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How does it work - Part 2

A project log for The Toaster

Restoring/Repairing a fancy (and expensive) toaster

tim-savageTim Savage 07/16/2018 at 13:380 Comments

In this part I investigate the high voltage and mechanical side. All of the high voltage circuits are controlled via the relay board MT2F15-4A (while working out a schematic for this board I all filled out all the pin details from Part 1). 

This board is hard wired into the unit and I didn't really want to remove it, I did however, wire up a quick control mockup on a breadboard and use my bench-top power supply to power the low voltage side. Next I (very carefully!!!) applied mains power to board to determine the exact function of each relay. This also confirmed that all of the heating elements are working and that the motor to raise and lower the lifter runs.

Missing from this picture is the plastic bracket that sits above the motor frame, this encloses the high voltage relay board. The bracket also provides structural rigidity when the motor is in operation (see the video at the end of this log for it running).

Next I wanted to understand the wiring of the heating elements, the unit does allow for the outside heating elements to be controlled independently to allow the cooking of crumpets where you don't wish to toast one side as long. Complicating things I discovered that transistor Q5 is a dead short (this was the first control pin I investigated as it controlled the motor). This will need to be replaced, I guess it's about time I took on surface mount components. A lot of care needs to be taken when testing this board there is not a lot of electrical isolation between high and low voltage sides.  
The outcome is the following schematic describing the operation of the unit. Low voltage side to the left and the mains side on the right.
PinUsage
1+12V roughly, this is actually the raw voltage out of the rectifier, probably closer to 16V.
2Ground
3Centre only, applying 5V turns the outer heating elements off. Controls both NC relays 4 & 5.
4Motor on, applying 5V turns on the motor. Controls relay 1
5 & 6Heating elements on. This looks to be a safety feature, both pins require 5V to turn on
both NO relays 2 & 3. These switch both the Active and Neutral.

And finally the mechanical motor lift. Note the two limit switches tripped by the rotating cam.

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