Resistance matching problem when transistor is used as switch!

Kkk-

Resistance matching problem when transistor is used as switch! [Copy link]

 

Recently, I used a circuit like the one below, which uses a 5551 to control MJD127, and then controls the on and off of 36V.

What I don't understand now is how to select the resistance value of R2 to make Q1 enter the saturation state. Assuming that the maximum power consumption of the output +36V_OUT is 500mA,

maychang

"What I don't understand now is the choice of the resistance value of R2. How large should it be to make Q1 enter the saturation state?"

This depends on how large the current gain β of Q1 is.

When T1 is saturated, the current flowing out of the base of Q1 is approximately 36V/R2. If this base current multiplied by the Q1 current amplification factor β exceeds the Q1 load current, Q1 enters saturation.

maychang

In practical applications, if it is found that Q1 cannot enter saturation (the tube pressure drop is large), then R2 can be reduced.

是最帅的啊

I also want to know. Continue to pay attention

Kkk-

maychang published on 2020-9-10 10:00 In actual applications, if it is found that Q1 cannot enter saturation (tube pressure drop is large), then reduce R2.

This is a screenshot of the Q1 data sheet. I don't quite understand the meaning of Vce and Ic in the test conditions.

maychang

Kkk- posted on 2020-9-10 10:25 This is a screenshot of the Q1 data sheet. I don’t quite understand the meaning of Vce and Ic in the test conditions

In the test conditions, Vce is the voltage between the collector and the emitter (it does not change during the test), and Ic is the collector current.

What is said here is that when the voltage between the collector and emitter is -4V and the collector current is -4A, the current amplification factor is at least 1000. When the voltage between the collector and emitter is -4V and the collector current is -8A, the current amplification factor is at least 100.

Kkk-

maychang published on 2020-9-10 11:01 In the test conditions, Vce is the voltage between the collector and the emitter (it does not change during the test), and Ic is the collector current. Here it is said that when...

Then teacher, I want to ask, if I want to calculate the resistance value of R2 when the transistor is in saturation state, how should I choose β?

maychang

Kkk- Published on 2020-9-10 11:04 Then teacher, I want to ask, how should I choose β when I want to calculate the resistance value of R2 when the transistor is in saturation state?

The current gain is taken to the minimum possible value.

PowerAnts

Kkk- posted on 2020-9-10 10:25 This is a screenshot of the Q1 data sheet. I don’t quite understand the meaning of Vce and Ic in the test conditions

This is a chart with reference value. When Ic=500mA is at room temperature, hFE is about 2200. Considering the lower limit of -20 degrees outdoors in winter or even -40 degrees for industrial products, hFE will be as small as 1250/1200. Taking 2 times the oversaturation coefficient, it is best to provide 0.833mA Ib. R1 has consumed 0.14mA, so the current flowing through R2 needs to be about 1mA, that is, R2 should be 33K.

PowerAnts

51K is not impossible, but some products may not work properly under low temperature conditions.

Kkk-

PowerAnts posted on 2020-9-10 13:00 51K is not impossible, but some products will not work properly under low temperature conditions

Thanks, now I understand

Kkk-

PowerAnts posted on 2020-9-10 13:00 51K is not impossible, but some products will not work properly under low temperature conditions

Master, this is the output waveform of MJD127 when it is turned on in this circuit. I asked the transistor technical support about this pulse, and he said it is the characteristic of the tube. He didn't know exactly what it is. Master, is this the characteristic of the Darlington tube when it is turned on?

maychang

Kkk- Published on 2020-9-23 08:47 Master, this is the output waveform of MJD127 when MJD127 is turned on in this circuit before. I asked the transistor technical support about this pulse, and he said it was the tube...

The horizontal axis scale cannot be seen in the oscilloscope screenshot, that is, the pulse period cannot be seen, making it difficult to judge.

Generally speaking, this damped oscillation is mostly caused by the load (including line distribution parameters) rather than the characteristics of the transistor itself.