Friday, November 29, 2013

Toast-R-Reflow: revised schematics

I've revised the schematics a bit. First, the power board:
 There's no actual change to the circuit here, just improvements in the layout.

The controller:

The change here is that a bunch of passive components have been added to the thermocouple input. There is a 1M resistor from the negative pin to ground. I'm not 100% sure why, but it's what the data sheet recommends. The RC network on the thermocouple pins is recommended to prevent RF from affecting the readings. Other than that, it's the same.


jjhannon56 said...

I was curious about the 1M resistor too...
I found out that the 1M resistor is for Thermocouple break detection.

From the data sheet:
The inputs of the AD849x are PNP type transistors, which means that the bias current always flows out of the inputs. Therefore, the input bias current drives any unconnected input high, which rails the output. Connecting the negative input to ground through a
1 MΩ resistor causes the AD849x output to rail high in an open
thermocouple condition.

Nick said...

You're right. And, as it happens, failing to a high output is a good thing for an oven controller, as it will prevent the elements from switching on. Failing low would cause the elements to switch on 100% and potentially overheat.