Understanding the standby circuit in LED-LCD television

When troubleshooting switch mode power supply (LCD/LED TV) it is very important to check the status of the standby voltage.

If you open the LED/LCD television and disconnect both the main board and the inverter board if they are separate, you will notice that only one outlet will have a voltage and the rest will be zero.

This voltage is usually 5Vdc and is called the standby voltage, if this 5 volts standby is not generated means the SMPS is defective and therefore you should not continue with any other test before you know what is blocking this voltage.

With that introduction of standby voltage let us learn a little about this very important voltage.

As you can see in the diagram above, when the AC is applied to the television, the standby circuit output 5vdc to the dc-dc board.

This voltage then passes the DC-DC board and enters the main board. Once on the main board the 5Vdc is converted to 3.3 Vdc by the regulator IC (I.C5702) shown in the diagram below.

The 3.3Vdc is then used to power the system control unit, power ON led, and the IR receiver.

This means whenever you see power ON led is on means the standby circuit is okay and the 3.3 Vdc regulator is okay and therefore don’t waste time trying to test them and therefore you should concentrate on other outlets.

Thank you guys

See you in the next class