1
\$\begingroup\$

My mother owns a device for drying fruits and stuff.

fruit drier inside

Now every year or so, the thermal fuse breaks. I'm always checking if the fan is damages or something else, but the device is always fine. It is just the thermal fuse which breaks for no known reason.

thermal fuse with thermal switch

So instead of replacing it with the same type of thermal fuse every time. I was wondering if I could use a resetting thermal fuse.

The current thermal fuse is labeled DYE ZRJG DF110S which seems to be a DF type thermal fuse made by Limitor with a function temperature of 110 °C. The tolerance is listed as +0/-5 K and the maximum rated current is 16 A.

The device has a rated power of 600 W, so the fuse needs to handle at least 2.6 A. It is a resistive heater, so that should be about it.

As I'm not really familiar with these temperature cutoff switches (is that the right product name?) is there a reason why I should not replace the fuse with a cutoff switch?

I could imagine, that if there finally is a failure in the device and it gets turned on and off by the thermal switch, it might cause more damage.

What I want to achieve is, that the device will run for more than a year without having to replace the thermal fuse (but it should still be protected, so a wire is not a solution).


Sorry - should have done this the right way the first time:

schematic

simulate this circuit – Schematic created using CircuitLab

After the request by @jsotola for better pictures I took the device apart again. I also removed the control unit cover to understand how the temperature control is implemented.

What I found was: a micro switch is turning on and off the whole device. A (probably bimetallic) bending strip is the temperature control, the knob controls how much pressure is on the strip and thus controls the tripping temperature of the strip.

main micro switch temperature controller

Close to the thermal fuse (on the other side of the picture I took first) there seems to be another thermal switch, which should probably act as a first safety measure.

\$\endgroup\$

closed as off-topic by Arsenal, Nick Alexeev Aug 25 '18 at 18:39

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Arsenal, Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 5
    \$\begingroup\$ maybe you need to remedy the reason why the thermal fuse is activating .... maybe the dehydrator air flow is being blocked because of improper use \$\endgroup\$ – jsotola Aug 24 '18 at 19:38
  • \$\begingroup\$ @jsotola that is unlikely the case. I've instructed my mother quite clearly and as technical devices are not her domain, she trusts me and follows my advice closely. And having enough clearance was one of them (and don't put it near something which produces heat and don't use the highest temperature setting). \$\endgroup\$ – Arsenal Aug 24 '18 at 20:09
  • 1
    \$\begingroup\$ the problem may be the direct transfer of heat from the heating element to the thermal fuse ..... also, why did you not think about turning the fan blade so that it does not cover the fuse and switch assembly ... please post the bottom picture again .......... i think that something else is failing \$\endgroup\$ – jsotola Aug 24 '18 at 20:35
  • \$\begingroup\$ @jsotola sorry about the pictures, at first this question was not planned, so the pictures I took were just thought to be used if I didn't remember where to put which wire. I'll try to get a better picture up next morning when there is decent light again. \$\endgroup\$ – Arsenal Aug 24 '18 at 20:49
  • 1
    \$\begingroup\$ "I've instructed my mother quite clearly and as technical devices are not her domain, she trusts me and follows my advice closely." That totally works for me in my household. Oh yeah \$\endgroup\$ – Henry Crun Aug 25 '18 at 1:19
1
\$\begingroup\$

You could try a "Bimetal Cutout". (It's a bimetal switch, which performs a thermal cut-out.)

Bimetal cutout component
Image from cantherm.com

These act like you might expect: They are normally closed, and open when the temperature rises above a certain threshold. They close again after the temperature drops down (including some hysteresis).

You can find them for 250V+, 10A+, and in the 110C range.

Note that you may have a difficult time fitting them in the same tight space.

\$\endgroup\$
  • \$\begingroup\$ @Arsenal These are self resetting, so what they end up doing, is acting as a temperature regulator, holding the inside of the device at overtemperature (110C) for a prolonged period - which is probably not what you want. If you used one of these with a buzzer that screams when it is over temp might make it safer. If normal is 40C, then having this at 70C would be OK, and you could leave the thermal fuse as failsafe for when the contact of this switch weld up. \$\endgroup\$ – Henry Crun Aug 25 '18 at 1:14
  • \$\begingroup\$ @HenryCrun yes that what I was thinking. There is a self holding type made by Limitor which would return to a closed position only after power was removed completely. Anyways the question changed drastically after my analysis and should be closed. \$\endgroup\$ – Arsenal Aug 25 '18 at 9:21
1
\$\begingroup\$

This is a safety device, so tampering with it may compromise the safety of the product. It's the last watchman that prevents a potential fire (say if the inlet air flow is blocked). The thermal fuses are very simple and reliable and (most important) are very unlikely to stick 'on'. That's why they are used in this application.

I have some sympathy with your position, we have a very nice Krups sandwich press which similarly has failures (particularly if it is accidentally left on for a long period of time). I changed the position of the thermal cutoff slightly internally and it works for longer now.

You could consider going slightly higher (eg. 114°C or 115°C) in cutoff temperature, but of course that does not comply with the original designer's intent (nor the testing that was performed when the safety agency allowed its mark to be put on the device). So, I'm not recommending that.

But you definitely should make sure the way your mom is using the dehydrator is not contributing to the failures. I know you put the device on a carpet to photograph it, but if it was actually on something like that it could obstruct the air flow and lead to higher temperatures inside, and the fuse would do its job and shut down the appliance.

\$\endgroup\$
  • \$\begingroup\$ Thanks. I'd never consider increasing the temperature cut off. 110 °C is pretty close to get some plastics melting, so I definitely don't want to go higher. The idea to move it a bit is actually not a bad one. There is a cutout in the plastic carrier, maybe it was intended to be placed more inside that cutout (and so a bit further away from the next heating winding). Yeah I was repairing it on the carpet, it was used on a flat smooth surface. I will tell my mother again to make sure that the airflow is not obstructed. \$\endgroup\$ – Arsenal Aug 24 '18 at 20:45
1
\$\begingroup\$

You need to do some measurements, and establish likely causes. I would imagine that normal operation of a dryer only involves air at 40-50C, so thermal fuse failure implies a major fail.

Either

  • she is overfilling it up, and thus restricting the airflow (or do so some other way)
  • the fan is failing to start or stopping (bearing lube failure, or something sticks down and catched the blades)

These two seem less likely:

  • the design simply runs too hot and should have the heater power reduced.
  • normal operation involves a high heating wire temperature vs air temperature, and that is making the temp switch much hotter than the airflow, by direct radiation

A PTC protection/heating thermistor, or the thermal switch Bort shows, may be able to regulate the temperature of the airflow, to something far below 110C (e.g 70-80C). You would leave the thermal switch in as a failsafe.

Another alternative is to get a cheap little temperature controller from CN like W1209 and again set a lower control point.

Since the likely root cause is failure of the airflow, have a buzzer to alert her that it has gone off. It might be able to be across the switch, or certainly it would be easy to add to a w1209 type controller (some have a place for the buzzer to be fitted)

\$\endgroup\$
0
\$\begingroup\$

After my further analysis (thanks @jsotola) I came to the conclusion, that the device should already have a self resetting thermal switch.

I hence took the idea from @spehropefhany and bent the thermal fuse a bit inside the plastic carrier in order to gain some distance to the heating winding.

So maybe that fixes it, but if not I end up replacing the fuse every year.

This question is a good example why repair questions are not the best fit for EESE, as my edit would probably change the answers quite drastically.

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.