Sunday, November 17, 2013

Toast-R-Reflow schematics

Here are the schematics for the two boards that comprise the Toast-R-Reflow project.

This is the controller board. In the upper left corner is the thermocouple amplifier. Below that is the 5 volt power supply. The input jack is a standard 2.1mm barrel connector. The rest of the circuit is the micro controller and its I/O. In essence, the controller takes in the temperature from the thermocouple and controls two digital output lines that go off to the other circuit (below). There are 330 ohm resistors on those two lines because they're intended to drive an optoisolator input, meaning that you want around 20 mA of current or so - enough to light up an LED. There is an i2c connector on the board that leads off to an OpenEVSE or AdaFruit RGB LCD backpack/shield, which is also how we get a pushbutton input (used to start the reflow process).

This is the power board. It's a separate board because it has 120 volt power running through it. Note that the path from the hot terminal through the triacs and to the output terminals are specified to carry up to 750 watts of power, so they're going to need to be very wide traces on both sides of the board.

The triacs are also going to need fairly substantial heat sinks. Even more so because they're going to be in proximity to the actual heated space of the oven. I think we can safely assume an ambient temperature of 50 degrees C, at least. The triac data sheet says that at the design current, we can expect it to dissipate 10 watts of thermal energy. The heatsinks I've ordered have a 5 degrees C per watt rating. Combined with the triac mounting terminal spec, the online calculators I've seen suggest that this ought to be a combination that should be acceptable.

The power board is a separate board so that it can be built-in to the oven. The two isolator input lines and ground can safely be routed out of the oven to the controller board, along with the thermocouple cable.

The three large terminals on the schematic are actually in the SparkFun electro-mechanical eagle library. They're #4 holes designed to be electrically connected. #4-40 hardware will be used to attach QD male connectors directly on the board. The QD female connectors on the wires of the oven will plug right in.

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