15
8. SEQUENCE OF OPERATION
7.1 POWER ON
When power switch is turned on, 208/240 volts is applied to the two transformers.
1.) The 240Volt to 12 Volt transformer, which supplies 12 Volts for the
computer board.
2.) The 240 Volt to 24 Volt transformer which supplies 24 volts for the Motor
and Element Relays.
208/240Volts is also made available to the hot sides of the Motor Relay and the
Element Relay.
The 12 Volts is supplied to the computer board which will start the computer and
interface board. The interface board display will read “ENTER”.
The 24 Volts from the transformer goes to the interface board on the yellow wire,
number 2A. When the oven is turned on, the computer is set to turn on the fan motor.
This is done at the interface board by the “fan on signal” which then applies the 24
volts from the interface board to the motor relay coil, through the purple wire, number
3A. The current path for the relay then goes to the door switch through the blue wire,
number 10. With the door closed the fan on signal energizes the motor relay, and
turns on the motor. The motor will remain on until; the computer turns off the fan,
the door is opened or the power is turned off.
7.2 PRE-HEAT
When a product button is pressed the display will read “P
REHEAT
”. At this point the
computer board signals to turn on the heat element on. This is done at the interface
board by the “heat on signal” which applies 24 volts from the interface board to the
element relay coil, through the brown wire number 4A. The current path for the
element relay then goes to the door switch through the blue wire, number 11. With
the door closed the heat on signal energizes the element relay, and turns on the
element. The element will remain on until; the computer turns off the heat on signal,
the door is opened or the power is turned off.
There is also a safety temperature thermostat in the circuit of the element relay. If the
oven should get too hot the thermostat will open causing the element relay to open
and turn off the element. The fan in this case would continue to run.
7.3 TEMPERATURE & TIME CONTROL
The time and temperature are controlled by the computer according to the product
programs stored in the computer memory. An RTD probe in the oven is used by the
computer to determine the oven temperature. The resistance of the probe will
increase as the temperature increases. The computer compares the probe reading with
the programmed temperature and turns the “heat on signal” on when the oven
temperature drops below the programmed temperature set point, and off when the
oven temperature exceeds the programmed temperature set point.
