Understanding the start up circuit in LCD monitor

In electronics we encounter circuits which are used to switch on or off particular circuit and in this lesson I am going to explain the start- up circuit in LCD monitors.

This circuit is used to turn on or off the inverter circuit; to accomplish this transistors are used as switch.

Below you can see a diagram of a start –up circuit of an lcd monitor

  Click to enlarge

A signal is send from the MCU and goes to the base of transistor Q201 which is a NPN transistor.

When no signal at the base of this transistor, the transistor which is acting as a switch is open.

It is like the switch which is on “OFF” position and therefore current does not flow from collector to emitter.

After applying the signal at the base of this transistor Q201 (see the green line) this will cause the transistor (switch) to close and therefore this will in turn allow current to flow from the collector to the emitter.

Now note that the collector of this transistor Q201 is linked to the base of transistor Q202 which in this case is PnP transistor.

As you know or maybe you have known now to turn on a PNP transistor is done by taking its base to the ground.

This is now accomplished by transistor Q201 and therefore Q202 is now turned “ON” and this will cause the 12Vdc on the emitter to flow to the collector pin of transistor Q202.

The collector pin of this transistor Q202 is connected to the pin 9 of the ic TL1451A.

This pin is the Vcc pin of this ic and therefore with the VCC present at pin 9 of this ic, this ic will start to conduct and a signal will appear at pin 10 of this ic(see pink line)

This signal goes to the base of Q206 which is NPN transistor. This will cause transistor Q206 to close and therefore take the collector Vcc (12Vdc) to the emitter of transistor Q207.

This signal will then appear at the Gate of the mosfet Q755 and this turn it “On” and hence fire up the inverter circuit.

For more info on how LCD monitor works kindly click on the book cover below.