Book Analog Interfacing to Embedded Microprocessors
temperature measured at the thermistor reaches the desired point. It then
turns the heater off. When the temperature drops below the setpoint, the
heater is turned on again and the heater temperature goes back up. The tem-
perature oscillates around the setpoint.
Figure 5.3 shows the actual temperature of the heater and the temperature
of the thermistor. As you can see, they don’t quite match, either in time or in
amplitude. When the heater is first turned on, it overshoots the setpoint by
some amount, then oscillates around the desired temperature. The key
reasons for this are:
Output Control Methods 105
Figure 5.3
On-off control system.
• The coupling between the heater and the heated object is not perfect. The
heater temperature must be higher than the object it is heating, to be able
to transfer heat into it.
• The object being heated has some thermal mass, so it doesn’t heat up or
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