Could you please advise what is wrong with this circuit?

灞波儿奔

Could you please advise what is wrong with this circuit? [Copy link]

 

The circuit below is placed behind the 5V output for voltage regulation, why does Q1 always burn out? Where is the problem in the circuit?

maychang

"The following circuit is placed behind the 5V output for voltage regulation"

It doesn't look like a circuit that is "put behind the 5V output for voltage regulation".

maychang

"Why does Q1 keep burning out? Is there something wrong with the circuit?"

Regardless of whether the circuit is correct or not, if the soldering is consistent with this diagram, Q1 will not burn out.

Fred_1977

Looking at this circuit, the three-pole will not burn. Is it possible that the components are soldered incorrectly? Your circuit should be a high-voltage protection circuit, not a voltage stabilization circuit.

chunyang

When the input voltage of this circuit is higher than 5.6V, D1 is turned on, and then Q1 is turned on, which makes Q3 cut off, Q2 is cut off under the bias of R5, and the output is turned off. It can be seen that this is not a "voltage stabilization circuit" but an input overvoltage protection circuit. The presence of a voltage regulator in the circuit does not mean that it is a voltage stabilization circuit, and the specific function needs to be analyzed. In addition, D2 is useless, and R4 and R6 can also be omitted.

From the circuit, there are two possibilities for Q1 burning out: one is that there is a short circuit in the Q1 related circuit, such as a short circuit between the base and the power input. The other is that the connection relationship of the components in the actual circuit is different from the main post diagram, that is, the actual circuit is wrong.

tsmt001

qwqwqw2088

When the output voltage is higher than VD1+0.7V, Q1 is turned on, Q3 and MOS tube are blocked, limiting the high voltage. When the voltage is low, Q is not turned on.

Q1 What tube is used?

tsmt001

lugl4313820

tsmt001 published on 2022-4-11 23:36

The analysis is accurate. Is this simulation software used?

灞波儿奔

maychang posted on 2022-4-11 17:09 "The following circuit is placed behind the 5V output for voltage regulation" It doesn't look like a circuit that is "placed behind the 5V output for voltage regulation".

Very surprised,

Why is it not a 5V voltage regulator circuit? I want to hear the teacher's analysis

灞波儿奔

Fred_1977 posted on 2022-4-11 18:37 Looking at this circuit, the three-pole will not burn. Is it possible that the components are soldered incorrectly? Yours should be a high-voltage protection circuit, not a voltage stabilization circuit.

Thank you, I am now checking if there is any problem with the circuit assembly.

I would like to ask, "This should be a high-voltage protection circuit, not a voltage stabilization circuit." On what basis do you judge this?

灞波儿奔

chunyang published on 2022-4-11 19:00 When the input voltage of this circuit is higher than 5.6V, D1 is turned on, and then Q1 is turned on, so that Q3 is turned off, Q2 is turned off under the bias of R5, and the output is turned off. It can be seen that...

Thank you, if the circuit is OK, check the circuit component parameters and welding normally.

灞波儿奔

tsmt001 published on 2022-4-11 23:36

You calculated very accurately.

This analysis is great, much more thorough than the theory.

It is a voltage stabilizing circuit, greater than 6.3V voltage stabilizing protection circuit

I have another question. It seems that the high-voltage protection of this circuit is not very precise. The teacher said that 431 can be added to accurately control the overvoltage protection. How to modify this circuit to achieve precise overvoltage protection?

maychang

灞波儿奔 posted on 2022-4-12 09:08 I am very surprised. Why is it not a 5V voltage regulator circuit? I want to hear the teacher's analysis

"Why is it not a 5V voltage regulator circuit? I want to hear the teacher's analysis."

Chunyang's analysis on the 5th floor is clear enough.

zhangdaoyu

The analysis on the 5th and 8th floors is so good, they are teacher-level figures

chunyang

There was an oversight in my analysis on the 5th floor. I did not take into account the emitter junction voltage drop of Q1. The protection starting voltage should be 5.6+0.7=6.1V. In fact, it will be slightly higher due to the existence of R3, but this oversight does not affect the circuit principle analysis and conclusion.

tsmt001

灞波儿奔 posted on 2022-4-12 09:17 Your calculation is very accurate. This analysis is great. It is much more thorough than explaining the theory. It is a voltage regulator circuit, greater than 6.3V voltage regulator protection circuit&n ...

This circuit is only for overvoltage protection, not voltage regulation. If it exceeds 6.3V, there will be no output, and if it is lower than 6.3V, it will start to supply power.

tsmt001

lugl4313820 posted on 2022-4-12 08:44 The analysis is accurate. Is this simulation software used?

Thank you. I modified it using the image editor Paint in W10 system and quoted some special symbols.

灞波儿奔

chunyang published on 2022-4-11 19:00 When the input voltage of this circuit is higher than 5.6V, D1 is turned on, and then Q1 is turned on, so that Q3 is turned off, Q2 is turned off under the bias of R5, and the output is turned off. It can be seen that...

There is indeed an error in the welding, and the transistor is no longer burned.

This circuit should be lower than 6.3V, Q1 transistor is turned on, Q2 and Q3 MOS tubes should not be turned on to disconnect the system output. Is this analysis correct?

灞波儿奔

tsmt001 posted on 2022-4-12 22:28 This circuit is only for overvoltage protection, not voltage regulation. There is no output when it exceeds 6.3V, and it starts to supply power when it is lower than 6.3V.

There is indeed an error in the welding, and the transistor is no longer burned.

This circuit should be lower than 6.3V, Q1 transistor is turned on, Q2 and Q3 MOS tubes should not be turned on to disconnect the system output. Is this analysis correct?