Question about the delay circuit composed of transistors and capacitors

xiaxingxing

Question about the delay circuit composed of transistors and capacitors [Copy link]

As shown in the delay circuit, after power-on stabilization:

1. Isn't the voltage at point A 12V?

2. Isn’t the voltage at point B 12-0.7=11.3V?

Why are the simulated values of the voltage at points A and B inconsistent with the above calculations? Thank you all!

1nnocent

The 0.7 given in the book is just a value within a range. Different transistors have different Ube values, and some have 0.6.

chineseboyzxy

The voltage drop of R3 and the voltage drop of the CE junction of Q1 will cause the voltage at point B to be low. The voltage drop of R1 causes the voltage at point A to be low.

maychang

1. Isn't the voltage at point A 12V?

No. Because the transistor base current flows through R1, a voltage drop will occur on R1. The specific amount depends on the size of the base current.

maychang

2. Isn’t the voltage at point B 12-0.7=11.3V?

No. The voltage at point B is the voltage at point A minus 0.7V.

xiaxingxing

maychang posted on 2021-5-14 13:07 2. Isn’t the voltage at point B 12-0.7=11.3V? No. The voltage at point B is the voltage at point A minus 0.7V.

Hello, senior! In the absence of R3, can the voltage at point B be calculated as follows:

12-Vceo, where Vceo is the saturation voltage drop of Q1. Thank you!

maychang

xiaxingxing posted on 2021-5-17 15:02 Hello, senior! In the absence of R3, can the voltage at point B be calculated as follows: 12-Vceo, where Vceo is the saturation voltage drop of Q1. Thank you...

This calculation cannot be done without R3 (short circuit).

Because the transistor will not saturate in this circuit without R3 (short circuit).