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Popcorn Pumper and the thermal fuse
Doug Dornbos 2
Hi all. I burnt out the thermal fuse in my Popcorn Pumper so I bought some thermal fuses on ebay. My popper works again but runs WAY too hot. I have no explanation of why this would happen. Which is the correct thermal fuse for the Popcorn Pumper? or, can anyone think of why this might happen?

The ones I bought are labeled: SEFUSE, SF240E, JET 240?C, 10A, 250V, C1582.

Thanks in advance for your thoughts.

BTW, here is someone's take on the ethics of using thermal fuses in low-cost appliances. I found it interesting and provocative but mostly disagreed with the opinion expressed. http://www.evonet...&ID=34

The popcorn makers sold in Europe are mostly using self resetting bimetal thermostats, usually set for 180 C.
They are mounted with a tin bracelet somewhere in the middle of the roasting chamber.
This means the temperature in that point never should exceeds 180 C, which is not enough for coffee roasting. That's why all the popcorn modding articles start with the thermostat removal, and place instead the temperature control device, SSR or dimmer/variator.
What you bought as replacement is tripping at 240C, which is a good temperature limit for roasting coffee, but not good for the roast starting, the drying phase is advisable to be performed under 200 C. More about this here:

You can solve easily this dilema, without a fancy roasting controller, by placing a diode in in series with heater and a switch in parallel with the diode, to shortcut it.
You start roasting with the switch open, the diode will reduce power to half, and temperature will top in the 160 - 175 C ballpark for an 1200 W popper. After the drying phase, i.e. 3-4 minutes, you switch to full power, shorting the diode, and the thermostat will keep temperature at 240 C until the end of roast, matching nicely the Schulman recipe.
The diode must be chosen according to your power requirements, I am using at least 8-12A for 1200W/230V = 5A current, fitted with a minimal heatsink.
@renatoa: curious to know how a diode reduces the power in half. Aren't diode supposed to clamp voltage to a specified value?

Also, in my EU Severin popper, the fuse @ 240C is a simple thermal one, and if it blows you need to replace it.
Placing a diode in series with an AC circuit will let pass only one half sine, either positive either negative, and will block the other.
Thus a halving of effective power, much more precise than you can do with other methods.

The 240C thermal fuse is on the heater plate, and is no resettable, for fire emergency situation.
There should be another one thermostat style, not found it ?
Something as you can see here, starting with 00:20:


renatoa wrote:

Placing a diode in series with an AC circuit will let pass only one half sine, either positive either negative, and will block the other.
Thus a halving of effective power, much more precise than you can do with other methods.

Now that you explain that it's so obvious :)

About the fuse, nope, mine just had the bimetallic strip one that I had to short.
This is weird, because without the 180 C thermistor the popcorn will burn at +200C
Perhaps the heater is intentionally underpowered, 900W or so, or air blow much powerful, to limit the temperature by design.
Wiz Kalita
If you use the diode trick, be careful when sourcing the components. The diode will need a peak reverse voltage higher than the peak mains. This is 41% higher than the average mains voltage.

You also need to make sure the switch can take high power AC loads. If you use an underdimensioned switch there can be a hazardous arc discharge when you switch it, which will likely destroy the switch. I've seen wall plug outlets partially melted from people plugging in water kettles that are already switched on.
The switch must short the diode, one time during the roast, and no current will be cut. One time only action during the roast.
The melting you are talking I think could be due to hesitant plug, instead a firm insert in a single move, so multiple connections/disconnections happened, and the arc conditions are met.
If this would be a real concern then we should see this happen for the coaxial ring system in the kettle bottom, that brings all the current to the heater.
Wiz Kalita
It's not a concern in commercial products because they've been designed not to blow up. Anyway it's not a huge engineering challenge, you just need to use something with the right voltage rating.
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