555 timer icPower SuppliesZener Diode

Switch-Mode 555 Supply Schematic Circuit Diagram

Switch-Mode Power Supply Using a 555 Timer IC

This switch-mode power supply, designed around a 555 timer IC, delivers a maximum output voltage of 40 V from a 12 V input. The output voltage can be easily configured using a Zener diode, with the requirement that it must be higher than the input voltage, with a minimum output of 12 V. The NE555 IC operates unconventionally in this setup. Typically, the output of this oscillator IC is low longer than it is high, but in this specific configuration, the output can be high for a shorter duration than it is low.

Operation and Energy Transfer

In this setup, the NE555 controls the switching of FET T1. When T1 is conducting, energy is stored in inductor L1. Upon T1 ceasing conduction, this stored energy transfers to capacitors C1 and C2 via Schottky diode D1, causing the voltage across these capacitors to rise. Zener diode D2 limits this voltage. If it surpasses the Zener voltage, the current flowing through the Zener diode triggers the conduction of T3. Consequently, the voltage on pin 5 of the NE555 decreases, shortening the duration of the high level on pin 3. Consequently, T1 conducts for a briefer period, reducing energy storage in L1 and stabilizing the output voltage.

Switch-Mode 555 Supply Schematic Circuit Diagram

Current Limiting and Voltage Regulation

Current limiting in this circuit is achieved through the combination of resistors R6, R5, and transistor T2. When the voltage across R6 exceeds 0.6 V, T2 starts conducting. This, in turn, triggers T3 into conduction, leading to a decrease in voltage and effectively limiting the current flow.

Soft-Start and Component Values

A soft-start effect is introduced by components C5 and R7. The value of resistor R1 can vary from 22 kΩ for an output voltage of 15 V to 10 kΩ for an output voltage of 40 V. It’s crucial to cap the Zener voltage at a maximum of 40 V for safety. T1 and T2 should be rated for a maximum of 50 V. While the FET choice is not overly critical, ensure it can handle the required current. If the coil becomes warm, it indicates the core is too small or the wire used is too thin. Among all components, the Schottky diode is the most crucial. Using an ordinary diode is not advisable as it can overheat. Old computer power supplies often have Schottky diodes suitable for this application.

Power Output and Safety Measures

This power supply can deliver around 200 W and accepts input voltages between 7 V and 15 V. It’s essential to note that the NE555 can handle a maximum voltage of 15 V. Lastly, it’s important to highlight that this power supply circuit does not have inherent short-circuit protection. To mitigate risks, a slow-blow fuse on the 12 V side is highly recommended.

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