[TriEmbed] Coffee roaster sanity check

Shane Trent shanedtrent at gmail.com
Tue Jan 7 12:22:48 CST 2020


I did not look at the photos but I have disassembled a few hot-air popcorn
poppers and they all used the same approach with the line voltage dropped
via a small heater coiled wired to a discrete diode bridge attached to the
motor leads. I opened the first one up to investigate automation and had a
few poppers die on me after extended coffee roasting. The ancient popcorn
popper shown in the post below has been going strong for YEARS. They really
do not make them like they used to.

Most recently I had to work on the internals to by-pass a low-temperature
thermal switch attached to the metal chamber. It seems some manufacturers
do not want their hot-air poppers to be used for roasting coffee (I use
these poppers only outside, on concrete, at least 12 feet from the house, I
had an Alpen Roaster (linked below) experience a software glitch that
caused a fire so I am cautous and always keep an eye on the roaster). I
needed a fix for the poppers because I had bought a box of four.

I did expect that using an Arduino's pin 5 or 6 with a 960 Hz PWM, when
combined with the inductance of the motor, the inertia of the fan blades
and thermal inertia of the heater, would provide a response to varied PWM
duty cycle that is more than smooth enough for the application.

The greater failure in my recommendation was a failure to point out that
the MOSFET must be rated to standoff the peak rectified DC voltage from the
120 VAC line, roughly 170 VDC, that will be present on the circuit when the
MOSFET is off. So you would need something like a 200 volt DC rating for
the  MOSFET or DC solid-state relay in this application.

So all in all, it does look like an  AC Solid-State relay would be the
simplest way to go for modulating the second heater coil and fan!


On Tue, Jan 7, 2020 at 12:56 PM Brian via TriEmbed <triembed at triembed.org>

> Annnnd just a bit more:
> I realized that my thoughts on PWMing an unfiltered DC waveform are
> unfounded, after playing around with a circuit in a simulator[1].  If
> the PWM period is sufficiently higher than the power waveform's period
> (a couple orders of magnitude at least), then the noise averages out.
> I stand by my suggestion of using an AC switching element instead of
> trying to tie in between the bridge rectifier and the motor, though,
> simply on the grounds of ease of work.  It's a lot easier to just cut a
> wire. :-)
> -B
> [1] Check out https://falstad.com/circuit
> here's the circuit I used:
> http://tinyurl.com/yfssghhk
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*Shane D Trent*
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