[TriEmbed] Coffee roaster

[Shane Trent]

Craig,

Power MOSFETs include a reverse bias body diode that will conduct on the
negative half-cycles of the AC when used as an AC switch. To turn off the
AC to the heater you would need to put two MOSFETs in series with one of
them reversed. This would let them stop AC but you would still need to use
higher voltage MOSFETS. The AC line's nominal 110 V rating is RMS (kind of
the effective power delivery vs a DC signal of the same voltage). The
actual peak voltage on the AC line is closer to 170 volts. You also cannot
stack the voltage rating of the back-to-back MOSFETs because one of the
body diodes will always be forward biased. So each diode must be able to
tolerate 170V minus one body diode drop (0.6 V nominal). So you could use
two MOSFETs rated for 200V wired back-to-back in series. You would want low
Rds devices to reduce the power dissipation but they will be more expensive
that your typical parts bin MOSFET. The MOSFET could do great PWM control
but you would want to drive them with a opto-isolator with a photovoltaic
output. The link below show's Digikey has 47 isolators (some multiple
channel) in normal stock that would fit the bill. This would let you drive
them with a microcontroller while keeping the microcontroller, and your
computer and you isolated from the AC mains.

https://www.digikey.com/products/en/isolators/optoisolators-transistor-photovoltaic-output/903?FV=ffe00387%2Ca400b4&quantity=0&ColumnSort=0&page=1&stock=1&nstock=1&k=opto+isolator&pageSize=500&pkeyword=opto+isolator

Your SSR would work well or you could use a Triac for controlling the AC
voltage. The SSR includes it's own internal opto-isolator but if you use
your own Triac you would need to add a triac-output opto isolator. Digikey
lists 513 triac opto isolators that would fit the bill. One issue
difference between using MOSFETs verse a SSR or Triac though. The MOSFETs
would have a very low power dissipation due to their low Rds when fully
saturated. While a triac will have a fixed forward voltage (1-1.5V) and
will dissipate V * I power when conducting. So at Triac conducting 5 amps
would dissipate 5 to 7.5 Watts, where the pair of mosfets at 10 mOhms each,
would dissipate 2W. The triac would be much cheaper though.

https://www.digikey.com/products/en/isolators/optoisolators-triac-scr-output/904?k=triac+optoisolator&k=&pkeyword=triac+optoisolator&pv41=719&sf=0&quantity=&ColumnSort=0&page=1&stock=1&nstock=1&pageSize=500

On other thing to watch is that removing the fan motor from the auxiliary
heater circuit means that heater will see 16 volts higher than it's design
specification and would dissipate around 37% more power. You may want to
avoid driving the axuulary heater at 100% duty. You can either use a
rheostat or just PWM at a reduced duty cycle.

(110^2)/(110-16)^2

I look forward to reading about your experiments
Shane





On Mon, Oct 29, 2018 at 11:23 PM Craig Cook via TriEmbed <
triembed at triembed.org> wrote:

> Since I can't upload pictures here, you should be able to see some here:
> https://drive.google.com/open?id=1qrpDKCUUwSSoPdv_TVqjslM3eq2wf0U_
>
> I think the suggestion is the red wire goes to a heating coil using 110
> Volts.  The black wire coming back into the motor (that drives the fan) has
> been dropped to around 16 Volts.
>
> I need a way to disconnect the heater coil and control it. That's where
> the solid state relay S216S02 comes in. One side will have 110V, the other
> receives a signal from the Arduino telling it to turn on and off.
>
> I also need to disconnect the fan motor and drive it using the Arduino.
> That's where I need an independent 16V applied to the fan motor.
>
> Actually, given I am only dealing with 110V, is there a better solid state
> relay choice?
>
> I have some of these "IRF510N IRF510 Power MOSFET N-Channel 5.6A 100V" Can
> I use one for the heater coil and another for the fan motor?
>
> I also have some of these "039N04L MOSFET"
>
> Thanks
>
> Craig
>
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