diodesnpnpnppower supplytransistors
Hello guys, I want to know what is purpose of the transistor T3 (BC547B) in this circuit. I understand it provides base current to the series pass transistor T5 (2N3055). Also I want to know:
What is the function of transistor T4 (BC557B) and T2 (BC547B) and T1 (BC547B)?
Is D1 (1W, 15V) a zener diode?
![[img]https://i.ibb.co/9wxFb3C/variable-Copy.png[/img]](https://i.stack.imgur.com/0YRt6.png)
What is BZ1 (24V) in this schematic?
Actual data sheet circuit is as shown at end of this answer.
Note that gate is connected to source in each case, not floating.
Note that the transistors are depletion mode JFETs which behave somewhat differently than MOSFETs or any enhancement mode FET would in this application.
Q1 and Q2 are both obsolete parts which will be hard to find and expensive if found. There are other ways to do the same job - see below.
2n3822 data sheet here
Q1, Q2 are depletion mode J-FETs. When the gate is connected to source they are ON and need gate to be driven -ve relative to source to be turned off.
When connected as shown they form a constant current source. It is more important that the current is approximately constant than that the current be an exact value. This is fortunate as for eg the 2N3822 the zero gate voltage drain current is specified as 2 mA minimum and 10 mA maximum. (See datasheet page 1)
LM317(1) acts as a variable current limit. Q1 provides a constant current to the 1k//(D1+D2) string. Operation of the circuit is described by figure 23 on page 9 of the datasheet - see below. Q1 can be replaced by any constant current source circuit that works with the available voltage and which will provide below -1.25V at the bottom of D2. Accuracy and actual current are not especially critical.
LM317(2) acts as a controlled voltage source. Here Q2 constant current is usually sunk by D3 + D4 which act as a negative voltage reference of 2 x diode drop or about - 1.2V to allow the wiper of potentiometer "adjust 2" to be pulled below ground by that much if desired to allow Vout of LM317(2) to reach ground.
If the load requires high current and it is attached directly to the zener then resistor R must be very low because all the load current must pass through it. The result is the current through the zener will also be quite high, making it hot and/or requiring a high wattage zener (for extra money).
Adding the transistor separates the zener current from the load current. So resistor R can be high, zener power can be low, and the heat becomes the problem of the transistor, not the zener.
Best Answer
Bz1 is a self-oscillating warning buzzer, and the circuit around R1, T1 & Bz1 is an over-current warning.
The current from the power supply drawn by the load you connect to it ends up returning through R1. When the current increases sufficiently to cause enough of a voltage-drop across R1, T1 begins to turn on which in turn causes Bz1 to buzz at you.
You are correct that D1 is a 15V 1W zener. It is the reference voltage source for the power supply circuit.
T4 and T2 together provide negative feedback to control the output voltage of the power supply, based on the voltage from P1 and D1.
P1 is simply acting as an adjustable voltage-divider to allow you to adjust the voltage from 0V to 15V.
If the voltage at the output exceeds the voltage at T4's base exceeds around 0.6V, T4 will start to conduct. This in turn causes current to flow through R6 - raising T2's base voltage - causing it so to conduct.
T2 pulls the voltage at T3's base down, causing it to start to turn off (or conduct less), resulting in the output voltage being lowered.