What is the correct replacement?

Hi Joel,

I could not get your pictures to load. Try replying to your own post and use the picture icon on the tool bar to insert your pictures into the text. In my experience the thermal fuses blow for two reasons. First of all if there is an over heat situation and the second is if their leads do not have a good mechanical connection (Rivets) A bad connection produces heat and this travels up the wires and fuses the thermal link. Typically there are three ways to break the circuit. #1 is the thermostat which controls the temperature. The back up for this is usually a self resetting thermal cut out and the final protection is a thermal fuse. If the part you describe is the thermal cut out its failure could have caused the unit to over heat and take out the thermal fuse. Soldering will not usually work as the heat will eventually cause the solder to crystallize and make a bad connection. It is probably too late but the best way is to cut the original thermal fuse leads and use metal crimp splices to hook the new thermal fuse to the old leads. I look forward to seeing your pictures. While I have never worked on a dehydrator it sounds very similar to some of the dental lab heaters and sterilizers that I used to work on.

John

Hi Joel, The pictured switch is the Auto Setting thermal cut out. It will open up if the element gets too hot due to lack of air flow and automatically close again when it cools down.. It should test with low ohms when it is cool all the way up to cut off temperature. Unless you know what you are doing do not try to test it with power applied. I would suspect clogged filters or fan problems if this were cutting out during use. The thermostat sensor is probably located in the air passage somewhere. You have already located the thermal fuse which may be close to the thermal cut out or at least somewhere near the heater. The material that you are dealing with in the picture is Mica as opposed to asbestos.

John

try a magnet in where on opposite said of the coils

Hi Crystal,

Thank you for your input. Can you give me more information on your idea. I do not see a connection between magnetism and Joel's problem as described.

John

Not sure how a magnet would factor into this... it is not a motor coil.

In the original post’s photo you can see the entire heat ring made of mica, not asbestos (thanks John).  To the cutout’s right along the ring is the power source connections, to its left and up around the ring is the thermal fuse (which I carefully soldered in using multiple heat sinks so the fuse is still contiguous), and directly across the ring from the cutout is a simple copper? wire jumper bridge.

Attached below find (hopefully) a closeup of the cutout.  A probe is seen holding the copper strip up so the contact points disk can be seen attached to it underneath.  In my experience points come in pairs, but no twin disk is seen on the brass bottom section.

My take now is that a point set here is normally closed allowing continuity through the heater unit.  The dark metal strip buried deep inside may be a heat spring material designed to change when overheated and pry the points apart until things cool to an acceptable temp.  There is some sort of short and narrow raised bar transversing the bottom brass behind where a disk would be if it wasn’t missing.  The bar appears made of cast material like a disk is.  Perhaps one end of the spring strip is supposed to ride on that, supporting the copper strip, I don’t know.  Regardless, the spring strip (if that’s what it is) seems to be laying loose on the “floor” of the cutout behind the bar.

My reasoning is that the unit won’t heat because the cutout has a missing contact disk and the dark metal strip is mispositioned somehow.  That is why I was asking if a replacement cutout could be found.  But it hinges on my interpretation of the device’s construction being in the ball park.  Not being familiar with such things, I have to guess at how they are supposed to look vs. what I’m seeing now.  Perhaps there’s not supposed to be a second disk, and the bar serves to support the copper strip behind.  Maybe my trying to probe in there just deformed the copper strip so it no longer lays down where it should.

i think the closest modern device might be a TH21?   GVZ Components - Klixon TH11, TH21

Hi Joel,

You definitely need to replace the cut out. Unless there is another sensor for a thermostat it is possible that it was being used as a thermostat. Are there any temperatures marked on it?

John

Hi Joel,

I found this switch on ebay that cuts out at 105C. I do not know if this is going to work but it is the closest replacement that I have seen. If you have to replace rivets do it with screws, washers and nuts as this will make a good connection that is not susceptible to heat failure like solder is.

https://www.ebay.com/itm/Bimetalic-Switch-Thermostat-Over-Temperature-Switch-221-Deg-F-in-USA/172517131295?hash=item282ad2841f:g:bkIAAOSwnHZYl5Px

John

I was considering getting a Klixon TH21 but now believe from its specs and description at carbonbrush.com that it doesn’t self-reset.  It’s dimensions are exactly the same as the old one’s, and it’s 20v, but each time it got hot enough to pop it’s points I’d have to shut the unit off and on to get them to close again and keep heat to the dehydrator.  When you let the unit run overnight that’s not going to be particularly convenient, and unless checked often you wouldn’t know when the heat was off because you hear the fan running regardless.  I’ve sent an email to carbonbrush.com inquiring as to a proper replacement “thermostat”.  No, I can see no identifying writing on the faulty cutout, but it may be on the underside, which cutting it off the board might reveal.

John, re your comments on heat and solder, I didn’t think of using crimp connectors on the thermal fuse and power source repairs I made.  I’ve believed since my 30’s that solder joints are more complete contact-wise than manual crimping.  Before than I crimped most wiring, but they often loosened or corroded.  I originally tried prying open the eyelet-wire connectors on the mica board, but they didn’t cooperate and I only damaged the mica board in the process.  So I opted to solder wires to the crimps, kind of in iffy solution.  I may desolder them all and see if I can get crimp connectors to fit both over the crimp on one end and smaller wire on the other.  The crimps I have are different sizes of automotive-type;  hopefully they’d suffice, or are their some better suited for higher voltage alternating current?

Rather than try drilling out the holddown/contact rivets and look for new ones, probably destroying the mica board in the process of replacing them, my thoughts go to cutting off the brass ends of the cutout with my Drexel to remove it, leaving the rivets and their crimps to the heater coil ends intact.  Then simply placing the replacement cutout’s holed ends over the rivets and bolting them down to the board with small brass nuts and screws through the rivet holes.  Is that a plausible idea?