[TriEmbed] Coffee roaster sanity check

[Brian]

On 12/30/19 9:15 AM, Craig Cook via TriEmbed wrote:
> Did some more learning.  Removed the heater and opened it up.

Thanks for posting the pictures!  That's extremely helpful.  I've 
whipped up a schematic of your popper from what I could tell from the 
front and back views of the heater assembly.  This gives us some crucial 
information.  Here's a link to the file:

https://drive.google.com/file/d/1e8TdR3RkUTkr46Wf_fazHuBwjvwShSG6/view?usp=sharing

or https://tinyurl.com/wt8q627

More discussion on that in a moment.

> 
> Turns out there is a Microtemp G4A01240C fuse connected to the hot AC line. That goes to a 5W2K7J resistor, which goes to the coil wire.
> 
> I let the magic smoke out of the fuse. You should not run the heater by itself without the fan cooling it down.

Oops!  Armed with the knowledge of the schematic, we can see why that 
happened, too.

> 
> There is something else connected the 5W2K7J resistor. I guess that is some sort of safety switch.

It's a temperature-controlled switch.  Based on what I can see, it's 
normally closed and opens when it gets hot.  What this does is shorten 
the heatup time of the main heater.  The heater gets all the current it 
wants until that switch senses some temperature and opens, at which 
point the 2700-ohm 5-watt resistor is placed in series and limits heater 
current.

And this is my guess at why the thermal fuse blew when the popper was 
run opened up--due to the lack of airflow, that switch didn't get hot 
enough, so the 2700-ohm resistor was never connected.  So the fuse did 
its job.

On to other discussion of the schematic!

There are two open-air coils in your heater assembly.  The larger, 
heavier, outer coil is the main heater.  The smaller-gauge inner coil 
functions as a power resistor for the motor.  It "wastes" a lot of power 
to give the motor the voltage it wants, but that heat just goes into the 
popping chamber, so it's not "wasted" from a practical sense.

It looks like the power resistor assembly is also in line with the 
motor.  This could have been a design decision made to help stabilize 
chamber temperature: when the switch opens, the heater starts producing 
less heat, so the fan slows down so as not to cool off the chamber.

Don't try to siphon power from this device to run your Arduino.  You're 
much better off powering it from an isolated supply.

Ah, I see your newest e-mail about your new SSRs on order.  Good choice.

One possible deviation from the author's design would be to insert a 
switching element inline with the fan, rather than disconnecting it and 
powering it externally.  You wouldn't want to use an SSR here, as many 
SSR's have extra circuitry that plays havoc with any attempt to drive 
them with PWM.  A circuit involving a triac and an optically-isolated 
triac driver is one option, which could sit on the AC side of the bridge 
rectifier, interrupting one of the white wires to the fan motor.  Or you 
could stick closer to the original design and power the fan externally 
with the circuit you already have.

Anyway, I hope my ramblings have been helpful!

Cheers,
-Brian