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Thermostat Bypass on Poplite
NewRoast
Hello all,

I have noticed that ever since the ambient temperature has gotten warmer, the thermostat on my modified Poplite goes off and on constantly during the latter stages of a roast, making it very difficult to regulate temperatures and get to 2nd crack.

When I separated the fan and heater, I did not bypass the safety thermostat and I am having a hard time finding specific instructions on how to do it. Ideally, I'd like to keep the inner and outer coils active, but leave the thermostat out of the circuit to eliminate the above issue.

Does anyone know where I can find a step-by-step guide for bypassing the thermostat on a Poplite unit?

Thank you!
JackH
ChicagoJohn's has an excellent and detailed Poplite modification. There may be something there you can use for yours.

http://forum.homeroasters.org/forum/v...post_56179
---Jack

KKTO Roaster.
NewRoast
Thanks. The wiring diagram he used did not bypass the thermostat, so what I ended up doing is soldering the contacts together so they cannot separate once the temperature reaches the threshold. This particular thermostat kicks in at 140C, making it a nuisance when trying to control the roast.
ChicagoJohn

Quote

NewRoast wrote:

Hello all,

I have noticed that ever since the ambient temperature has gotten warmer, the thermostat on my modified Poplite goes off and on constantly during the latter stages of a roast, making it very difficult to regulate temperatures and get to 2nd crack.

When I separated the fan and heater, I did not bypass the safety thermostat and I am having a hard time finding specific instructions on how to do it. Ideally, I'd like to keep the inner and outer coils active, but leave the thermostat out of the circuit to eliminate the above issue.

Does anyone know where I can find a step-by-step guide for bypassing the thermostat on a Poplite unit?

Thank you!


What is affecting yours, I think, is the bimetallic strip. If your unit works well in cooler weather, you might find the problem will be obviated by simply drilling some larger holes around the housing as illustrated in the image of the mod I posted. You may just not have enough ambient air coming in to keep it cool.

There is redundant protection in the form of an aluminum fuse that melts permanently at 240C and opens the heating circuit. I discovered this accidently twice :) If you open your unit up you will easily be able to locate the bimetallic contact points. You could bend this to keep it permanently closed regardless of temperature. You would still have the aluminum fuse in the circuit to prevent a catastrophic heating event (such as will occur if you try to operate it without the fan running, or if the fan fails during operation). This takes place within a matter of seconds without air flow, so it's best to have at least the aluminum fuse in the circuit.

In my development work, I've managed by my various errors to smoke the aluminum fuses on two boards. I have modified one of these to totally bypass both the fuse and the bimetallic strip. The problem with it is that you can't use solder because that will soften and melt at these temperatures. So I disassembled one end of the 50 ohm resister that previously went to one of the AC connections for the full wave bridge diodes on the motor, and then I connected that to the termination of the main 11.4 ohm coil using a small nut and bolt. Ideally a solderless crimp connection would be the way to go, but I used what I had and will check it periodically to make sure it doesn't loosen up.

I picked up 10 of the aluminum fuses on Amazon for around $5 and will be crimping one back in place to restore another assembly.

So if adding more air holes in the housing doesn't work and you want to have some measure of protection, leave the aluminum fuse in place and bend the bimetal strip to insure continuous contact. Otherwise, if you make a chaff collector as I've described, you can actually try using it indoors, but it sounds like you are trying to achieve some pretty dark roasts, so you may wind up with lots of smoke. I try to stop at around 215C.

Good luck, and always put safety first !!!
ChicagoJohn

Quote

NewRoast wrote:

Thanks. The wiring diagram he used did not bypass the thermostat, so what I ended up doing is soldering the contacts together so they cannot separate once the temperature reaches the threshold. This particular thermostat kicks in at 140C, making it a nuisance when trying to control the roast.


Your bimetallic device isn't set properly if it disconnects at 140C even for a popcorn popper. That is only 284F, and I doubt you would ever have been able to make popcorn, let alone roast coffee at that temperature. So something doesn't sound right there. I routinely cut off my roasts at 215-220C and first crack isn't until somewhere after 200C. So your number of 140C is quite low.

Common rosin core 60/40 Sn/Pb alloy melts around 183C. This would be what I use for electronics. That would be the range for common solder alloy. You can do hard soldering "silver soldering" with solder that goes up to maybe 450F / 232C, which I have not done, but my wife, a metalsmith has, and she told me you need to use a torch which I would not recommend with the composite they use to hold the coils together.

I personally would not trust common solder anywhere around the coil assembly. Hopefully it will work for you but probably worst case is it will soften and open up. You could always hard-wire it in that case. Best of luck!
NewRoast
Thanks for the input.

The rating I mentioned is embossed on one of the thermostat terminals, so based on what you said, the 140C may mean something else. I have included a picture showing how I soldered the contacts together. It's a bit difficult to see, but I suppose the worst that could happen is that the solder does not hold and I would need to hard wire it. To do that. I assume that I would need to remove the bimetallic component and then bridge the gap with wire or bend the bimetal strip. That would leave the aluminum fuse in place.

I will also try drilling holes in the base for improved airflow as you suggested.

Thanks again for your replies.
NewRoast attached the following image:
img_20150701_161947_659.jpg
NewRoast
After drilling holes in the base and soldering the thermostat contacts, I tried some roasts this evening and believe these may be the best results I have had yet. No thermostat kicking on and off, which gave me the time and temperature control I was looking for. Easily made it to 2nd crack with some nice Columbian beans.

Happy 4th everyone.
ChicagoJohn

Quote

NewRoast wrote:

After drilling holes in the base and soldering the thermostat contacts, I tried some roasts this evening and believe these may be the best results I have had yet. No thermostat kicking on and off, which gave me the time and temperature control I was looking for. Easily made it to 2nd crack with some nice Columbian beans.

Happy 4th everyone.


Happy 4th to you too! As to your results -- "Cool beans" ))
Looks like you resolved your problem; happy roasting!
And thanks for reporting your results.
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