Hot Plate part (Fuse thermal axial) replacement help

[edy]

Hi,

I have a hot-plate (one of those warmers with a temperature control knob) which is basically used to keep food warm, something like you would see at a conference seminar room or hotel function where they put aluminum foil trays with rice or meat on.  Something like this:



Anyways, the thing stopped working and I was able to trace it to a loss in continuity across the axial thermal fuse, which is basically connected right on the mains 120V wire heading towards the control knob/switch. I have attached photos. It says:

SHENG PING
SPF 109
TF 113 C
10 A 250 V
15 A 125 V
074

I'm in Toronto and usually buy replacement from Sayal.com (walk-in store). Is it just a matter of matching up the 15A 125V rating and also the temperature to be 109? Or do I use 113? I'm not sure what the SPF and TF refer to, and the closest thing I can find is 109 and 117 at 15A. Second question is, why did the fuse blow out and if I simply replace it, is there a chance it will blow again? I can check the resistance of the heating element but if anything is wrong it will increase the resistance, unless there is a "cross-over" somewhere shorting the element which would have caused a reduced resistance. On the other hand, did the fuse blow because it exceeded the current (15A) or because the temperature of the appliance exceeded the cutoff rating? Or am I over-thinking this?

[Gyro]

Regarding the blowing, yes, I think you are probably over-thinking it. With thermal fuses the rating is more of a maximum carry rating than a 'fuse rating'. As long as the element reads a sensible value and hasn't got continuity to ground, I would think it's a nuisance fatigue failure.

It's hard to think that 4'C either way is going to make too significant a difference, probably chose according to your personality (optimistic vs pessimistic). I'm a natural pessimist - but I'm probably more likely to experience another nuisance trip! 

Don't forget to either crimp the leads or heavily heatsink them if soldering.

[Armadillo]

Manufacturer says you cannot leave the device powered ON and above hold temperature [84 degC for this model] beyond 168 hours.
If the ohms do not deduced a shorts then it must be thermal + operating current + time issue.

[SeanB]

Those thermal fuses do fail with time, they are designed that way as the fail range is that they cannot work above the rated temp, so will fail when within 4C below it, or with prolonged time at near the rated temp. Replace with another that is 113C, and crimp it in like the original, and put any sleeving that was on the old one onto the new one, and put back in the same position as well. Likely it died from old age, or from vibration when moved hot. If it blows again then the thermostat is faulty, allowing the plate to get too hot ( it will then not keep food hot but burn it as well) and either you change it or get a new hot plate. If all you can get is 109C use that instead, and note the plate should then not be run at full for long periods, or at least in most cases you should not use it to warm food, just to keep hot food warm over 70C so it does not go bad.

[edy]

Thanks for the help. I think I'll buy the one rated at 109 which has these part numbers on Sayal:

THERMAL FUSE 109C 15A   NTE8108
NTE Electronics Inc.   FRA-388
148581

(I've attached a product spec sheet to this post... TCOs.pdf)


The next one up is at 117 which I guess is also an option, I don't think it would damage the device if allowed to creep up a few more degrees:

THERMAL FUSE 117C 15A   NTE8115
NTE Electronics Inc.   FRA-428
148660

(this one doesn't have much data in the spec sheet attached as far as holding temp, etc.)

Why do they not use a reset-type thermal fuse in this situation, like is often done in microwaves? Is it a cost issue, reliability, space, or usage scenario? It seems that it would make more sense given that many people would not bother repairing something like this and just toss it in the garbage (the only reason I am working on it is because it is my friend's and he knows I can help, otherwise he would have dumped it). The only reason I can think of is that in Microwaves they are usually short-duration, temperature goes up and then cools off... whereas a Hotplate/warmer would go up and stay operating continuously for a long period of time (relatively speaking compared to microwave) which could present a problem with a reset-type thermal fuse which has some hysteresis (will turn off at a higher temperature than will switch back as it cools). The hystersis would not allow as precise control of the cutoff perhaps or there may be drift in the sensor over time since it is continuously operating near the threshold for prolonged periods.

[Teledog]

If you want something re-settable, perhaps a "thermal switch"? Assuming you have the real estate.
A variety of types out there (N/C, N/O, temps, etc.)
Lots on fleabay..local dealers probably want more $

Edit;
Sayal calls them a "thermostat"

[Gyro]

You normally want the safety trip to be a one-time device (or at least something that needs inaccessible manual reset. It probably already has a thermostat of some kind to regulate the element.

The other issue is that these thermal fuses are normally very specifically placed, eg. in a fibreglass sleeve clipped to the heater metalwork. It would be difficult to adequately match the thermal coupling of the original design and time-constant with something else.

[edy]

Ok, I've been having some issues with hotplates recently. I'm not sure if this is a fluke or related to some other issue... Now another 2 hot-plates (different) broke and one I "fixed" also broke again but at a different component (I replaced the thermal fuse but it is fine).

I've attached the wiring diagram and will attach a few photos showing new points of failure. Any help would be appreciated. The device is as simple as can be.... I don't see why things wouldn't work as they should. I will explain.

I changed the thermal fuse on one of the hotplates. Everything worked great for a few hours, then it stopped! (or at least the "light" burned out"). My wife unplugged it, I don't know whether it was still hot or not, she just unplugged it (maybe assumed it was not working?) and plugged in a 2nd hotplate.

I opened the 1st hotplate which I had previously repaired last week by changing the thermal fuse, and the thermal fuse was OK... Instead I noticed a melted plastic around the indicator bulb/resistor area (pictured below). However, based on the wiring, it is connected in parallel to the element... If anything, it would not affect the heating, just the light wouldn't go on. It was NOT a short through the light/resistor either, by removing the light/resistor the element worked normally, I just didn't have an indicator it was on/off.

So the question is, I need to replace this indicator... what is the best/easiest way? The element is 400 W and I measure about 30 ohm across it, but at 125V I am calculating it would get around 520 watt.... Does that bulb/resistor circuit draw some of the current I would need to replace with something similar to avoid the element from getting too much current? To bring it down to the rated 400 W?

Second hotplate that she used also burned out in a matter of a few hours, opened it up and confirmed it was the thermal fuse. This was a NEW HOTPLATE, and thermal fuse already gone? Something strange going on.

Anyways, back to the first hotplate.... The photos I attached show a bulb... not a conventional filament bulb, looks like it is a gas type and resistor I cannot tell the type, it had heat-shrink black insulation over it, I removed as carefully as possible. I am now measuring 55k ohm across resistor and 0.6 M ohm across bulb.... Both of which I suspect are not right. That would indicate why indicator bulb not working.... But did it cause the hotplate to stop functioning or was my wife just confused because the light went off and it was still "hot" to touch but she did not know if it was cooling down or perhaps completely off since she had no way to know whether the temperature was remaining steady or not?

And why did the new hotplate thermal fuse burn out already? And did my repair of the first hotplate perhaps put one of the wires too close to the element and that burned out the indicator light?

Would I just use something like this, and mount it outside the case (I'm worried it is not designed to withstand the heat):

https://secure.sayal.com/zinc/zinc_SEARCH.asp?txtSEARCH=211633

[edy]

I'm attaching a few more photos... As you can see, the wiring is very simple. The element (400 W is directly in line with mains. Whether I measure across the element directly or across the plug, I am getting around 34 ohm, which calculates at 120V to about 3.5A across the element or 420-430 Watt. I am assuming that "indicator light" (not shown as it is removed already in the photos) which is attached in PARALLEL to the element draws some of the current so that the element stays below 400 W.

The other photos show how the other wires attach in parallel to the element and should lead off to the indicator bulb/resistor that I attached to the previous post. I need some kind of indicator to know the hotplate is working, but I want to make sure it is (A) not going to melt, otherwise I will have to mount it fully outside the hot plate, and (B) if it is drawing current, what do I use to make sure to reduce the element amperage down to not exceed the 400 W rating, or (C) can I simply replace exactly as is... but I don't know what kind of lamp and resistor was in the circuit before as it was all embedded in shrink-wrapped insulation and plastic and impossible to see any writing or markings.

Thank you again for your help.

[SeanB]

Neon lamp, basically open circuit till the voltage across it exceeds 90V or so, when it turns on and glows. Resistor is almost any value from 22k to 100k, there to limit the current through the neon lamp as it essentially turns into a low resistance when on.  New resistor and all is fine, the 600k you measured was likely your fingers gripping the meter leads, or the lamp was dirty. you could put the sayal indicator in as well as it is the same thing new.

That the thermal fuse also blew on the other simply is that you are turning it to the max level all the time. The resistance of the element increases with temperature, it is made with a wire with a rather high temperature coefficient so the power used is when it is hot.

[edy]

Thank you for the help.

I will go out and purchase the 125V green indicator lamp shown here (it says in description NEON):

https://secure.sayal.com/zinc/zinc_SEARCH.asp?txtSEARCH=241464

As opposed to this one which is amber 125V lamp but it does not mention NEON in the description:

https://secure.sayal.com/zinc/zinc_SEARCH.asp?txtSEARCH=211633

There is also this one which appears to be larger 12mm instead of 7mm, also NEON which I can use I assume:

https://secure.sayal.com/zinc/zinc_SEARCH.asp?txtSEARCH=241485


I'm not sure what type it is, but I don't believe it is meant for high-temperature applications so I will need to connect it external to the hot-plate or find some way to insulate it well if I mount it internally and make sure it is away from the element.

I will also need some thermally-insulated wires (like the ones used in the hot-plate) as I am running out of length inside of the pre-existing wires as I've had to cut and strip and re-crimp some ends a few times. I'll also have to get some wire "crimps" along with the thermal wires.

Like I said before, it seems like an easy-enough wiring schematic. Why the 2nd hot-plate thermal fuse burned out (brand new out of the box only a few hours later) is a mystery, a freak coincidence perhaps? I can't imagine it has anything to do with our outlets. I will replace that one as well and see what happens. Meanwhile, the 1st hotplate I will need to replace the lamp indicator and see how that goes... Will need to be careful to ensure the wires are all tucked away and not near the heating element.

Yes this is a very simple design... It's either 0% or 100%, no thermostat inside, no bimetal thermostat, nothing in this one. I assume you are saying that as the heater element heats up the resistance increases so whereas it starts around 34 Ohm, it will increase up to maybe 40 or 50 ohm and that will effectively drop amps passing through it, which will drop the wattage as well and remain below 400 W when in actual maximum temperature? The indicator lamp basically does minimal to affect the element and can be even kept completely out of circuit without creating any potential to overheat the element, is this correct?

[edy]

Hi again,

I can't seem to figure this out....    .   I fixed the thermal fuse in the 2nd (brand new) hot-plate after it burned out only on the first use of this device.

Ok, the 2nd use it worked for about 1 hour, then the LIGHT burned out (neon lamp) melted just like the 1st hot-plate..... But I monitored it closely and I am positive that the hotplate continued to work for a while after the light was no longer functioning.... Until finally the newly replaced thermal fused burned out AGAIN!

So now I am back to the same situation again.... I have to replace the thermal fuse a 3rd time, and now change the neon bulb. What is happening? 


Here's the only possible scenario which my wife believes to be the culprit. Can someone please explain if this could possible do it:




So we have 2 sheets like the one pictured above.  One is teflon-coated and darker in color and made from STEEL, and the other one is ALUMINUM. She is noticing that the hot-plate burned out since she started using the ALUMINUM and never before using the steel teflon-coated one like this:




So I am about to now change the thermal fuse and also get a new neon bulb "indicator" light (which I will mount OUTSIDE the hotplate on the side as I am afraid to mount it inside for fear it will melt), and my wife will no longer use the aluminum sheet. Could the aluminum baking sheet be causing this to happen? It is not verified but the only thing that is different seems to be this baking sheet. After I fix this 2nd hot-plate we will go back to the steel baking sheet and see if it happens again, but I won't necessarily know if it is the sheet, my opening/closing the device and the bulb melting because of the wires not being placed back exactly right (which I doubt).

However, I can tell you that when I was touching the hot-plate when it was using the aluminum baking sheet it definitely feels hotter... the entire enclosure, including the top glass and the sides and everything else gets VERY HOT to the touch, no wonder it would eventually melt the plastic neon indicator light housing and eventually take out the thermal fuse.

Here is the warming tray I am talking about (we have 2 of them):





Any ideas would be welcome!

BTW - Reading the reviews on Amazon didn't instill much confidence. The design is simple, but there is no heat regulation... It basically just stays on and hits some max temperature based on equilibrium of the heating coil properties and "heat loss" into the surrounding air and food by the glass top and the surrounding metal base (which also gets scorching hot when on)... Yet even so, if you tip the "balance" thermally it will lead to the thermal fuse burning out and seemingly easily. You can see by the interior photos I posted earlier that the wires get very close to the heating element and are not tucked away far from it (and the run that goes from the element to the neon lamp seems to go directly above the element). A few reviews have noted similar "burn outs" of their warming tray and also for some they had the glass top "explode".

[drussell]

You're letting this poorly designed unit get too hot.  It obviously needs to have a significant heat load (food) on it to sink away the power fast enough since it has no thermostat.

Add a thermostatic switch of some kind, even just one of those simple fixted-temperature thermal switches, to properly control the element, thus rectifying the cheaped-out design flaw, and be done with it. 

[Gregg]

You are fighting an uphill battle against a really poor design, useless for anything but a fire starter.  The heating element seems to be fastened to the base so that the whole device has to get hot before the glass top gets hot.  Then it has to dissipate 400 watts somewhere without a thermostat to keep it in check.  (Note the toasty brown of the zip tie in the picture.) 
First choice would be to return it and not purchase another of the same brand.
Second choice would be to connect it to an external dimmer to manually regulate it; there are many prepackaged ones for things like router speed control, or you can use a standard incandescent light dimmer in your own housing. The problem with trying to build an internal temperature control is the excessive heat of the whole hotplate. 
Best solution would be to send it to AvE for a proper evaluation on is lack of skookumness.