Power supply

Thread Starter

Prophet Bill$

Joined Dec 2, 2008
22


Parts:
P1____________500R Linear Potentiometer
P2_____________10K Log. Potentiometer
R1,R2___________2K2 1/2W Resistors
R3____________330R 1/4W Resistor
R4____________150R 1/4W Resistor
R5______________1R 5W Resistor
C1___________3300΅F 35V Electrolytic Capacitor (see Notes)
C2______________1΅F 63V Polyester Capacitor
D1,D2________1N5402 200V 3A Diodes
D3_____________5mm. Red LED
Q1____________BC182 50V 100mA NPN Transistor
Q2____________BD139 80V 1.5A NPN Transistor
Q3____________BC212 50V 100mA PNP Transistor
Q4 __________2N3055 60V 15A NPN Transistor
T1_____________220V Primary, 36V Center-tapped Secondary 50VA Mains [COLOR=blue! important][COLOR=blue! important]transformer[/color][/color] (see Notes)
PL1____________Male Mains plug
SW1____________SPST Mains switch​

HEY TECH BUDDIES
I AM A TECH STUDENT AT THE COLLEGE OF THE BAHAMAS AND I AM BUILDING THIS VARIABLE DC SUPPLY BUT MY PRESENTATION IS IN TWO DAYS AN I NEED TO KNO THE PURPOSE AND ROLE OF EACH OF THE COMPONENTS.
PLEASE HELP A.S.A.P.
THANKS IN ADVANCE
 

Audioguru

Joined Dec 20, 2007
11,248
You posted a copy of the project from Electronics-Lab that has errors. The original correct project is here: http://www.redcircuits.com/Page36.htm .

It is a very simple circuit using only 4 transistors. You need to learn about how transistors work in order to understand the function of each part. Ask your teacher.
 

mik3

Joined Feb 4, 2008
4,843
In simple words:

Q4 is the transistor which controls how much current has to flow as to keep the output voltage constant. Think of it as an electrically controlled variable resistor (actually is not like that).

Q3 senses the output current and limits it to the limit you set with P1 (pot).

Q1 senses the output voltage as to be constant at the value set by P2 (pot). If the output voltage varies due to changes on the load or input voltage from the transformer, then it gives the appropriate signal to Q2 and then Q2 controls Q1 as to keep the output voltage constant.
 

Thread Starter

Prophet Bill$

Joined Dec 2, 2008
22
THE CAPACITOR ACTS AS A SMOOTHER TO SMOOTHE OUT THE RIPPLE EFFECT RIGHT, SEEING THAT IT IS VERY HIGH (1UF) OH! AND HOW DO I CALCULATE THE RIPPLE FACTOR/EFFECT? ANDWHY IS R5 SO LARGE?..ALSO I CHANGED C2 TO 1uF 100V AND T1 TO 120V PRIM. 12 CENTER TAPPED AND 25VA WOULD EFFECT WOULD THIS HAVE ON THE CIRCUIT AS A WHOLE?
 

Thread Starter

Prophet Bill$

Joined Dec 2, 2008
22
I CHANGED C2 TO 1uF 100V AND T1 TO 120V PRIM. 12 CENTER TAPPED AND 25VA WOULD EFFECT WOULD THIS HAVE ON THE CIRCUIT AS A WHOLE?
 

Audioguru

Joined Dec 20, 2007
11,248
I CHANGED C2 TO 1uF 100V AND T1 TO 120V PRIM. 12 CENTER TAPPED AND 25VA WOULD EFFECT WOULD THIS HAVE ON THE CIRCUIT AS A WHOLE?
C2 is 1uf in the project. Its voltage rating does not affect how it works.

The original project has a 36V center-tapped transformer that makes an unregulated +24.5V. Your 12V center-tapped transformer will make an unregulated +7.5V.
The original project has a max output of almost 24V. Yours will have a max output of almost 7V.

AND WHY IS R5 SO LARGE?..
It is rated at 5W. The max output current of the original project is 2A. the power dissipated by R is (2A squared times 1 ohm)= 4W.
 

Thread Starter

Prophet Bill$

Joined Dec 2, 2008
22
I didnt find a 500ohm linear potentiometer but i have managed to find a 1k linear potentiometer. What would be the effect of this to my circuit?
 

mik3

Joined Feb 4, 2008
4,843
I didnt find a 500ohm linear potentiometer but i have managed to find a 1k linear potentiometer. What would be the effect of this to my circuit?
If you use the 1K pot, Q3 may not be able to divert all the current needed as to keep the current limited to the set value. I haven't analyzed the circuit to know exactly but to b sure use the 1K pot and put a 500R fixed resistor between pins 1-2 and another 500R fixed resistor between pins 1-3.

where

pin 1=wiper pin
pin 2 and 3=the other two pins
 
Last edited:

mik3

Joined Feb 4, 2008
4,843
It won't have any effect on the circuit. The purpose of this capacitor is to provide power to the circuit if it causes a rapid change in current and the transistors can't respond such fast.
 
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