Could you please help me analyze what the Darlington tube does in this circuit? Thank you.

ena

Could you please help me analyze what the Darlington tube does in this circuit? Thank you. [Copy link]

This picture is from someone else. In the picture, there is 18V AC power in front of the rectifier bridge.

I analyzed it this way: at the moment of power-on, Q2 is turned on, and there is voltage at the input end of U2. At this time, the input voltage is greater than the base voltage, and the transistor is turned off, and this action is repeated. It is equivalent to a pulse input. What is the effect of this? What is the advantage compared with direct input (without Darlington tube)?

仙猫

A voltage regulator circuit with Q2 as the adjustment tube is connected in series in front of the voltage regulator block KA7802. The output voltage = the breakdown voltage of D6 - 0.7V ≈ 14.3V .

The result is: Q2 shares the heat generated by the input voltage exceeding 14.3V, and the input regulation rate of the two-stage series voltage regulation is better than that of a single-stage.

cruelfox

This is a simple voltage regulator circuit added before 7812 to improve the performance of the entire circuit. The base voltage of Q2 is limited by the voltage regulator D6 and is basically constant. The emitter voltage of Q2 is one VBE lower than this voltage, so the fluctuation range is small.

maychang

1N4744 is a nominal 15V voltage regulator, so the voltage between the emitter and ground of Q2 is about 14.3V. After rectification and CE4 filtering, the average voltage of the 18V AC voltage is estimated to be about 18V DC, and Q2 reduces the voltage of about 18V to about 14.3V.

maychang

Q2 is an emitter follower. Its function is to reduce the output voltage of KA7812 and reduce the heat generated by KA7812. In fact, Q2 shares part of the total heat.

qwqwqw2088

The reason why Q2 is used for linear voltage regulation

It also reduces the heating of the subsequent circuit