Several safety-critical solenoids in a lasermeasurement system on an automotive-assembly line required protection from internal overheating during normal operation. After a 60sec activation, the solenoids required 180sec to cool before their next activation. One apparently straightforward protection circuit would comprise a timer based on a microcontroller, some support components, and a short program written in C++. However, the project would require evaluation and selection of a suitable microcontroller, purchase or rental of a device programmer, and considerable time in programming the microcontroller and evaluating its operational hazards.
As an alternative, I recalled the words of my tutor: "Decrease the number of dangerous components to decrease the risk of danger." A simple analog circuit would be safer, smaller, and easier to maintain. The circuit in Figure 1 uses a traditional analog method of measuring time: the charge and discharge behavior of a resistancecapacitance circuit.
Figure 2 highlights the circuit's timing components. Capacitor C2, a tantalum electrolytic with ±10% tolerance, diode D1, and resistors R2 and R5 constitute a double-RC (resistorcapacitor) circuit. During solenoid activation, R2 provides a charging path for C2, and diode D1 prevents C2 from discharging through the solenoids. When the solenoids are off, the discharge path comprises R2 plus R5, which provides a longer time constant. The difference between the two time constants determines the solenoids' activation and recovery periods. A Schmitt trigger designed around one-half of IC1, an Analog Devices (www.analog.com) AD822 dual operational amplifier, senses the voltage across C2 and defines the solenoids' cutoff- and turn-on-timing intervals. An intermediate buffer stage, IC1B, drives a Microchip (www.microchip.com) TC4432 MOSFET driver, which in turn controls the gate of Q1, an Nchannel power MOSFET that drives the solenoids from 24V.
When Q1 switches on, the voltage level across C2 increases, and, after 60sec, the output of the Schmitt trigger falls from 12 to 0V. The buffer stage drives the cathode of diode D2 to 0V. The voltage at D2's anode reaches 0.7V and is insufficient to trigger MOSFETdriver IC2. Q1 now switches off, removing supply voltage from the solenoids and reverse-biasing diode D1. Capacitor C2 starts to discharge through R2 and R5, and the input voltage you apply to the Schmitt trigger falls at a slower rate than during the charging interval. After 180sec, the Schmitt trigger's output rises to 12V, and the circuit awaits arrival of another external trigger pulse through resistor R3.
