EEVblog #1172 - TRIAC Testing (WEP Meltdown Part 2)

A follow-up to the Yihau/WEP SMD Station Meltdown, testing the heater drive TRIAC and looking at the brushless DC fan motor.
And how a TRIAC and SCR works.

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[gamerpaddy]

3.5-16v supply voltage ic, on a 24v fan together with the missing RC filter and diode, i wonder that not more burned up like that

[Ultrazaubererger]

This is not actually a bug, but a feature!
The MCU detected a fan failure so it blew up the heater while you were in the vicinity
so it wouldn't burn your house down in case you left it plugged in.

[fzigunov]

I'm using DaveCAD and so far I love the software! Simple, clean! No whackers!

[VMFRD]

The MOC opto-coupler gate driver fails as well. Just this week I replaced a TRIAC that was conducting without external signal and that fail to correct the problem, it was only when I replaced the MOC3041 that the behavior went back to normal. Testing the whole driver circuitry is a good idea...

[GadgetUK]

I still think that the lack of airflow meant that the thermocouple was measuring possibly 1 hundred degrees or so less than the actual temperature.

The other factor might be the magnetic switch when you put the thing down - because if the fan failed, you might think it had switched off when seated back in the cradle. You wouldnt be aware that what had happened is the fan died, and the station thought the tool was still being used (ie. not disabled by the reed switch - I often find I have to reseat mine a few times to get it to register that I've finished using it). That coupled with the temperature measured as much less than actual temperature perhaps caused ramp up in current provided to the element in order to compensate as it thought it was too low...
In any case a bad design, these things need to consider failure of each individual component and how it might be fatal.

[CliveChamberlain]

The fan chips data sheet said max 16v and somebody gave it 24v - Ooops! That YHFS fan was NOT labeled correctly...

[robertw]

If I owned it, I’d definitely be adding a bi-metal thermal overload in series with the heating element... couple dollars and 5 minutes work might save your house or life...

[ElmerFuddGun]

10:42 - _"We can control the turn on and turn off time of this TRIAC."_ You can only control the turn on time. The TRIAC will continue conducting until the "end" of the phase when the current becomes too low. Oops... ;-P

[MegaQwerty]

Should have had a three wire fan. The third fan wire for sense or feedback. The heat element would not have been allowed to come on or remain on without receiving the Fan sense or feed back signal from the fans third wire.

[novafawks]

Thanks for explaining what a triac is, it helps not so experienced viewers like me understand everything!

[wp]

I'll add my vote for a video on going down the rabbit hole on the workings of a TRIAC. Explaining the quadrant thing etc. Cheers

[Equiluxe]

I still dont think there was any failure other than the fan, the rest of it could still function correctly and overheat to the point of melting the plastic. I suspect that if you just put the mains power on the heating element without any control the element would burn out very quickly possibly before the thing got so hot as to melt the plastic that much.

[ElmerFuddGun]

IF the TRIAC failed that requires more failures to also happen to cause this. The fan does *NOT* stop until the sensed temperature is below 100° (I believe. At least on my 858D+.) thus the fan would continue going full speed (whatever it was set to) until it has cooled down.

A thermocouple error is a different story and could cause this. Note that even when set to the max temperature of 450° AND max air flow that the heater is not on continuously. Now consider if the flow was set low but the thermocouple is now reading low (and thus never getting to the set point) how hot it will get with 100% heater duty cycle. The situation gets even worse if a small nozzle is installed. I believe this is what may have happened.

[WalidIssa]

Hi Dave, This testing circuit will not work properly ... You will feel it is going from ON to OFF suddenly .... The reason is A1 and A2 ... A1 should be down and A2 is up ... The gate is referenced to A1 not A2 so the voltage should be higher than A1 to trigger properly the Traic .... Good Luck

[TheLenstaa]

Your wide-range precision resistance substituter has surpassed its re-calibration due date 😉

[richfiles]

Guys... Everyone touts DaveCAD as some great, free, open source CAD tool, but don't y'all realize it's free because of _micro-transactions!_ It's totally true!
The software might be free to play, but you have to keep buying the DaveCAD media! That's where the paper industry makes all it's DaveCAD cash!
I've seen prices from $0.95 a piece, all the way up to big lootboxes (with random colored media) that cost $15 and up! Crazy, I tell ya! :P

[dalehorton]

Looks like some very loose windings on one of the coils in the fan, did one break away?
I'm guessing the handset was thrown down into its rest, which dislodged one or more of the fan coils, and likely damaged the thermocouple. I believe if the thermocouple goes open circuit, the controller will just deliver 100% power to the heater constantly.

The drive circuit is quite similar to the one in my cheap chinese desoldering gun, which was shipped to me with a broken heater, and the seller shipped me a replacement heater element (with built in thermocouple). On installing the new heater, i failed to notice that one of the precrimped thermocouple wires was not stripped properly, and crimped down on the insulation, making it open circuit. When i turned it on the thing glowed white hot. I yanked power and attached the thermocouple properly and it works great.

[akhtarkh]

Isn't the temperature sensor a thermocouple, not a thermistor ?

[m.k.]

A silly question:did you check the optocoupler?

[drkastenbrot]

The failure mode with these is quite simple. The fan is individually controlled and there is no feedback. The thermal regulation is done by a microcontroller using the couple and triac. When airflow stops, the airflow from the heating element to the thermocouple also stops, making the thermocouple read low less, causing the heater to switch fully on. Since the heat will never quite reach the thermocouple to get it to the set temperature, this thing will go full meltdown.
All it would have needed is fan rpm sensing and maybe even a thermal fuse in the heater block.