[TriEmbed] Coffee roaster sanity check

Rick nm3g at triad.rr.com
Tue Jan 7 16:43:38 CST 2020

Good evening all ...

Your Mosfet must withstand the peak rectified DC voltage PLUS the 
turn-on/turn-off transients when the Mosfet switches.

While you can soften the Mosfet turn-on and turn-off some by a gate 
current limiting resistor ... you are asking for problems if you don't 
snub the Mosfet Drain-Source, AND provide sufficient voltage withstand 
to protect the Mosfet from Gate-Drain punch-thru and subsequent 
destruction. You can't depend on the snubber alone ... it's job is to 
quench those transients so you aren't radiating lots of noise onto the 
AC line. Snubbers do fail and you don't want this to be your sole 
protection device. You can't depend on the Mosfet blowing open ... a 
short circuit keeps your heater power on 100% ... and hopefully you've 
included a thermal fuse to prevent an unplanned fire event!



On 1/7/2020 1:22 PM, Shane Trent via TriEmbed wrote:
> ...
> 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.

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