How to understand the power indication circuit composed of triodes and MOS tubes?

lingking

How to understand the power indication circuit composed of triodes and MOS tubes? [Copy link]

 

As shown in the figure, the general function of this circuit is that the LED is lit after the system is powered on, indicating that the system has been powered on. But what is the function of the transistor next to it, and how does this circuit work?

chunyang

When VDD is powered on and the gate voltage of Q27 reaches the turn-on voltage, Q27 is turned on, the LED lights up, and R162 in the circuit provides the base voltage to Q28. When the current flowing through the LED reaches a certain level and the base voltage of Q28 rises enough to make Q28 start to conduct, the gate potential of Q27 will be pulled down, thereby reducing the current flowing through the LED. When the component values are reasonable, the circuit will eventually reach a dynamic balance, making the current flowing through the LED basically constant, regardless of VDD.

chunyang

To put it more simply, this circuit is a current limiting circuit that limits the maximum current flowing through the LED.

chunyang

To extend this further, can this circuit use only one transistor?

lingking

chunyang posted on 2018-12-10 16:43 When VDD is powered on and the gate voltage of Q27 reaches the turn-on voltage, Q27 is turned on, the LED lights up, and R162 in the circuit provides the base voltage to Q28. When the current flowing through the LED...

So this is the case

lingking

chunyang posted on 2018-12-10 16:48 To extend this a little, can this circuit only use one transistor?

I know the method of using a transistor, but how should the two resistors in this circuit be calculated? The nominal current of the LED is 20mA. Should we assume that 20mA flows through the resistor below? In this way, the voltage between the B and E poles of the transistor is 2.4V.

star_66666

It's so classic.

lingking

star_66666 posted on 2018-12-10 23:27 Very classic

How are these two resistors calculated?

star_66666

Q27 is turned on first, providing voltage to terminal B, and then Q28 is turned on. It's very simple.

star_66666

Circuits for children

lingking

star_66666 posted on 2018-12-10 23:40 Q27 is turned on first, providing voltage to terminal B, and Q28 is turned on. It's very simple

But what is the specific method for calculating the resistance values of these two resistors?

star_66666

At the beginning, it is VDD/(Rled+R162) and after it is turned on, it is VDD/Rled. It is that simple.

maychang

lingking posted on 2018-12-10 22:51 I know the method of using a triode, but how should the two resistors be calculated for this circuit? The nominal current of the LED is 20mA, ...

It is estimated that either the LED current is incorrect or the resistance value of 121 ohms is incorrect. If the resistance of 120 ohms is correct, then the current through the LED is 5.8mA. For indication, this large current is sufficient.

lingking

maychang posted on 2018-12-11 12:39 It is estimated that either the LED current is incorrect or the resistance value of 121 ohms is incorrect. If the resistance of 120 ohms is correct, then the current through the LED is 5.8m ...

That should be the working current of the LED in milliamperes, and 20 mA should be the limit parameter

maychang

lingking posted on 2018-12-11 13:39 That should be the working current of the LED in milliamperes, and 20 milliamperes should be the limit parameter

Yes. The resistor value in the original design is probably chosen to limit the current through the LED to a few milliamperes.

chunyang

lingking published on 2018-12-10 22:51 I know the method of using a triode, but how should the two resistors be calculated for this circuit? The nominal current of the LED is 20mA, ...

The nominal current may not be the current required for actual operation. If it is calculated as 20mA, of course it can be done, but because the transistor emitter junction potential cannot be arbitrarily high, the resistance value should be a voltage drop of 0.7V (silicon tube junction voltage drop). According to the values in the original poster's figure, the working current of the LED is less than 20mA.

chunyang

As for the value of R161, this resistor determines the collector current of the transistor, and the gate input impedance of the MOS tube is very high, which can be considered as an open circuit (that is, it does not affect the calculation of the surrounding resistance), so only the tolerance of the transistor needs to be considered, and the minimum value is the resistance corresponding to the maximum collector current of the transistor, but in order to reduce power consumption, the actual value of the resistor can be much larger than the minimum value. Of course, from the perspective of anti-interference, it is not advisable to take a value that is too large, otherwise the MOS tube may malfunction under electromagnetic interference, so the value of 47K or slightly larger can be taken, and it is best to take the same value as other resistors in the same range in the circuit, so as to reduce the number of specifications of the components used. Generally, in the peripheral circuits of MCU, resistors of tens of K are often used for pull-ups. If there are any, the same value can be used.

chunyang

lingking posted on 2018-12-10 22:51 I know the method of using a transistor, but for this circuit, how should the two resistors be calculated? The nominal current of the LED is 20mA, ...

The method of using a transistor is to connect it in the form of a current source, which is suitable for occasions where the power supply voltage is high and the rate of change is large.

lingking

chunyang posted on 2018-12-12 15:12 As for the value of R161, this resistor determines the collector current of the transistor, and the gate input impedance of the MOS tube is very high, which can be considered as an open circuit (that is, it does not affect...

It makes sense

15818739085

chunyang posted on 2018-12-10 16:43 When VDD is powered on and the gate voltage of Q27 reaches the turn-on voltage, Q27 is turned on, the LED lights up, and R162 in the loop provides the base voltage to Q28. When the current flowing through the LED...

Hello, why does the current flowing through the LED rise slowly?