Thermostat Switch question ..#2

Thread Starter

NadaZait

Joined Nov 15, 2022
2
Why? The switch/thermostat should reset it's self on cool down. A manual rest before cool down would just have the switch turn off again because it is already at temperature.
"A thermal switch (sometimes thermal reset or thermal cutout (TCO)) is a device which normally opens at a high temperature (often with a faint "plink" sound) and re-closes when the temperature drops" From; https://en.wikipedia.org/wiki/Thermal_cutoff
@shortbus I'm fairly sure this is my problem but I think the previous person put in the wrong thermal cutout so not sure what part to get. Can you please help/advise?

I think I need normally open but not sure of parameters.

Mod: link to old thread

https://forum.allaboutcircuits.com/threads/thermostat-switch-question.162472/post-1424101
 
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MisterBill2

Joined Jan 23, 2018
13,838
The problem is that the temperature sensing path is not correct. The thermostat is switching off because the temperature at the sensing position is not adequately cooled by the water being heated. So the thermal coupling to the water portion is not adequate. So better thermal conductivity to the water side is required.
 

shortbus

Joined Sep 30, 2009
9,766
@shortbus I'm fairly sure this is my problem but I think the previous person put in the wrong thermal cutout so not sure what part to get. Can you please help/advise?

I think I need normally open but not sure of parameters.
This is an old thread, you need to start one of your own stating your problem. There is no way of knowing your real problem without you describing it.

But this may help you, it's a PDF of different Klixon thermal switches. https://www.sensata.com/sites/defau... current capacity thermostats-datasheet_1.pdf
 

MisterBill2

Joined Jan 23, 2018
13,838
Direct shutoff control of a heating element needs a special thermostat that opens on temperature rise and stays open until mechanically reset. That is how the thermostat in those really cheap coffee brewers work. Press the start button resets the thermostat to ON, and when the water is all gone then the temperature rises and switches it OFF, where it stays until the next batch of coffee. But if it switches off too soon, then there is too much heat transfer to the thermostat from the heater and not the water.
It may not be possible to fix that problem. Try doing smaller batches .
 

MrSalts

Joined Apr 2, 2020
2,625
Commonly, a device like a coffee maker will have a thermal fuse in series with the thermal switch. If the thermal switch fails, the thermal fuse triggers to shut things down permanently. If the switch failed, it is likely the fused failed as well. You'll need to replace both. The thermal fuse typically triggers 20-50° above the thermal switch's trigger point.
 

shortbus

Joined Sep 30, 2009
9,766
Direct shutoff control of a heating element needs a special thermostat that opens on temperature rise and stays open until mechanically reset. That is how the thermostat in those really cheap coffee brewers work. Press the start button resets the thermostat to ON, and when the water is all gone then the temperature rises and switches it OFF, where it stays until the next batch of coffee
Think your wrong about that. If you refill the water within a very short time it will start heating again, because the Klixon switch is cooled back down. Never saw one that you need to turn the main switch back on. If you get past the point when the high limit switch opens then maybe, but has nothing to do with normal heat control Klixon switch.
 

Tonyr1084

Joined Sep 24, 2015
7,210
I had a coffee pot similar to what is being described in both the other thread and in this one. The one in my coffee pot failed after a few years of operation. Built in obsolescence. The way the thermostat worked on that coffee pot was a thermal switch that - at rest - was off (normally open). When you pushed the button you actually pushed the bimetallic strip over its trip point to turn the contacts on (temporarily closed till it reaches temperature). Only when it got hot enough it would break back and shut off. Pretty cool idea, but actually not very long lasting.

As for water level sensing - coffee pots cook until there is no more water in them. When the water is gone the pot begins to heat above waters boiling point. When that's reached the thermal switch opens and shuts power off. In this particular instance it remains off until someone pushes the button again.

Typica thermostatic switches will reach their cutoff point and go open (in most cases). After the thermostatic switch cools down to some lower point it will click back in and begin operation again. Those are not user interfacable. The kind of switch being talked about in the original thread (and probably this one too) requires a user to command an ON condition, switching the thermostatic switch to the closed position. It will open when the temperature spikes after all water has been boiled out. If there are chemicals in the water, those may also boil out. Minerals will likely be left behind, eventually clogging the boiler. When I say "Boiler" I'm talking about a coffee pot boiler which consists of a nichrome wire in an aluminum housing bonded to a water tube. As the wire heats the water in the tube boils. As the water boils it extracts heat away from the wire. When the water is depleted there is nothing to draw heat away, and the wire continues to heat to whatever level the thermostatic switch is designed to open at.
 

Tonyr1084

Joined Sep 24, 2015
7,210
Direct shutoff control of a heating element needs a special thermostat that opens on temperature rise and stays open until mechanically reset.
Agree!

@shortbus They DO have TC's that require a user to press a button - a mechanical link that literally clicks the switch over. When temp is reached it clicks back. It will not reset as long as the temperature is higher than the reset point. But adding cold water is likely going to cool things down rapidly, thus allowing the user to brew another pot (or cup) of coffee. Mine was a single brew unit. It failed and I scrapped all the parts in it. The thermal fuses are still good. The failure point was the TC itself. It wore out from the amount of pressure needed to trip the bimetallic strip over. I could have replaced that TC Switch, but the pot was cheap as heck. And we often brewed more than one cup a morning. So we got a regular brewer.
 

shortbus

Joined Sep 30, 2009
9,766
@shortbus They DO have TC's that require a user to press a button - a mechanical link that literally clicks the switch over.
Any coffee pot I've ever owned(I don't do single serve pots) have a few Klixon type switches in them. For the water heating part of the pot, there is the thermal cut off switch, that shuts down power when the water is gone from the water heating area. Then there is the one that keeps the coffee pot warm and at a certain temperature in the pot. Then you have a high limit switch that protects the whole maker from overheating. That is usually a thermal fuse not a switch.

It's funny that all of the pots I've ever had would still stay on even after the coffee pot is empty. No resetting of the switch. I know because when still working I sometimes would forget to shut the maker off before going to work. And when getting home the pot was empty, from evaporation, and the light was still on on the machine.
These were all of the Mr. Coffee type pots, the one I'm using now is a Black and Decker and that's how it works too. And If I pour water in the pot it starts to heat that water without ever turning the switch off and back on, just add cool water back into the chamber.

Thermal fuses come in many temps and types and are saftey fuses not a switch. Once the blow they're just like any other fuse, they have to be replaced. https://www.swe-check.com.au/editorials/thermal_fuses.php


And don't know how this changed over to coffee pots. The OP was about electric water distillers, not coffee pots.
 

shortbus

Joined Sep 30, 2009
9,766
For the non believers-
"The coffee maker's switch turns power to the heating element on and off. To keep the heating element from overheating, there are also components such as sensors and fuses. In coffee makers, sensors detect if the coil is getting too hot and cut off the current. Then, when it cools down, they turn the current back on. By cycling on and off like this, they keep the coil at an even temperature. Fuses simply cut the power if they sense too high a temperature. They're there for safety reasons, in the event that the main sensor fails." From - https://home.howstuffworks.com/coffee-maker.htm
 

Tonyr1084

Joined Sep 24, 2015
7,210
The OP was about electric water distillers, not coffee pots.
Yes. And your entire post is accurate. But I personally had one of those covfefe pots that had the exact type of thermal switch the TS first brought up. Wish I still had it simply for the sake of taking a photo of it. But no need. There are probably images on the net. Thought he's talking about a distiller, the switch mentioned is the same thing. Press the reset button, the heat comes on. When the fluid is gone the temp rises above the set point and the thermal switch breaks current. Will not self reset. Must be reset manually. It's a poor design, but it functioned for a little while. Don't recall how long before failure, but the wife would usually make two cups a day. And I don't think the brewer lasted much longer than one year. If even that long.
 

shortbus

Joined Sep 30, 2009
9,766
Have been around many coffee pots from the 10/12 cup Mr.Coffee to the big banquet type coffee urn. Never saw one that the switch needed to be reset.
 

Tonyr1084

Joined Sep 24, 2015
7,210
Have been around many coffee pots from the 10/12 cup Mr.Coffee to the big banquet type coffee urn. Never saw one that the switch needed to be reset.
Neither have I, not on the big name brand units. But this pot wasn't a pot - it was a small one cup boiler. The pot was a coffee cup.

This might be the brewer I had. Read the reviews. Complaints about short lifespans and boiler not fully emptying before it shuts off. Can't be certain this is the unit we bought, but it has a single button on the side which may need to be pushed in and a solid click is felt. That's the thermal switch being set to ON. I think this may be it. It was dirt cheap and in the end was worth dirt.
 

Tonyr1084

Joined Sep 24, 2015
7,210
HOLEE SCHMITZ - You'll never guess what I found!

I was rummaging through my stock, looking for a 2X AAA battery holder when I came across this:

THEE thermal switch we've been talking about. This is the coffee boiler. The upper tube is the water tube while the lower aluminum housing houses the nichrome wire. The switch is attached to the side (you can obviously see that), and off to the right, almost out of frame, is the thermal fuse. This boiler - the fuse is good but the thermal switch is no longer resettable. There! Proof of the existence of such an animal.
1668972693801.png
 

Tonyr1084

Joined Sep 24, 2015
7,210
Only because was a single cup coffee maker.
Can be any application. As long as there is water to boil the temperature will never go above - maybe 240˚F. But when the water is gone the nichrome wire will continue to heat the aluminum housing to its switching point, maybe ≥ 250˚F.
 

Thread Starter

NadaZait

Joined Nov 15, 2022
2
The problem is that the temperature sensing path is not correct. The thermostat is switching off because the temperature at the sensing position is not adequately cooled by the water being heated. So the thermal coupling to the water portion is not adequate. So better thermal conductivity to the water side is required.
Thank you for this tip. The bracket attaching the reset switch was almost touching the water tank and hence why the cutout was occurring when the water itself had not reached its desired temperature.
 
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