Bed PWM stuck on full, hardware fault?

[mrwizzard]

So i've been building a voron and used an ssr i had used before but different board. i'm using the duet 2 wifi. so when I used this on bed output DC in on the ssr like usual, it works and the bed heats but the average voltage is the full 24v regardless the pwm setting so it's pulling like 700w, lol.

however when i tested it on another heater on the duex, e2 i think...before i finished the config it seems the mosfet poped and it's suck on. while the bed output isn't on the main board i feel like this SSR broke something or is preventing proper pwm.

Any ideas where to look? i have a new mosfet for the duex but perhaps this is an issue with U11? i haven't checked if my hotend has the problem which is probably a good idea. i have some extra replacement chips for U11 but.... i rather not if i don't have too.

Thanks

[jens55]

@mrwizzard, SSR's are known to fail in the fully on mode. Fist thing I would do is to apply no input to the DC side of the SSR and see if the output is on or off.
Next step would be to verify wiring and that in fact I was using a DC SSR and not an AC SSR.

The Duet output (+) that drives the SSR is supposed to be on all the time. The Duet switches the negative side to ground and not the postive side.

If you heard a pop or are smelling magic smoke then you are screwed and the MOSFET is dead so you need to replace it.

Short of a wiring error or a fat finger situation while you are measuring things (that is my fav way to kill mosfets), there is nothing that could cause the mosfet to go pop that I am aware of.
Edit: this is in context of using the mosfet with the SSR. Too high a current or a high input capacitance on a different load can in fact pop the mosfet.

[dc42]

@mrwizzard is it a DC-DC SSR (driving a low-voltage bed heater e.g. 24V) or a DC-AC SSR driving a mains-voltage bed heater?

The cheap DC-DC SSRs you get on eBay are basically useless. The Fotek ones are fakes, and they overheat and burn out at a fraction of the supposed rated current. The only good low cost DC-DC SSRs I know of are the Auberin Instruments ones.

Cheap DC-AC SSRs are sometimes but not always fakes too. The fake ones use a triac that is rated at maybe half the rated current printed on the label. So they may work without overheating is you run them at very much less than the rated current.

You can tell whether the Duet is commanding the SSR to deliver power by looking at the LED on the Duet that is associated with the heater. Some SSRs also have an LED to indicated that they are being commanded to deliver power.

[A Former User]

This post is deleted!

[MikeS]

@mrwizzard for my 2.4 i bought the suggested omron. It costs around 25€ and it's certified. No need to go cheap with it because it's a real safety issue if it fails. Here's the oredring codes from the sourcing guide for suggested SSRs:

[mrwizzard]

Seems like just a config issue on my part, crazy right? somehow a e2 heater got uncommented because it was connected to a chamber light so it turned on (was never on before!), then the config-override had 1.0 as the PWM which i never looked at and i did delete but i restored from a backup as i messed something up and completely forgot.... it was a long day. everything appears to work fine now.

However, i did get the panasonic SSR anyway as the omeron is out of stock (probably because people are building vorons). i figured the other one is fine as it's rated for 40A. (probably not but it might at least be 10A).