Table of Contents
Advertisement
r
"
C
AVM930-MWNS
r
I
,~
1. Circuit description
C
The circuit diagram can be divided info the following main parts:
~
Microprocessor
Voltage regulator and reset
r
LED driving circuit
\
Display
'-
Key detection circuit
Buzzer drive circuit
('
Function drive with LEDs
.,
Temperature probe interface
2. Microprocessor
C
The microprocessor
controls the complete operation of the oven.
3. Voltage regulator and reset
r
The vortage regulator and reset circuit consists of the zenerdiode 6001 and the transistor 7001 and the resistors 3005
\,-
and 3006. If the voltage ac ross the circuit is lower than the zener voltage, the transistor 7001 is off and holds the reset
input of the microprocessor
7002 low.
-:
C
'
When the vortage rises above the zener voltage plus Ube of 7001, the reset input of 7002 goes high and the vortage bet-
ween U1 and UO is stabilized to approx. 5 V. The regulator circuit is supplied with current partly via the resistor 3001 and
the zener diode 6002, and partly via the current generator 7044 and the array of LEDs 6046-6049, 6051-6055.
-
r
The zenerdiode 6002 provides the current generator 7044 with a constantvoltage
of -5 V with reference to UO. The transi-
stor 7044 gene rates a constant current of approximately 13 mA through the LED array. Each LED is shunted by a transi-
stor. The transistors 7045-7047 are driven via the extra driver stages 7041-7043 since the outputs R70-R72 are of the
C
type sink open drain,
while the outputs
P51-P53,
P60-P63
are of the type source
open drain.
The outputs
P51-P53,
P60-P63
are provided
with internal
pull down resistors
of typically
80 kQ.
Due to the base currents info the shunt transistors the current through the LED 6055 will be approximately
19 mA when
this LED is lit in the AWD-mode.
("
"-
4. Display
The display is of the FTD type (Fluoroscent Tube Display). The display segments and grids are driven directly tram the
microprocessor
outputs P1 0-P13, P20-P23 (segments) and P40-P43, P50 (grids). These outputs can drive a high voltage
(
and are provided with internal pull down resistors. The filament of the display also forms the cathode, and the filament
-vortage
is clamped to U2 on the power supply assembly. The display is driven in multiplexed mode, and the grid outputs
also provide the scan signals tor the key-scanning.
.
(
'-
5. Key detection circuit
The keys form a matrix consisting of Jive row inputs, provided by the display grid signals, and two column outputs, which
are detected by the transistors 7031 and 7032. The key switches are isolated tram the display segments by diodes in
C
order to prevent the switches tram affecting the display. The door switch is also apart of the switch matrix.
-If
one ofthe switches 1031, 1032,1034, 1036 or 1038 is closed there should be negative going pulses on the input KOO of
the processor.lf one ofthe switches 1033,1035,1037,1039
orthe door switch is closed, pulses can be observed on the
(
input K01.
I
~
~-
6. Buzzer drive circuit
r
The buzzer is driven by transistor 7004. The signal is taken tram port R91 of the processor, and this output must be high
\.
when the buzzer is off in order to prevent excessive power dissipation in the resistors 3024 and 3027.
.
e
7. Function selector with LEDs
(
The position of the function selector switch is indicated by the LEDs 6011-6014. The switch position is detected by the
inputs K02 and K03 of the microprocessor.
The microprocessor
can only distinguish between grill positions and FA.."
positions.
C
8. Temperature
probe interface
The resistors 3083 and 3084 compensates tor the non-linearity
of the temperature probe. For the temperature range
35 -95 degrees centigrade the vortage at pin 2 of 7082 is linear within ::!:1 degree. The vortage divider 3065 -3067 provi-
(
des a high and a low reference vortage forthe A/D-converter. The resolution ofthe A/D-converter is two units per degree
-centigrade.
t.~
Data is transferred in se ries tram the A/D-converter to the microprocessor. The A/D-converter is selected tram port R83
n
which is connected to the /CS input. The dataflow is controlled by the crack signal tram port R92, and data are received
~
on port R92. Normally the output DO on pin 6 on the A/D-converter is in tri-state, so the input R92 on the microprocessor
will be held low by 3082.
n
If a temperuture probe is connected, 3081 will cause this input to go to U1. In this way the processor can detect the pre-
,
sence of a temperature probe. If a temperature probe is connected, but the measured resistance corresponds to a tem-
perature below -10 degrees centigrade,
it will be assumed thai the probe is broken. The oven will go to temperature
mode, but will then immediately go back to stand b:'.
/
\
Ed. 01 -May 91
C
Advertisement
Table of Contents
