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