What is the function of R109 when this transistor is used as a switching tube?

西里古1992

What is the function of R109 when this transistor is used as a switching tube? [Copy link]

 

Is there any relationship between its value and base resistor R108? Second question, this transistor model is MMBTA06LTIG, does the value of base resistor R108 determine how fast the transistor turns on? Do I need to know the level of the control signal to calculate the specific injection current? If I want to slow down the turn-on time, do I need to connect a capacitor between the base and the emitter? How do I choose the capacitance value? I checked the datasheet of the transistor. Can the information circled in the picture determine the value of the base resistor?

lcofjp

Add a pull-down to stabilize the level and prevent misoperation caused by interference signals or uncertain signals

chunyang

R109 is a static bias resistor, which is used to ensure that the transistor will not be mis-conducted due to electromagnetic interference or other reasons when there is no input.

西里古1992

chunyang posted on 2018-9-11 13:25 R109 is a static bias resistor, which is used to ensure that the transistor will not be mis-conducted due to electromagnetic interference when there is no input.

Is the resistance value selected based on experience? What is the relationship with the base resistance?

chunyang

Sirigu1992 posted on 2018-9-11 13:31 Is the resistance value selection based on experience? What is the relationship with the base resistance?

The base input resistance and the bias resistance form a voltage divider. The result of the voltage division between the two must still ensure that the transistor will be saturated and turned on when driven by logic 1, so it can be calculated.

maychang

"Does the resistance value of the base resistor R108 determine the conduction speed of the transistor?" The value of the base resistor R108 determines the saturation depth of the transistor. If the transistor is too deeply saturated, there will be a delay in exiting saturation, but R108 has basically no effect on the conduction speed.

maychang

"Do I need to know the level of the control signal to calculate the specific injection current?" Of course I need to know the level of the control signal.

maychang

"If you want to slow down the on-time, do you need to connect a capacitor between the base and the emitter? How do you choose the capacitance value?" Why do you want to "slow down the on-time"? How much time do you want to slow down? These are uncertain, and it is impossible to say where the capacitor should be connected and how to calculate the capacitance.

maychang

"Can the information circled in the figure determine the value of the base resistor?" The blue box shows the relationship between the delay time, rise time and fall time and the collector current.

shtb8501

The switching frequency of relay ctrl is usually slow and related to magnetism, which can easily induce surges, leading to unstable voltage between the base and emitter of the transistor, affecting the output characteristics of R-. The purpose of adding a resistor should be to eliminate this influence. Is that right?

xiaxingxing

maychang published on 2018-9-11 18:02 "Does the resistance value of the base resistor R108 determine the conduction speed of the transistor?" The size of the base resistor R108 determines the saturation depth of the transistor. Transistor saturation...

Senior, hello, may I ask, won't this R form an RC delay with the junction capacitance between the base and emitter of the transistor? The larger this R, the greater the delay? Thank you!

maychang

xiaxingxing posted on 2018-9-29 18:52 Hello, senior, may I ask, won’t this R form an RC delay with the junction capacitance between the base and emitter of the transistor? The larger this R is, the greater this delay...

"Excuse me, won’t this R form an RC delay with the junction capacitance between the base and emitter of the transistor? The larger this R is, the greater this delay is?" Yes, in the first post, after R108 and the transistor emitter junction resistor are connected in parallel (note that it is not just R108), they form an RC delay with the junction capacitance of the transistor emitter junction. But in normal operation, the emitter junction is forward biased, and the transistor input resistance is quite small, and of course it is also quite small after being connected in parallel with R108. The RC delay circuit composed of a relatively small resistor and the emitter junction capacitance has a relatively small RC delay time. In fact, R108 is connected in parallel with the transistor emitter junction resistance. The resistance value after parallel connection mainly depends on the emitter junction resistance, and R108 has a very small effect on it.

xiaxingxing

maychang posted on 2018-9-29 19:16 "Excuse me, won't this R form an RC delay with the junction capacitance between the base and emitter of the transistor? The larger this R is, the larger the delay will be...

Very detailed, very good, I understand, thank you!