[TriEmbed] Coffee roaster sanity check

[Brian]

On 1/9/20 9:08 PM, Craig Cook via TriEmbed wrote:
> I appreciate everyone's advice. I think I added to the confusion of what I am playing with.

We aim to please and/or confound! XD

<snip>

> For the Fan I was basing my ideas off this:
> https://itp.nyu.edu/physcomp/labs/motors-and-transistors/using-a-transistor-to-control-high-current-loads-with-an-arduino/
> 
> It seems to control a 12 VDC motor using a FQP30N06L 60V LOGIC N-Channel MOSFET
> 
> 
> The fan I am using seems to be running from 20 VAC with a bridge rectifier.
> 
> Plan A
> 
> Use an external 19 VDC, 2A power supply with a FQP30N06L to do PWM to control fan speed. Based on my understanding of the nyu tutorial, I think that should work. The onboard 20 VAC supply will not be used.

This is definitely the best approach if you have the external DC power 
available.  Much simpler and safer.

> 
> Plan B
> 
> Use the built in 20 VAC supply with something else to do PWM control for fan speed.
> 
> I believe the consensus for Plan B is to use an AC Solid-State relay.
> 
> Given that I know 20 VAC and 58.3 ohms are involved with the inner coil, I believe that means there is 0.34305 amps involved.

Not quite.  You need to add up the impedance of the inner coil AND the 
motor, and then divide that into 120 VAC to get your current.  Also note 
that AC impedance isn't quite the same as DC resistance, but for heavily 
inductive loads (like motors and coils), the actual impedance is higher, 
so your DC math will give you a good "worst case" cushion around the 
actual AC current.  Then double that to spec your parts.

> 
> If I want to use the existing AC line, I could use one of these to control it https://www.digikey.com/product-detail/en/ixys-integrated-circuits-division/CPC1706Y/CPC1706Y-ND/3077519 (along with resistors to drop the ardunio digital control into the 1.2 VDC range)

Actually, no.  That SSR is rated for a load voltage of maximum 60 V. 
But when your fan circuit is turned off, no current is flowing through 
the inner heater coil, so no voltage is dropped across it; therefore 
your switching element will see the full 120 VACrms (~170V pk) when it 
is in the "off" state.  I'm sorry to say I've forgotten who mentioned it 
first; someone did a few layers back in the thread.

> For both Plan A and B I was planning to leave the bridge rectifier in place.

No harm in that.

-B