Manual Popper Controller

[gazza]

******WARNING******
The following post contains photos and description of modifications a to MAINS powered appliance.
******WARNING******

This started as a small project for my nephew, he has already moved on to a coretto but I thought it was interesting enough to finish it anyway.

The popper is a $12 Aldi job, the element is 1200W and the fan is 18V DC that is derived from  ~ 20VAC from a tap on the heating element. Out of the box the fan in my unit was running at 16V.

The controller is based on a Picaxe 08M2 micro and it provides a 125hz PWM signal to a Mosfet for the fan and a slow (about) 1hz PWM signal to a 25Amp Solid State Relay (SSR) to control the heater element.

The DC supply for the fan and the micro is an old laptop power supply rated at 20V DC 3A, this gives a bit of extra air flow at the start of the roast but may result in that early demise of the fan motor.

Adjustments are made with a couple of pots on the front panel. The fan can be adjusted from about 50-120%, the heater from about 10% to 100%.

The heater element is isolated from the mains by a separate relay that is powered by the DC supply to the fan so that the heater cant be on if the fan is not running.

So far I have successfully done a few batches of 150gms and they turned out OK. I have tested the agitation with 200gms and the beans were mixing OK so it might be able to do that much if you have a suitable extension chamber.

I am having a few problems with finding the best place for the RoastMonitor thermocouple. In the current spot the probe is in the bean mass but the readings are quite a bit higher than Im used too.

Learning when to adjust which knob is going to take a bit of practice, so far it is fan on full to first crack and only use heat control to adjust the temperature, after that it gets interesting. Slowing the fan to stop the beans going everywhere increases the heat and it is very easy to over compensate with the heat control.


Gary

[shapeshifter]

Re: Manual Popper Controller

Gary sounds like you are getting there with the control.

It closely resembles how mine is made and Ive been using it for almost a year now.

I have also just got some feedback from a pro roaster who says Im on the right track with my roasting, so for all the doubters out there, it is possible to roast very well with a modified popper

[kettle]

Re: Manual Popper Controller

It is very hard to get top quality roasts with an unmodified popper, but with fan and heat control, you can get excellent results. I used to roast with a popper, but sadly the fan died.

From memory it was full fan and low heat to dry beans, then steadily reduce fan to keep beans circulating as they get lighter and manage rate of rise using the heat control.

One warning, your interlock to avoid burning out the element is only partially effective. If the fan dies while DC active, the heater element can also go. I know this from experience

[gazza]

Re: Manual Popper Controller

One warning, your interlock to avoid burning out the element is only partially effective.

I know but I tried a few different methods of sensing the current flow in the fan motor bit the PWM made it unreliable so I went with something easy that was better than nothing.

The most promising version is a split toroid with a hall sensor followed by a peak hold circuit but it still didnt work the way I expected.

I have purchased a different hall sensor and will give that a go at some stage.


Gary

[commanda]

Re: Manual Popper Controller

I know this is not an electronics forum, AND I havent seen your circuit. But, maybe an integrator rather than a peak hold, a rail to rail opamp, and a Faraday cage around the toroid to keep out stray magnetic fields. Been there, done that. I now use a specialist low side current sense IC (not a popper fan though).

Amanda
EEs love coffee too.

[shapeshifter]

Re: Manual Popper Controller

Amanda, no reason why the electronics cant be discussed though for people that want and can go this way

We are just about to embark on our UCR Mark 2, it will basically be an air roaster based on a popper but with a new electronic design, much more refined and controlled than what we did with the modified popper.