In this note, we are going to know about EHT Supply in TV and Its Circuit Diagram. Welcome to Poly Notes Hub, a leading destination for engineering notes for diploma and degree engineering students.
Author Name: Arun Paul.
What is EHT Power Supply?
The EHT (Extra High Tension) power supply in a TV is the high-voltage supply that powers the cathode ray tube (CRT) in older televisions. This source generates extremely high voltage, often between 10 and 30 kV, which is required to accelerate the electron beam towards the phosphor-coated screen and produce images.
Key Function of EHT Power Supply in TV
- It delivers the high voltage required to accelerate electrons generated by the cathode in the CRT towards the screen.
- The voltage level influences the image’s brightness and clarity.
- Works in conjunction with other circuits to guarantee optimum electron beam focus and deflection.
Circuit Diagram of EHT Supply in TV
Here is the Circuit Diagram of EHT Power Supply in TV and Its Working Principle –
An anode voltage of the order of 15 kV is needed for sufficient brightness in black and white pictures tubes. This is known as HV or EHT (extra high tension) supply.
To obtain such a high voltage by stepping up the mains voltage with a transformer is almost impossible and prohibitive in cost. A novel method used for obtaining EHT source is illustrated in fig. during retrace intervals of horizontal scanning, high voltage pulses of amplitude between 6 to 9 kV are developed across the primary winding of the horizontal output transformer. As shown in the figure these are stepped by an autotransformer winding to about 10 to 15 kV and then fed to a high voltage rectifier. The output of the rectifier is filtered to provide required dc voltage.
Such an arrangement does not loss very much the horizontal output stage because the current demand from this high voltage source is less than 1mA.
The horizontal output circuit is so designed, that in addition to providing EHT source, the energy stored in the horizontal deflection coils during retrace is tapped through a diode called damper diode to charge a capacitor. The voltage thus developed across the capacitor, actually adds 200 to 300 volts to normal B+ voltage to give a boosted B+ supply of 400 to 700 volts. This voltage is also suitable for first and second anodes of the picture tube. This arrangement makes the horizontal deflection circuit very efficient.