SSR Failure Modes

Ok, so I have a big commercial oven.

The heating elements are currently driven by an Omron temperature controller that I configured to do PID temperature control. At the time I knew that it was going to end up smoking the 3 pole mechanical contactor. Almost 3 years later (not bad really), that time is nearing as I hear a bit of a buzz when the contactor engages.

As I have been eyeing replacing the Omron controller with an Arduino (straight PID falls down in certain cases) I'm going to replace the mechanical contactor with a 40amp 3 pole SSR. In anticipation of replacing the Omron controller with an Arduino, I'm buying a DC controlled SSR and a cheapie 120v-240v to 12v power supply to power it off the old line.

3 pole ssr:
http://www.ebay.com/itm/DC-AC-40A-5-32VDC-380VAC-3-Phase-SSR-Solid-State-Relay-w-Heat-Sink-Vducx-/230855685653?pt=LH_DefaultDomain_0&hash=item35c0123615

Power supply:
http://www.ebay.com/itm/Waterproof-6W-9W-LED-Driver-Power-Supply-660mA-DC-12V-AC-85V-265V-SLM-0066-/221116036947?pt=US_Lighting_Parts_and_Accessories&hash=item337b8af753

So, I know that SSRs tend to "fail closed". What does this specifically? Does it mean that it's going to short across the poles? Does it mean the circuit will remain closed so the heating elements end up frying themselves? i have seen a mention of using i2t fuses, but what's the point of this? It sounds like i2t fuses help protect the SSR, but don't help in the event of a failure, do they?

Anyone have any suggestions so I can sleep at night? :slight_smile:

clearchris:
So, I know that SSRs tend to "fail closed". What does this specifically? Does it mean that it's going to short across the poles? Does it mean the circuit will remain closed so the heating elements end up frying themselves? i have seen a mention of using i2t fuses, but what's the point of this? It sounds like i2t fuses help protect the SSR, but don't help in the event of a failure, do they?
Anyone have any suggestions so I can sleep at night? :slight_smile:

In my experience, they fail "open" -- open circuit.
A good quality module should only fail, IMHO, if subjected to abuse but not from use.
See "Emphasis on Quality" -- Opto22 - About Us

.

i have seen a mention of using i2t fuses, but what's the point of this? It sounds like i2t fuses help protect the SSR, but don't help in the event of a failure, do they?

Why not, if it protects the SSR it also opens and protects the load from melting.
But yes most times an SSR fails open.

Wierd. All the literature mentions they usually fail closed. Perhaps thats old information? I’ll go with real world experience any day though.

So maybe I run a few MOVs across the 3 phases and call it good? Of course, MOVs have their own problems…

Unfortunately, my budget isn’t Opto, it’s HongKongSpecial. :frowning: Hopefully in two years I can pull this out and put an Opto in.

I replaced my domestic oven thermostat with a PID controller and an SSR. Now oven temperature is controlled to within 1 degree and my better half's cake baking is no longer a hit-&-miss game. A highly recommended modification if you feel up to it. But nothing to do with Arduino.

I used to deal with many of these SSR's while working for a big electronics company. In my experience they failed open, but not always. Also, when they fail unlike a contactor or relay there is usually alot of smoke. I would always add an overtemp safety cutout on any kind of heating oven SSR or not. Those contactors weld together too.