Design Ideas: June 22, 1995

Microcontroller watches, controls ac power

Samuel Kerem,
Infrared Fiber Systems, Silver Spring, MD

For monitoring and controlling a power circuit from a microcontroller (µC), Fig 1 replaces the conventional technique of a low-value resistor and differential amplifier (with gain set for the expected current value). A control line turns triac Q₁ on and off via optoisolator IC₂. When an appliance is plugged into J₁ and turned on, the voltage drop across Q₁ is independent of the current value and sufficient to turn on optoisolator IC₃. The phototransistor current of IC₃ charges
C₁to a logic 1, which is sensed by the µC. When IC₂ is on, but the device in J₁ is actually turned off (by its own switch), there is no current through Q₁, and the sense line is at logic 0 because C₁ is discharged through R₄.

When the µC turns Q₁ off (even though the device in J₁ is still switched on and therefore conducting), current flows through pins 1 and 2 of IC₃, as well as R₃ (a positive-temperature-coefficient resistor with a large resistance change resulting from a small temperature change). In this case, R₃ sharply increases its resistance to keep the current flowing through it at about 15 to 20 mA. However, even though C₁ is charging to logic 1, the µC "knows" the device is not really on, because it has set power switch Q₁ to the off state via the control line. (DI #1722)