Abstract: A simple pulse width modulator speed control for a brushless DC "pancake" type fan motor utilizes a type 555 timer for driving a transistor switch, connected in series between the DC voltage supply and the fan motor, at about a 10 Hz rate. The duty cycle of the pulsed output of the timer is controllable by a variable resistor, which in the preferred embodiment comprises a thermistor for controlling the fan speed as a function of temperature. The fan motor is restarted on each cycle of full voltage amplitude pulses and consequently will start under all operating conditions.

Abstract: A constant current battery charger includes a bucking regulator comprising a power switch transistor, a diode, an inductor and a voltage sensitive switch with adjustable delay for controlling the power switch transistor. The hysteresis or delay is obtained with a Schmitt trigger and the effect is a stable pulse width modulator in which the duty cycle of the power switch transistor is varied. A temperature sensor adds resistance to change the sensitivity of the switch and reduce charging current when a predetermined battery temperature is reached. A speed up circuit consisting of a shorting transistor is connected across the base-emitter of the switch transistor.

Abstract: An automatic voltage doubler switch includes a triac for converting a rectifier arrangement from a direct to a doubler mode of operation. Drive current for the triac is supplied by a resistor and capacitor circuit connected across one of the rectifiers in the bridge network. A self-biasing SCR trigger circuit senses the AC line voltage and has its anode and cathode coupled across the capacitor. When the AC line rises to a predetermined level, the SCR fires and shorts out the capacitor to render the triac nonconductive. The capacitor delays drive current on start up until the SCR trigger can sense the AC line voltage. The capacitor also maintains the triac conductive during opposite polarities of the AC line. The capacitor also maintains the SCR in conduction during phase reversals of the AC line. A switchable RC delay is activated only during the voltage doubler mode to delay operation of the SCR trigger.

Abstract: An automatic voltage doubler switch includes a Triac having a gate electrode that is driven by a relaxation oscillator that includes a Diac switch. A capacitor, charged from a voltage that is indicative of the AC line voltage, triggers the Triac through the Diac switch to complete a conductive path for converting the power supply from a direct operating mode to a voltage doubler operating mode. A cutoff transistor senses the AC line voltage and functions to disable the relaxation oscillator when the AC line is at a particular level. A hysteresis circuit reduces the resistance in the base circuit of a line voltage detector transistor to keep it in conduction despite further drops in line voltage. A hysteresis inhibit transistor operates on power up of the power supply to override the effects of the hysteresis circuit.

Abstract: A start-up circuit for a switch mode power supply comprises a relaxation oscillator including an RC charging circuit for developing a trigger voltage for firing a Sidac connected in series with the primary winding of a small ferrite core transformer. The secondary winding of the transformer is resonated with a tuning capacitor and the voltage coupled to a rectifying circuit for developing a charge across a trigger capacitor. When the trigger capacitor voltage achieves a trigger potential, an SCR is fired for starting a pulse width modulator in the switch mode power supply. The relaxation oscillator is rendered ineffective by a feedback voltage supplied from the power supply when it starts.

Abstract: A start-up circuit for a switch mode power supply includes a transformer supplying a full wave rectifier and developing a reference potential from a zener diode connected at the output thereof. An SCR has its anode-cathode circuit connected between the rectifier output and the input terminal of the switch mode power supply and its gate coupled to the zener diode. The power supply applies a boot strap voltage, higher than the reference potential, to its input terminal which back biases the gate of the SCR. The transformer is, therefore, effectively disconnected from the switch mode power supply after start-up. The start-up circuit also can function as a voltage regulator in which the rectifiers conduct as needed during the AC voltage ripple peaks to maintain an output voltage determined by the reference potential.

Abstract: A low voltage shutdown circuit for a PWM power supply includes a voltage divider supplying bias for a transistor having a load resistor that is part of an integrating network. The capacitor in the integrating network is coupled to a voltage sensitive switch (Diac) which feeds an LED detector in a photo optic coupler. Under normal conditions, the transistor is in saturation and the capacitor is discharged. The transistor comes out of saturation when the voltage level across the divider drops to a predetermined level and the capacitor charges to the trigger voltage of the Diac, which fires the LED in the photo optic coupler for developing a shut-off signal.