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EHT Over And Under Voltage Protection

IP.com Disclosure Number: IPCOM000101094D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 106K

Publishing Venue

IBM

Related People

Clitheroe, AM: AUTHOR [+4]

Abstract

A low-cost circuit is described that provides protection against EHT overvoltage and undervoltage excursions in CRT displays by inductively sensing such conditions and shutting down the monitor power supply accordingly. Using the EHT as the sensor provides enhanced power supply protection since any failure of the analog IC supply rails would be inferred by the resulting collapse of EHT. Measures for protecting cathode-ray tubes from exessively high final anode voltages (EHT) are necessary because: (1) Hazardous levels of X-ray radiation are emitted from the CRT under such conditions. (2) CRT manufacturers specify a maximum final anode voltage above which the probability of the CRT internally conducting an abnormally large current, a condition known as "flashover", is increased.

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EHT Over And Under Voltage Protection

       A low-cost circuit is described that provides protection
against EHT overvoltage and undervoltage excursions in CRT displays
by inductively sensing such conditions and shutting down the monitor
power supply accordingly.  Using the EHT as the sensor provides
enhanced power supply protection since any failure of the analog IC
supply rails would be inferred by the resulting collapse of EHT.
Measures for protecting cathode-ray tubes from exessively high final
anode voltages (EHT) are necessary because:
(1)  Hazardous levels of X-ray radiation are emitted from the CRT
under such conditions.
(2)  CRT manufacturers specify a maximum final anode voltage above
which the probability of the CRT internally conducting an abnormally
large current, a condition known as "flashover", is increased.
Flashovers reduce CRT life or, in severe instances, cause immediate
and fatal damage, with the possibility of secondary failures in the
drive circuitry.

      The design innovation utilizes the EHT as a sensor, detecting
analog circuit faults which the normal power supply protection
circuitry would be insensitive to.  For example, a failure in either
of the monitor's low voltage module supplies would render the EHT
generator inoperable but not necessarily be detected by power supply
protection. The circuit is useful in high-performance color monitors
which are able to display pictures in a range of raster-scanning
frequencies and also in monitors in which the EHT is regulated by
some closed-loop control mechanism to maintain constant brightness
and picture geometry between different scanning modes.  If the
control system fails the EHT will either fail with it, or rise to the
flashover point of the CRT.

      EHT overvoltage is commonly detected using a secondary "sense"
winding built into the EHT transformer assembly. When the DC-restored
voltage rises above a certain threshold level a trip switch circuit
operates which cuts the drive signal to the horizontal scan and EHT
circuitry.  The monitor power supply operates regardless of the EHT
trip mechanism activating in this case.  Analog IC manufacturers tend
to incorporate this function into their scan processor modules.

      THE IMPROVED CIRCUIT For the addition of eight low-cost
components this circuit improves  ...