Simple Transformerless 5 Volt Power Supply

Description: An increasing number of appliances draw a very small current from the power supply. When designing a mains-powered device, one can typically choose between a linear power supply and a switch-mode power supply. However, if the total power consumption of the appliance is very low, transformer-based power supplies tend to be bulky, while switch-mode power supplies are generally designed to provide higher current outputs, leading to increased complexity, PCB layout challenges, and reduced reliability. A potential solution is to create a simple, minimal part-count mains (230 VAC primary) power supply without transformers or coils, capable of delivering approximately 100 mA at 5 V. A common approach could involve using an inefficient stabilizer that rectifies AC voltage and utilizes a zener diode to provide a 5.1 V output, dissipating excess voltage in a resistor. Even with a load requiring only about 10 mA, the losses could reach around 3 watts, resulting in significant heat dissipation for minimal power consumption. At 100 mA, the unnecessary dissipation could exceed 30 W, rendering this approach impractical. The primary challenge is to minimize heat dissipation and protect components from damage. The circuit described employs a JVR varistor for overvoltage and surge protection. A voltage divider consisting of resistors R1 and R2 follows the rectified 230 V. When the voltage is sufficiently high, transistor T1 turns on, preventing T3 from conducting. As the rectified voltage drops, T1 turns off, allowing T3 to conduct current into the reservoir capacitor C1. The point at which T1 turns off is determined by potentiometer P1 (typically set to about 3.3 kΩ), which adjusts the total output current capacity of the power supply. Reducing P1 causes T1 to react later, increasing current supply but also heat dissipation. Components T2, R3, and C2 form a soft-start circuit to limit current spikes, which is essential for controlling C1’s charging current during initial power-up. At a specified P1 setting, the output current through R5 remains constant, enabling load R4 to draw the current it requires while the excess flows through zener diode D5. By knowing the maximum load current, P1 can be adjusted to provide a total current through R5 that is just 5 to 6 mA above the load's maximum requirement. This configuration significantly reduces unnecessary dissipation while maintaining zener stabilization. Zener diode D5 also protects capacitor C1 from overvoltage, allowing for the use of low-cost 16 V electrolytic capacitors. Current flowing through R5 and D5, even when the load is disconnected, prevents excessive gate-source voltage rise at T3, protecting the device. Additionally, T1 does not need to be a high-voltage transistor, but its current gain should exceed 120 (for example, BC546B or BC547C may be used). It is important to note that the circuit is not galvanically isolated from the mains, and any contact with live components while the circuit is operational poses a risk of electric shock. This circuit should not be constructed or operated by individuals lacking expertise in mains voltage safety procedures.

The described circuit represents a practical solution for low-power applications where space and component count are critical. The use of a JVR varistor provides essential protection against voltage surges, enhancing the reliability of the circuit. The integration of a simple voltage divider and transistor switching mechanism allows for effective management of power delivery, while the soft-start circuit mitigates inrush current, protecting sensitive components from damage during power-up. The adjustable potentiometer enables fine-tuning of the output current, ensuring that the circuit can accommodate varying load demands without excessive heat generation. The zener diode serves a dual purpose, providing voltage regulation and protecting the reservoir capacitor from potential overvoltage conditions. The overall design emphasizes efficiency and safety while addressing the challenges posed by low-power mains-powered devices. Proper precautions must be taken during assembly and operation to ensure user safety and circuit integrity.An increasing number of appliances draw a very small current from the power supply. If you need to design a mains powered device, you could generally choose between a linear and a switch-mode power supply. However, what if the appliance`s total power consumption is very small Transformer-based power supplies are bulky, while the switchers are gen

erally made to provide greater current output, with a significant increase in complexity, problems involving PCB layout and, inherently, reduced reliability. Is it possible to create a simple, minimum part-count mains (230 VAC primary) power supply, without transformers or coils, capable of delivering about 100 mA at, say, 5 V A general approach could be to employ a highly inefficient stabilizer that would rectify AC and, utilizing a zener diode to provide a 5.

1 V output, dissipate all the excess from 5. 1 V to (230G—v2) volts in a resistor. Even if the load would require only about 10 mA, the loss would be approximately 3 watts, so a significant heat dissipation would occur even for such a small power consumption. At 100 mA, the useless dissipation would go over 30 W, making this scheme completely unacceptable. Power conversion efficiency is not a major consideration here; instead, the basic problem is how to reduce heavy dissipation and protect the components from burning out.

The circuit shown here is one of the simplest ways to achieve the above goals in practice. A JVR varistor is used for overvoltage/surge protection. Voltage divider R1-R2 follows the rectified 230 V and, when it is high enough, T1 turns on and T3 cannot conduct. When the rectified voltage drops, T1 turns off and T3 starts to conduct current into the reservoir capacitor C1.

The interception point (the moment when T1 turns off) is set by P1 (usually set to about 3k3), which controls the total output current capacity of the power supply: reducing P1 makes T1 react later, stopping T3 later, so more current is supplied, but with increased heat dissipation. Components T2, R3 and C2 form a typical soft start` circuit to reduce current spikes this is necessary in order to limit C1`s charging current when the power supply is initially turned on.

At a given setting of P1, the output current through R5 is constant. Thus, load R4 takes as much current as it requires, while the rest goes through a zener diode, D5. Knowing the maximum current drawn by the load allows adjusting P1 to such a value as to provide a total current through R5 just 5 to 6 mA over the maximum required by the load. In this way, unnecessary dissipation is much reduced, with zener stabilization function preserved. Zener diode D5 also protects C1 from over voltages, thus enabling te use of low-cost 16 V electrolytics.

The current flow through R5 and D5, even when the load is disconnected, prevents T3`s gate-source voltage from rising too much and causing damage to device. In addition, T1 need not be a high-voltage transistor, but its current gain should exceed 120 (e. g. BC546B, or even BC547C can be used). The circuit is not galvanically isolated from the mains. Touching any part of the circuit (or any circuitry it supplies power to) while in operation, is dangerous and can result in an electric shock!

This circuit should not be built or used by individuals without proper knowledge of mains voltage procedures.