In a digital control circuit, a clock pulse is counted so that when a predetermined timing is reached the driving of a load is controlled. The digital control circuit may preferably be an integrated circuit for controlling the combustion of a burner for a combustion apparatus such as a water heater or an air heater. The combustion control circuit performs the igniting operation for a predetermined period of time and the supply of fuel is stopped if the ignition can not be attained within the predetermined period. In the digital control circuit, a clock pulse having a predetermined bias voltage is counted by a time measuring circuit and whether the clock pulse has the predetermined bias voltage or not is judged by a detecting circuit so that the igniting operation is stopped when the clock pulse has the predetermined bias voltage, thereby avoiding a risk that, if the input of the clock pulse is interrupted within the predetermined period, the predetermined period may be prolonged indefinitely and the supply of fuel may be continued even though ignition has not been attained.

A solid-state relay/breaker that can replace mechanical units to control any type of AC or DC load. Power MOSFETs or other transistors control a load current. A bypass current sensing path monitors MOSFET current and causes a shutdown through signal processing in the event of an over-current condition. This shutdown resembles that of a slow-blow fuse where the rate of shutdown is proportional to the value of the current. This prevents shutdown on momentary spikes or in-rush. In addition, temperature and internal power supply voltages are monitored to determine additional operational conditions where over-temperature or voltages out of range can also cause shutdown. The MOSFET and current sensing path are turned on and off in staggered timing with different slew rates to provide built-in hysteresis. The device can be manufactured in any type of package to match any type of environment or existing replacement requirement.

A power failure indicator circuit for a pulse width modulated power supply compares the averaged voltage of the pulse width modulated signal to an input reference voltage to determine if power failure of the supply will occur. The reference voltage corresponds to a predetermined proportion of the maximum average voltage of the pulse width modulated signal. In a preferred embodiment, the pulse width modulated signal is inverted and filtered to generate an averaged signal corresponding to the DC input voltage to the modulator within the pulse width modulated power supply. A comparator compares the averaged signal voltage to the reference voltage to determine if the averaged signal voltage level has fallen below the reference voltage level. If the averaged signal voltage level does drop below the reference voltage level, the comparator generates a signal warning of power failure of the power supply.

A short circuit protection system that reacts to a voltage across an electrical load to prevent overdissipation of a semiconductor switch in the event of a short circuit. The system includes a capacitor having a charge state. When the capacitor is charged, the electrical load is turned ON via a transistor. The system automatically latches the electrical load ON until a short circuit occurs or until the capacitor discharges. When a short circuit occurs, the system automatically turns the load OFF.

In one embodiment of the present invention, across an electrical load to prevent over dissipation semiconductor switch in the event of a short circuit. The system includes a microprocessor with a terminal which is alternatively configurable as an input and as an output. When the terminal is configured as an output, the microprocessor can turn the electrical load ON and OFF via a transistor, under software control. Once the microprocessor turns the load ON, the microprocessor reconfigures its terminal to be an input. The system automatically latches the electrical load ON until a short circuit occurs or until the microprocessor turns the load OFF. When a short circuit occurs, the system automatically turns the load OFF. With the microprocessor terminal configured as an input, the microprocessor can detect when the system has automatically turned the load OFF due to a short circuit, thus being able to set a diagnostic code or take other appropriate action. A low-pass filter can be added between the microprocessor terminal and the transistor, reducing the inductive voltage spike which may occur when the load is turned OFF.