Transformerless 5 Volt Power Supply

  
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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
Transformerless 5 Volt Power Supply - schematic

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...



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