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Nostalgia Hot Air Popcorn Popper Build A,B, C
ocojo
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Posted on 02/10/2021 1:52 PM
Don't use a PID to control temperature. It switches too slow. Buy a thermocouple probe and meter. Probe must be in bean mass about 1/8" above bottom of roast chamber. I'd modify the popper in the following order. Option A and B uses fan to control temperature. Option A: Wire 15 ohm element to On/Off switch. Control the 100 ohm element and fan/diode bridge with a light dimmer (Westek 6077B 200-Watt Manual Dimmer Replacement Kit $6). Option B: Remove 100 ohm element from fan circuit. Control fan/diode bridge with light dimmer and a 115/44VAC transformer (Hammond Mfg 166J44 $40). Option C: Connect a PWM (WHDTS Adjustable Signal Generator $13) to the Gate of a MOSFET (IRFZ44N), Source of MOSFET to ground, Drain of MOSFET to minus side of solid state relay (Inkbird Solid State Relay 40DA $15). Connect +VDC to plus side of solid state relay. Switch 115VAC through the solid state relay and connect to 15 ohm element in popper. Control heat using the duty cycle of PWM. At this point you have spent about $200 which makes the Fresh Roast look like a pretty good option. I roast 70 grams in this popper and get pretty good results.
Edited by ocojo on 02/10/2021 3:38 PM
 
renatoa
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Posted on 02/11/2021 3:05 AM
Agree about "Don't use a PID", but not about the reasons.

Depends on PID... most models I know comes with a relay output setting for on-off, i.e. 20 seconds cycle.
If you switch them to SSR output, the cycle time is reduced to 2 seconds, which is exactly what you need for 1% resolution and 50 Hz mains.

What is wrong with using a PID though... the output PWM modulation happens only in a narrow band related to the P term value. Until that band is reached the output is 100%, which can led to roast damages.
For example if you have a set value of 150 C, and a P term value of 30 (as comes by factory default the popular REX C100 PID), the proportional band is 100%/30 = 3 degrees, so until you reach 150 - 3 = 147 C degrees, the power is 100%, then start to modulate and reduce to the value required to maintain the set value of 150C

You need a high class PID controller, priced even triple times than a full TC4 setup, to have this issue overcome, i.e. featuring a separate setting to limit the output value during the ramp toward set value.

There is another even lower priced option to the above listed, the $15-20 SCR regulator with digital control and display, you can buy from Amazon, ebay or other chinese sources, similar to the below link:
https://www.amazo...07Q6K4P2Y/
Edited by renatoa on 02/12/2021 1:45 AM
 
ocojo
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Posted on 02/11/2021 10:57 PM
Motor controls are an option but the comment sections aren't overloaded with positive reviews. The PID I was using was turning on and off in an appropriate manner but the measured temp would still drop 10 degrees before ramping back up again. I thought that was a thermal mass type problem so I screwed around with adding metal to the chamber (pennies in a false bottom, wrapping some steel sheet around the sides) but that only improved the temp swing marginally. I have found that the PWM with a MOSFET gives very tight temp control. The PID still makes a handy thermocouple reader so it wasn't a complete throwaway.
 
ocojo
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Posted on 06/27/2021 6:34 PM
The 115/44VAC transformer (Hammond Mfg 166J44 $40) burned up. When they say 1 amp max I guess they mean it. Use any of the 24VAC 2 amp tramsformers that are for HVAC and are cheap. Skip putting the 15 ohm and 100 ohm resistors in parallel. The 100 ohm is a cheap way to drop the voltage to the fan/diode and doesn't contribute much to heat. I've been using this gizmo for 6 months and other than the small batch size it works pretty much spot on.
 
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