Resistance furnace drive and measurement circuit

Resistance furnace drive and temperature measurement circuit: The resistance furnace
drive and temperature measurement circuit mainly implements the temperature measurement and
driving functions of the controlled object. The circuit
is shown in Figure 27-61. A Jinchromium-Constantan thermocouple is used as the temperature sensor
, and the gain of the operational amplifier is adjusted through a potentiometer
so that the output of the operational amplifier is 5v when the temperature is 200°C.
The output of the operational amplifier is simultaneously converted into a digital quantity through A/D conversion
, and then output to the driving device after D/A conversion
. The output voltage of the D/A converter can control
the PWM duty cycle of the TL494 output. The larger this voltage is
. The longer TIA 94 outputs high level. When
the TIA94 output is high, the MOC3041 optical coupling thyristor is turned on by zero-crossing triggering, and the 220V AC power is connected in series with the following thyristor,
so the resistance furnace can be heated. When the TIA94 output is low, the MOC3041 optical coupling thyristor is disconnected, and the 220V AC power Disconnect at the same time
to stop heating the resistance furnace. In this way, we can control the heating time of the 220V- AC resistance furnace
by controlling the output of the D/A converter , thereby controlling its temperature. There are two purposes of using MOC3041 in the heating circuit: one is to realize the isolation of strong current and weak current; the other is to realize the zero-crossing trigger
    of the bidirectional thyristor , so that the current waveform flowing through the thyristor is a sine wave and reducing the harmonics.