Challenges in Low-Power Appliance Design
Modern appliances often demand minimal power supply currents, raising a dilemma in choosing between linear and switchmode power supplies. Traditional transformers are bulky, while switchmode alternatives often cater to higher current outputs, leading to complexity and reliability issues. The challenge arises in crafting a straightforward mains-powered solution (230 VAC primary) capable of delivering approximately 100 mA at 5 V without relying on transformers or coils.
Inefficient Stabilization Approach
A typical approach involves employing an inefficient stabilizer to rectify AC, utilizing a zener diode for a 5.1 V output. Excess power dissipates in a resistor, but even with a 10 mA load, substantial heat dissipation (around 3 watts) occurs. Scaling up to 100 mA amplifies this dissipation to over 30 W, rendering the approach impractical. Efficiency takes a back seat here; the primary concern is reducing heat dissipation and safeguarding components from overheating.
A Simple and Practical Solution
The circuit presented here offers an elegantly simple solution to address the aforementioned challenges. It efficiently tackles the issues of heavy dissipation and component protection, making it a practical and effective choice for low-power applications.
Overvoltage Protection with JVR Varistor
In this circuit, a JVR varistor plays a key role in safeguarding against overvoltage and surges. The voltage divider formed by R1 and R2 processes the rectified 230 V power. When the voltage reaches a specific threshold, T1 conducts, preventing T3 from conducting. As the rectified voltage drops, T1 switches off, enabling T3 to channel current into the reservoir capacitor C1. The interception point, set by P1 (typically around 3k3), dictates the power supply’s total output current capacity. Adjusting P1 controls the current supply, but this also affects heat dissipation due to delayed reactions in T1 and T3.
Soft Start Circuit for Controlled Current
Components T2, R3, and C2 form a ‘soft start’ circuit, reducing current spikes during power-on, particularly essential to limit C1’s charging current. At a specific P1 setting, the output current through R5 remains constant. Load R4 draws the necessary current, and the excess flows through zener diode D5. P1 adjustment ensures a slight surplus (5 to 6 mA) over the maximum required by the load, minimizing unnecessary dissipation while preserving zener stabilization. D5 additionally shields C1 from overvoltages, allowing the use of cost-effective 16 V electrolytics. The current flow through R5 and D5 prevents T3’s gate-source voltage from rising excessively, averting potential damage. T1 doesn’t require high voltage capabilities but should have a current gain above 120, making transistors like BC546B or BC547C suitable choices.