Saturday, July 23, 2011

Understanding the x-ray protection circuit in Television

High Voltage shutdown commonly referred to as X-Ray protection circuit shut down the horizontal deflection in the event of a detected faults if that faults causes CRT radiations to exceed acceptable limits.

It is important to note here that HV can increase even if the power supply is okay; a good example is if the safety capacitor (snubber) opens or looses some of its original capacitance.

If any of the above faults occurs in Television, It will cause the Fly back to produce undesirably high secondary voltages. The good news is that as the HV increases all other output from the fly back also increases and therefore this circuit of X-ray protection is not attached to the HV output which could have caused more challenges to the designers but another output of low voltage is monitored.

Usually a voltage reference is taken from the FBT and rectified by a diode to get a DC voltage, this DC voltage is applied to a precision resistor divider, the values of these resistors are chosen to produce correct X-Ray protection trip threshold for each CRT.
Below you can see a simple X-ray protection circuit usually found in CRT Televisions.



If the voltage increases above normal (say 13 Volts) the Zener diode which in our case is a 12 Volts Zener will conduct and voltage will appear at the X-ray in pin. The X-ray protection pin is usually zero volts when things are normal and if this pin get voltage, the I.C internal shutdown circuit will trigger and shut the horizontal out signal and hence shutdown the Television.

This will then prevent dangerous X-ray being emitted which could have otherwise caused health hazard to the user or even break the CRT tube.

Conclusion
I think now you can see why you should never replace the safety capacitor with one with lower capacitance value because this will cause the HV to increase a lot and hence emit this dangerous X-ray radiation.

One sign that the snubber capacitor has lost some of its original capacitance is that the screen usually become narrow (E/W) and sometimes you can even hear cracking sound caused by this excess HV. When you get these symptoms this is an indicator that this capacitor has lost some of its original capacitance and replacing it usually cures the problem.

Wishing you all the best

Humphrey Kimathi

Author CRT Television Repair course

9 comments:

  1. HELLO MR KIMATHI

    i understood that:
    if HV increas by any reason specially the changeing the capasitance of snubber cap
    a zener doid in x ray protection circute will conduct voltages to the xray pin of internal ic and ic will shut down the tv . and this will protect the user from more absurbtion of harmful x ray and tv set from explosion
    am i right?
    beh

    ReplyDelete
  2. Hi Beh, i can't put it any better than that..you are absolutely Right.

    Regards Humphrey

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  3. Hi Humphrey,

    Thanks for this article now I understand how this xray protection works. I am not really familiar about the safety capacitor you mentioned in your conclusion can you help me understand this part? I am still new electronic repair and I found your blog really helpful for me to learn more in this field.

    Thanks you very much.

    ReplyDelete
  4. Hi Chris, Thanks for passing by Chris. regarding safety capacitor this is the capacitor which is placed between the collector leg of the H.O.T and the other leg on the emitter leg. you can learn more about this capacitor on this article i wrote on this site.

    http://www.bestelectronicarticles.com/consumer-electronics/crt-televisions/how-to-prevent-the-horizontal-output-transistor-h-o-t-from-getting-shorted-after-replacement.html

    all the best

    Humphrey

    ReplyDelete
  5. This is interesting Mr. Humphrey, thanks so much for sharing all this good secret about X-Ray protection.

    Regards
    Emmanuel.

    ReplyDelete
    Replies
    1. HI Emmanuel, Thank and glad you have learn something new
      Regards Humphrey

      Delete
  6. Hi Humphrey,
    Your blog of this kind help many techs out there,cause you explain in detailed language all types of trouble repairs particularly in TVs and Dvd players.I thank you for your generosity and your precious time in answering inquiries from all concerned in this field.May God always guide you to do good to others.
    Regards,
    Romeo

    ReplyDelete
  7. Thanks my friend Romeo.
    Regards Humphrey

    ReplyDelete
  8. Thanks my friend Romeo.
    Regards Humphrey

    ReplyDelete