Book Analog Interfacing to Embedded Microprocessors
The output
voltage will also be 6V, so the current through the heater is V/R, or 6v/15W, or
400 ma. This is a power dissipation of (P = I ¥ E) 6 ¥ .4, or 2.4 watts.
The opamp, operating with a supply voltage of 16v, has to supply the 400 ma current
to the heater. But since the supply is 16v and the heater voltage is 6v, the remaining
voltage (16v - 6v = 10v) is dropped across the opamp output stage. This means the
opamp is dissipating 10v ¥ 400 ma or 4 watts. Unless it has a good heatsink, it may get
hotter than the heater.
The total dissipation in this circuit is the sum of the opamp dissipation plus the
heater dissipation. This is 2.4 watts plus 4 watts or 6 watts. (You can also find this
by multiplying the power-supply voltage by the total power-supply current: 16v ¥
400 ma = 6.4W). The table below shows the power dissipated for various input/heater
voltages:
245
Figure B.1
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