Capacitive step-down power supply typical circuit
Check the drive circuit, should be carefully checked according to the circuit circuit is wrong, pay special attention to check the rectifier bridge (long foot is the positive output, the diagonal is the negative output, the other is the AC input) or rectifier diode and Zener diode The polarity is correct (printed with black or white end of the negative), as well as check the transistor or regulator IC three electrodes are wrong and so on.
C1 is a step-down capacitor (using a metallized polypropylene capacitor), and R1 provides a discharge circuit for C1. Capacitor C1 provides a constant operating current for the entire circuit. Capacitor C2 is the electrolytic capacitor, the voltage value depends on the number of LEDs connected in series (about 1.5 times the total voltage of its total voltage), its main role is to suppress the instantaneous voltage rise caused by power leakage, thereby reducing the voltage impact on the LED The impact of life. R4 is the discharge resistance of capacitor C2, the resistance should be increased with the increase in the number of LEDs.
As the capacitor step-down power supply is a non-isolated power supply, in the instantaneous power will produce a large current, that is, the so-called inrush current. In addition, due to the impact of the external environment (such as lightning) power system will invade a variety of surge signals, some waves will lead to LED damage. Therefore, to provide thermistor protection, this is mainly negative temperature coefficient thermistor protection (NTC thermistor, NTC is abbreviation of Negative Temperature Coefficient) and positive temperature coefficient thermistor protection (PTC (Positive Temperature Coefficient)) and then Transient Voltage Suppressor (referred to as TVS)
Negative temperature coefficient means a negative temperature coefficient, refers to the negative temperature coefficient of a large number of semiconductor materials or components, limiting the inrush current of the most simple and effective way is in line input in series with an NTC thermistor
Positive temperature coefficient current through the PTC thermistor caused by the temperature rise, that is, the temperature rise of the heating element, when more than the Curie point temperature, the resistance increases, thus limiting the current increase, so the current drop lead to component temperature, resistance value Reduce the circuit current and increase the component temperature and rise again and again.
Transient voltage suppressors are mainly used for fast overvoltage protection of circuit components. When the TVS tube bipolar by the reverse transient high-energy impact, it can 10-12s on the order of the speed between the two poles into a very low impedance, absorb high-energy surge, the voltage between the two tongs Located at a predetermined value to protect the components of the electronic circuit from the impact of various surge pulses and damage.