[TriEmbed] Coffee roaster sanity check

[Shane Trent]

Craig,

You can still power the cooling fan from the AC power (dropped by the
smaller heater coil) and use PWM to control the fan speed while isolating
the Arduino from the AC line if you want. There are actually a few ways to
it, two described below.

You could use a transistor output optoisolator to pull the gate of an N-FET
MOSFET up to turn it on (remember to include an appropriately sized
pulldown resistor to turn the gate off).
https://electronics.stackexchange.com/questions/294744/mosfet-switch-using-an-optocoupler


The 4N25 is a common choice but is rated for use up to only 30 volts while
the HCPL-181 looks to be around the same price and is rated for up to 80
volts.
https://www.digikey.com/product-detail/en/broadcom-limited/HCPL-181-00BE/516-1787-1-ND/1966493


Or you could use a Photovoltaic output optoisolator to drive the gate of an
N-FET MOSFET. The device below includes additional circuitry to speed up
the turn off of the FET to better support PWM. The extra speed is
unlikely to be needed for controlling a heater, particularly when you
consider the thermal mass of the beans being roasted, but it is nice to
know that you have faster PVI isolation options.
https://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/VOM1271T/VOM1271TCT-ND/3588699

http://www.vishay.com/docs/83469/vom1271t.pdf

Either option would require the addition of an anti-kickback diode, or
recirculation diode, to be installed reverse biased across the fan motor
leads to capture and redirect the current spike from the motor when the
MOSFET is turned off.

[image: MosMot1.png]
Let me know if you have any questions about implementing any of the ideas
mentioned. Good luck with your project!

Shane






On Wed, Jan 1, 2020 at 8:27 PM Craig Cook via TriEmbed <
triembed at triembed.org> wrote:

> >Oh wow, that totally slipped past me when I looked at it last.  That
> won't work because the Arduino's power supply is isolated (and that's a
> very good thing).  "GND" in your circuit is an isolated GND; there's no
> path through the MOSFET back to the neutral AC line.  The motor just
> won't run at all.
>
> Ah, I guess that's why the instructables version did not do this, it's why
> they used a dedicated DC supply for the fan.
>
> Getting the fan to work with AC may be a version 2 improvement. It's not
> as easy as I thought.
>
> >"Bresenham algorithm"
>
> I'll skip this as well.
>
> >Of course, since you're using an Arduino, you could also do some
> datalogging,
>
> Yes. That's what I'm working on now. Getting a ESP8266-01 working. I want
> to connect the system to my home wifi and send data to my desktop to create
> trend graphs. I want to see current temperature of the thermistor, roast
> start and end time, each time the SSR starts and stops, etc. I may add a
> DHT temperature sensor as well to see the current outside air temp to see
> how it affects roasting time.
>
> After about a year of using of the roaster the beans get over roasted
> (shortly after that the unit fails and the heater coil breaks). I suspect
> it is due to the fan motor getting worn out and not cooling the chamber. If
> I can graph the data for each roast I should see a trend over a few months.
> I suspect the roasting time will start getting longer when the fan gets
> worn out.
>
> Using the SSR should extend the unit's life.
>
> If I get all of this working after the next unit failure I may be able to
> predict when I need to order a spare popcorn maker.
>
> >I might have been too terse in describing what I meant.  The 10k pulling
> the SSR input to GND should stay, and a 250-ohm resistor should gobetween
> the SSR input and the Arduino pin
>
> Ah, I think I have it correct now. Updated the pdf and kicad files.
>
> Thanks
>
> Craig
>
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-- 
[image: photo]
*Shane D Trent*
Patent Agent

919-348-0061 | shanedtrent at gmail.com

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