This circuit concept is far from unprecedented, but when considering a trade-off between utilizing a compact, short-circuit resistant transformer or a capacitive voltage division (directly from 230 V alternating current) as a power source for a fan, it can prove exceptionally useful. When urgent cooling represents a secondary concern and options are restricted, it may be the only viable solution. At reduced currents, a capacitive divider occupies less space than a small, short-circuit resistant transformer.
Resistors R1 and R2 are incorporated to mitigate the initial surge current entering capacitor C2 upon activation. Due to the frequently unknown maximum operational rating of resistors available, two resistors are employed for current limiting. Similarly, resistors R3 and R4, designed for discharging C1, are utilized. If the circuit is connected to a power outlet, it’s imperative that a hazardous voltage doesn't remain on the outlet, hence the inclusion of R3 and R4.
Zener diode D1 plays a vital role in regulating the maximum voltage supplied. It operates as a voltage regulator, limiting the peak voltage and dissipating any excess power. Selecting a suitable value for D1 is crucial for efficient operation and component protection. The typical Zener diodes used in such applications are designed to operate reliably over a wide range of temperatures and currents, ensuring consistent performance.
Capacitor C1 dictates the maximum current that can be supplied. Beyond this limit, the power supply functions as a current source. When the current is lower than the specified threshold, Zener diode D1 limits the maximum voltage and dissipates the remaining energy. The optimal value for C1 should be determined based on the anticipated maximum current requirements. A common practice is to begin with the mains voltage when calculating C1.
The 12 V output voltage, the diode forward voltage drops across B1, and the voltage drop across R1 and R2 can be disregarded for simplification. The calculated value is then rounded to the nearest E-12 capacitor value. The capacitor’s impedance at 50 Hz is calculated as 1 / (2p50C). For instance, to supply 50 mA, the required impedance is 4600 ? (230 V/50 mA). Consequently, the capacitor value is 692 nF. This value is typically rounded to 680 nF.
To account for variations in mains voltage and neglected voltage drops, it may be beneficial to opt for the next higher E-12 capacitor value. Alternatively, the required capacitance can be achieved using two smaller capacitors. This approach might be necessary depending on the available space constraints. It's advisable to select a capacitor specifically designed for mains voltage applications, such as an X2 type, for optimal performance and safety.