Residual power fast discharge circuit

The excess power fast discharge circuit, that is, the discharge circuit, is used in the scenario where the power supply needs to be switched on and off quickly and repeatedly, and there is a large-capacity capacitor on the load circuit.

After disconnecting the power switch, if there is a large capacitor in the load circuit, the voltage on the load circuit will drop slowly. At this time, if the power switch is reconnected, the load circuit will be powered on again without being completely powered off, which may cause the circuit to fail to reset and start normally, and then the circuit will work abnormally, resulting in a freeze when starting up, etc.

So in life, when you restart an electronic device by turning on and off the power, such as restarting a router, you usually wait a few seconds after turning off the power and then reconnect it.

For some devices, such as televisions, the LED indicator light will take several seconds to turn off after a power outage, because the residual power is not discharged quickly.

The figure below is a circuit for rapid discharge of residual electricity, where C1 represents the large capacitor in the load circuit.

1. Circuit Description

The following analysis takes the input voltage of 5V as an example, which is divided into two cases: the switch SW1 is closed and open.

1. After switch SW1 is closed:

The voltage 5V-In becomes the voltage 4.7V-Out through the diode D1. Because the diode D1 uses 1N5819, which is a Schottky diode, its forward voltage drop is relatively small, about 0.3V.

At this time, the voltage of the b pole of transistor Q1 is 5V, the voltage of the e pole is 4.7V, and Q1 is not conducting.

2. After switch SW1 is disconnected:

The e-pole voltage of transistor Q1 starts to drop from 4.7V, and the b-pole voltage starts to drop from 4V, Vbe = -0.7V. Turn on Q1, and the voltage 4.7V-Out of the large capacitor C1 is quickly discharged to the ground through transistor Q1 and resistor R2.

2. Write at the end

Speaking of "discharge", the most powerful one is not the circuit above, the most powerful one is Fairy Zixia, let's feel it.

Finally, I hope you all enjoy learning about circuits!