Reflow oven controller

[dande]

Hi There!
I put together a simple USB based reflow oven controller. I am interested on your thoughts on the design, if there is any flaw in it. I am pretty newbie, so there might be. It uses a ATMega328P as the main controller and a FT232 for the USB connection. A MAX31855 is used to convert the thermocouple voltage levels to usable ones, it will also use a electromechanical relay to turn the oven on and off.
I am not quite sure if the FT232 3.3V out's 50mA is enough to drive the MAX31855, or if I should use a dedicated voltage regulator. Also unsure if the USB connection needs protection fuse and/or a ferrite.
Thanks all!

[bobcat]

The basic circuit looks OK.
The USB should be able to power this just fine if you use a solid state relay. I suggest that you use a solid state relay for the oven control. A mechanical relay will eventually fail because it will be switching a lot.
Filtering and ESD protection is a must on USB powered devices.
Don't forget to add hysteresis to the temperature programming!

[dande]

Hey, thanks!
So using a fuse on the V+ line is enough protection?

[bobcat]

A fuse not really necessary. ESD protection and some filtering is more important.

[dande]

Okay, I just realised something terrible. Since, the max31855 is 0-3.3V, I need to convert the voltage levels between it and the atmega chip, especially regarding the clock signal which goes from 0-5V from the atmega.
I believe the max31855 high output of 3.3V is understood by the atmega as high.

[bobcat]

You can operate the ATMEGA328 at 3.3v if you use a 10 Mhz or less oscillator.
10 Mhz should be plenty fast enough for this application.