Thermal fuses....why do coffee makers use two in series?

[nevetslab]

While getting my day started yesterday morning, and went to pour a second half-cup of coffee, having poured what I had in my cup back into the pot, I discovered the coffee pot was now just luke warm. Pilot light in the switch was still lit. I removed the pot, took a chance with quickly hitting the heating surface for the pot with my finger, finding it now just a little above room temperature. Rats. Having been down this road before, I once again had to do some basic shop equipment maintenance.

But first, since the bench was occupied, I removed the coffee maker, and fetched my small lab hot plate, plugged it in, set it near minimum temp, knowing it tends to heat up the surface quite high, like over 250 deg F. Monitored the hot plate for a while with a Fluke Temp Meter & surface temp probe. Got the pot hot again, so finally after completing the Peavey 6505 repair, I opened up the Hamilton Beach $20 coffee maker, found one of the two 229 deg C thermal fuses open.

Once again....two thermal fuses in series between the heater coil and the thermal switch to thermal-cycle the system. I thought I still had some similar rated thermal fuses, but had used them all up. I did have some PEPI 135 deg C resettable OOR thermal switches, and installed that, not sure if I'd just end up again with the second thermal fuse open. Working so far....at least I got this morning's half-pot full of coffee made.

Why do mfgr's use TWO thermal fuses in series? I've seen this in all of the coffee makers I've owned when looking for the cause of their ultimate failure, intended to make Mr. & Mrs. America throw it out and go buy another one.

[nickb]

Are they in significantly physically different places? If so I guess they are covering the case that one area could be excessively hot while the other is OK.

The simple answer is they had to do it to meet safety requirements.

[nevetslab]

Are they in significantly physically different places? If so I guess they are covering the case that one area could be excessively hot while the other is OK.

The simple answer is they had to do it to meet safety requirements.

No.....the one that failed was electronically welded to the output side of the heater element, it in series with the next one, it's body about an inch away, which was welded to one of the thermal switch blades, and the other coupled to the AC mains. Both elevated above the heat plate assembly, with the water chamber clamped down to the heat plate. I've never measured the voltage across them while it's in use. After I installed a replacement part, it NOT being a similar thermal fuse, I'm guessing the second thermal fuse will die at some point. I'll order some replacement parts in the mean time. With it pulling 800W when heating (~ 6.7A), I'm guessing there may be some voltage drop across them under load.

Note: I just checked to see what was available in these parts, found some 228 deg C 10A parts at Fry's Electronics, of all places, for $1.89 ea, in stock. That may be a misprint, but if not, it would be one of the first times something I needed was ACTUALLY AT a Fry's store! Price is right.

[Randall]

I helped a friend fix his clothes dryer this weekend, and noticed a similar thing. There were two thermal switches close to each other. One was tacked to the back panel up near the top, and one was closer to the element, with it's sensor side in the open. I assumed one was keeping an eye on the outside surface of the box, and the other was monitoring air temp as it came off the element.

[nosaj]

I helped a friend fix his clothes dryer this weekend, and noticed a similar thing. There were two thermal switches close to each other. One was tacked to the back panel up near the top, and one was closer to the element, with it's sensor side in the open. I assumed one was keeping an eye on the outside surface of the box, and the other was monitoring air temp as it came off the element.

Most likely what you are referring to is the High Limit thermostat(prevent temp from getting higher than what is stamped bu the L code I think) and the other is the cycling thermostat which turns the element on and off .
nosaj

[g1]

Maybe the second is a fail safe, for the same reason nickb stated, safety approvals?

[The Dude]

Here's the UL document that applies to coffee makers.

http://bbs.bzxzk.net/attachment.php?aid=16514&down=1

From what I've read, it's cheaper to use 2 lower spec parts than it is for 1 higher spec part in order to meet UL requirements. Section SA11.3 hints at it. In other words, it's a cost saving thing.

[vintagekiki]

Two thermal fuses located in series with the heater coil (before and after) purpose is protection from electric shock independently from which side is the L (Live) AC input terminal if the resistance heating element (heater coil) short circuit occurs towards the metallic tube (in which it is located)

[J M Fahey]

Havenīt seen yours, a picture might help, but in large power amps itīs not uncommon to have 2 thermal breakers , one on each heatsink assembly (say, left and right, or both 350W channels on a Hartke 7000, etc.) , for the very good reason that one may be melting and the other working fine, so the most stressed one triggers first.

Again, would love to see your coffee maker guts.

[Bill Moore]

I'm glad to hear that I'm not the only one who refuses to throw an appliance away, if it can be repaired!

[J M Fahey]

Well, it can be overdone
I live in a huge house, and itīs chock full of broken but "repairable" stuff. (in theory).
Plus not-broken-but-not-really-useful stuff such as lots of old CRT monitors, PC carcasses, dot matrix and inkjet printers, etc.
Oh well.

[nevetslab]

Havenīt seen yours, a picture might help, but in large power amps itīs not uncommon to have 2 thermal breakers , one on each heatsink assembly (say, left and right, or both 350W channels on a Hartke 7000, etc.) , for the very good reason that one may be melting and the other working fine, so the most stressed one triggers first.

Again, would love to see your coffee maker guts.

I didn't think to take photos of the insides of it when I had opened it up a couple days ago. The bottom cover screws are 3-point screws like a Philips, only not. Did manage to make a small Wiha bladed driver work on them. I'll take some photos on Saturday, after I stop off at Fry's to see if they actually DO have the thermal fuses I found on line. Figures the one nearest the heater terminal that had the thermal fuse welded to it was the one that opened. The other one, joined with the failed one using a brass in-line crimp-splice connects to an Elmwood-style OOR Thermal Switch, mounted to the water heater housing, thermally bonded to the pot heater plate. That connection is also welded, as is the AC mains lead using hi temp fiberglass insulation, with the opposite end of the heater coil connecting in similar fashion.

I looked thru the UL 1082 Safety Standard for Coffee Makers I'll have to re-read SA 11.3.1 again. It refers to UL 1020 standard for Thermal Cutoff's for use in appliances. Thanks, Dude!

At least once a month, I will arrive in the morning, to discover I again failed to switch the pot off, and have to recover from the boiled-out coffee remains in the pot. I'd think the thermal fuse would open under those conditions at some point, but, didn't until I had made a fresh pot a couple days ago. Stupid basket in the design.....never seen a No. 3 Filter, but that's the size designed into it. No. 2 is too small, No. 4 is too big, but folding a small edge back makes a No 4 fit the basket. Whaddya expect for a $20 unit?

[nevetslab]

Insides of the Coffee Maker's Hot Plate/Water Pipe/Heater Wiring

I stopped to open up the coffee maker before making this morning's pot.



Repairs already made: Replaced AC Mains Switch; Replaced one of the hoses to the Heater Pipe Assy (real PITA!); removal of the 2nd 229 Deg C Thermal Fuse, temp replacement with a 135 Deg C OOR Thermal Switch

Selco_Thermal Fuses_2017_SWTC_Series.pdf
thermalcutoffs.pdf

So far, so good.....still making coffee. Essential Shop Equipment!

[g1]

Where did you get the hose and what kind?

[Steve L]

Just for your information...

Before I retired, one of my responsibilities as an electrical engineer was protecting my large, wealthy corporation from insurance companies who wanted to blame us for fires to get their settlement money back, or attorneys for bad electricians trying to shift the blame off of their clients.
I would investigate the site where the damage occurred and then when all of the suspicious evidence was gathered we would meet in a lab somewhere to do the forensics.

Guess what were the number one devices, by far, that lined the shelves in all these labs where all of the evidence from different fires was stored.
Yes, coffee makers, especially the same kind shown in your pictures. Row after row of them. (space heaters were number two)

I never lost a case, but the coffee maker guys did in the past. That is why you see so many thermal cutoffs. You really should either remember to turn it off or at least put it on a 24 hour timer.
By the way, you could turn it off after it brews and microwave it when you want to drink it. It tastes better that way, unless you are one of those people who like it well done.

Steve