TV repair course

Lesson 1: Power supply types

1-Non-switching power supply

This is a very simple power supply type
The current passes through a power transistor (Q1) but it also passes through a resistor (R1)

The AC current enters by PT connector and is rectified by the diode bridge (D901 to D904), filtered by C905 and becomes DC current.

After the 120VAC is rectified it will result about 140VDC.

The 140VDC enters by the collector of Q1 and leaves by its emitter and goes to horizontal output (FBT and HOT)

There will always be 110VDC on the emitter of Q1, regardless of voltage variations in the AC outlet.

No matter if the voltage on the collector is 130V or 170VDC

As the current passes through Q1 it will produce heat which can damage Q1
To prevent Q1 to become too HOT, R1 was added to the circuit.
This way, good part of the current will pass through R1 and Q1 will regulate the power supply.
Notice that is not possible to plug this kind of power supply directly to a 220V outlet.
In order to plug this kind of power supply to a 220V outlet, a internal transformer is necessary.

Possible failures in non-switched power supply

1-The fuse blows every time it's replaced

The most common causes are shorted diode bridge or shorted Q1
In this kind of power supply when Q1 shorts, the HOT almost often shots too

2- Narrow screen

This failure is caused because the power supply becomes low
In this case, the current stops passing through Q1 and passes only through R1, causing it to become too hot
If you touch the transistor, it will be cool. This is the clue that the transistor is not working.

If Q1 is not working, check transistors and resistors which are linked to its base
Note: Avoid touching directly any part, because it is too hot, it will cause injuries to your finger

2- Switching power supply (series type)

In a switching power supply, the power resistor is not necessary
The current will pass all through the transistor

But how is it possible? Wouldn't the transistor become too hot and damage itself ?

Let me explain:

In a non-switching power supply, studied before, the transistor is called regulator transistor.
However, in a switching power supply, which we will study now, the transistor is called switching transistor.

The difference between the non-switching and the switching power supply is that in the first one the transistor drives all the time, that's why it becomes too hot

In a switching power supply, the transistor does not drive all the time.
Imagine the switching transistor like a switch which will open and close more than 25,000 times per second.

In the picture below you can see a switching power supply schematic which uses an integrated circuit IC801.
In the schematic you can see the switching transistor inside IC801

Notice that all switching power supplies use a start-up resistor
In the picture above, the start-up resistor is R804.

The start-up resistor has a side linked to the main power supply, which comes from the diodes bridge.
The other terminal is linked to pin 2 of IC801, which is linked to the base of the internal switching transistor.

As a little voltage is applied to pin 2 by the start-up resistor, the switching transistor starts to conduct.
Then a current flows through the internal coil of T801.

This current will induce voltages on terminals F1, F2 and F3 of T801, which will polarize pins 2 and 5 of IC801.

Possible failures in switched power supply

1- If the HOT or FBT shorts, the fuse will not blow

The switch power supply will just stop working

2- If the internal switching transistor of IC801 shorts, the fuse will blow

To prevent the fuse and other parts from damage because of shorts, a series bulb lamp device will be used

In the next lesson you will learn how build a series bulb lamp yourself, with cheap parts

3- Power supply is dead

Check for possible shorts in horizontal circuit
If no short is found , check the start up resistor

See you in the next lesson