slow the pwn?
I keep reading about bangbang mode but could find settings just for the bed.

I now have another 6 week wait for another SSR

DC/DC SSRs typically drop 2 volts at maximum current, which means that when driving a typical heat bed they dissipate between 20W and 40W of power. This calls for a massive heatsink. So I don't recommend using DC/DC SSRs. You could look at using a SevenSwitch instead.

DC/AC SSRs used to power mains voltage heatbeds also drop about 2V at maximum current, however the current is around 10 to 20 times less for the same power; so they can be use with little or no heatsinking.

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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].

This was latterly 20 seconds and the SSR did not get hot.

If I hook up a DC/AC SSR do I need to add any other circuits or just go from mains (240v) active into the SSR AC side and then to the heat bed?

I looked at the SevenSwitch but its only 15amp and my heater bed is 400w (24vx15a=360w) wont it just burn that out as well?

I think its odd that its the input side of the SSR that burnt out like it couldn't keep up with the pulse's of on off.

Quote
Kawobei
If I hook up a DC/AC SSR do I need to add any other circuits or just go from mains (240v) active into the SSR AC side and then to the heat bed?

You'd need to have a heated bed designed for AC and that voltage.

Quote
Kawobei
This was latterly 20 seconds and the SSR did not get hot.

You are saying there was smoke but not heat? How do you know that it didn't get hot? Something must have got hot to produce the smoke.

SSRs have polarised inputs, i.e. you need to get the input polarity right. Is it possible that you connected the input terminals the wrong way round?

Don't mess with an AC mains powered heated bed unless/until you know what you are doing or have advice from someone who does (preferably, a qualified electrician).

Edited 1 time(s). Last edit at 05/15/2015 04:25AM by dc42.

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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].

I confirm polarity with multi meter before hand as I knew they didn't like incorrect polarity.

I know it didn't get to hot cause I was holding its outer casing.

I have built the sevenswitch circuit and it seems to be working. Thank you.

Still would like to know what killed the SSR :/

Hi Kawobei,

Can you share some photos of sevenswitch which you made.

I made one but not working.

Also can you share the config file for Marlin. I mean whether to use bang-bang only or PID.

I destroyed another 2 mosfets before changing tatics.

My problem in the end was fairly strange, The Mega2560 I was using was rated for 25v input but I dont think it was keeping up. I changed to a Taurino and every started working just fine. Sure the MOSFET gets to about 80 degrees but the heat sink keeps up with that.

I rechecked all my SSR connections and bangbang is on checked duty cycle with multimeter but must have been a knock off SSR.

I will order one of those expensive ones as a backup next time.

Thank you.