Friday, September 20, 2013

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.




16 comments:

  1. Keep it up! It is interesting and educative!

    ReplyDelete
    Replies
    1. Hi Berry, Thanks and most welcome here.

      regards Humphrey

      Delete
  2. HI MR KIMATHI
    this is also a very good article about the use of transistors as switch in the electronic circuits
    thanks
    beh

    ReplyDelete
    Replies
    1. Hi Beh, Thanks and glad to see you here

      Regards Humphrey

      Delete
  3. HI HUMPHERY
    how is going in NBO?worry about you !
    beh

    ReplyDelete
    Replies
    1. I am safe Beh, thanks for your concern
      regards Humphrey

      Delete
  4. ok take care more. hope not attack any where else ,,,,,,,,,,,,,,beh

    ReplyDelete
  5. A good article my friend. long time no news. how is going in the hostage crisis in your country my brother. is it already finished. God protect you and your family.

    Zed

    ReplyDelete
    Replies
    1. Hi Zed,
      Sure it quite a long time since i blogged, too busy around this place, regarding the issue on attack, all hostages are now free.

      Praise be to God of Abraham

      Regards Humphrey

      Delete
  6. HI HUMPHREY
    i want to have this article on paper and i will be thankful of you if you send me this article printable.
    regards
    beh

    ReplyDelete
  7. HI HUMPHREY
    thank you for sending this article in printable version and :
    also i have this question about this signal that is coming from MCU ? is this signal a steady voltage that MCU generate? or is just a spike signal that for one time or momentarily coming to Q202 and then goes off?
    thanks
    beh

    ReplyDelete
    Replies
    1. Hi Mr Beh, Good question, this is a steady signal, this is what will keep the switch on(transistor)
      regards Humphrey

      Delete
  8. hi Humphrey
    thanks for reply
    beh

    ReplyDelete
  9. Replies
    1. Thanks and welcome tah
      regards Humphrey

      Delete