A Low Cost Visual Indicator for Detecting Ground Connection Failure of CRT Filter Screens

By Gene Chase and Jay Patel*

Bellcore
331 Newman Springs Road
Red Bank, New Jersey 07701-5699

*The Pennsylvania State University
University Park, Pennsylvania 16802

Abstract

In this paper we describe a low cost device that can be incorporated as a part of a CRT filter screen to indicate ground connection failure. This device can be used as an indicator to show the operator that the CRT filter screen has been properly grounded and is operational. It is based on a liquid crystal element that operates without external source of power.

Introduction

There is extensive use of cathode ray tubes (CRT) as display devices, particularly as an output device for computers. The increased use of computers particularly in homes, have made CRTs consumer items. CRTs are also extensively used in business offices, including those owned by the telephone operating companies.

There are a number of reports of Electrostatic Discharge (ESD) shocks originating from computer monitors, because the screen naturally develops an electric charge during operation. Some of these reports come from individuals wearing telephone headsets. The telephone operating companies employ a large number of people who provide telephone information services, by accessing computer databases linked to monitors. These operators almost always wear headsets, which are used for rapid call handling. In Operator Services Offices (OSCs) there have been reports of electrical shocks to headset users. This phenomenon is attributed to an ESD event whereby the monitor is individually coupled to the user. The personnel voltage can then trigger a field induced ESD to any nearby grounded conductor. Because of the high voltage developed on some screens (>15 kV), in some cases, these ESDs can be quite painful, causing the personnel to stop work and seek medical assistance

The recognition that the CRT screens can go to relatively high potentials, with respect to the earth ground has led to some measures to avoid screen induced ESDs, both to the human operator as well as the electrostatically sensitive electronic components such as computers. The approach has been essentially to provide a Faraday cage for the monitor. The easiest way to do this is to provide a grounded transparent conductor in front of the CRT screen. Such grounded computer monitor screens are effective in reducing the electric field radiation, or E-field radiation, directly in front of the monitor.

An effective measure against screen induced ESDs has been the use of grounded computer monitor filter screens with the additional feature of reducing glare and low-frequency electromagnetic radiation normally emitted from computer monitors. Originally these filters were designed to only reduce monitor glare and increase contrast for the operator. This was accomplished by using an anti-reflection coating on the glass surface. Some filters provide additional features such as privacy, which allows only the operator directly normal to the screen the ability to read it. Filters are manufactured for all sizes of computer monitors. There are millions of these filters in use.

A properly operating computer screen in front of the CRT would prevent screen charging and reduce the potential for ESD events. Thus the problem is that of faulty CRT filter screens that go undetected until the induced ESD event occurs. Although there are reported cases of screen induced ESDs from monitors with presumably properly grounded monitor filters, subsequent electrical testing indicated that the filter was not functional because its’ protective conducting layer was no longer connected to ground. Failure analysis of screens revealed that the metallic U-spring connector to the glass was open circuited.

These problems can be prevented if an improper bonding connection can be detected, and the operator alerted that the filter screen is no longer bonded to earth. Ideally this could be an indicator of some sort that would warn the operator that the CRT screen is not properly grounded. It would also be advantageous to have this indicator operate without external power so that it becomes part of the screen, and would be a small fraction of the cost of the screen itself.

Description of Indicator Device

We have devised such an indicator to assure that the computer monitor filter is functional. The indicator is a liquid crystal based device that operates without external power and is small enough to be integrated with or attached to the screen itself. Its power is derived from the screen itself, and failure immediately deactivates the indicator. The operation is schematically illustrated in the following figure, showing two CRT’s one of which has a failed screen.

Figure 1. Schematic representation of the screen failure indicator

 

The computer screen as described above consists typically of an anti-reflection feature, together with the conductive coating. While the anti-reflection feature is to reduce reflections, the purpose of the conductive coating is essentially to provide a Faraday cage around the screen to suppress of the electric field beyond the screen. The proper operation of the screen requires that the screen be properly bonded to ground as outlined above.

The bond wire from the CRT filter carries the field-induced current that originates from the computer monitor. This current can be thought of as a capacitive current, which is time dependent as the electron beam is rastered. This current can be measured as a voltage across a suitable resistor. For example, using a one megohm resistor the voltage can be detected with a laboratory oscilloscope. Schematically, this is shown in Figure 2. The oscilloscope tracings are shown in Figure 3 and 4. The different traces are for different information being displayed on the monitor. As can be expected, higher voltages are observed when the screen is brighter, in other words, when a large amount of charge is deposited on the inner surface of the screen. Thus Windows Software, which normally appears bright, had a higher voltage compared to the DOS screen, which are generally a few characters against the black background. The traces mimic the composite video raster. As shown in Figure 3a, taken at 5 mS/div with a bright screen, the 60 Hz vertical frequency dominates, with peak-to-peak voltages of about 1.2 volts when measured across a one megohm resistor. At 10 m S/div, Figure 3b, the 35 kHz horizontal raster displays about 1.1 volt p-p. Figures 4a and 4b show the raster traces for the dark DOS screen, where in both cases the voltage in about 1.0 volts p-p measured across a 10 megohm resistor.

Figure 2. Representation of the LCD connection to the screen and the test circuit.

Because of the capacitive nature of the voltage, the average voltage about the resistor oscillates about zero. The rms voltage however is several hundred millivolts, depending on the resistor. This voltage is however enough to

(a) (b)

Figure 3. Oscilloscope tracing of the voltage across the resistor shown in figure 2, during CRT vertical scan (lower frequency 60 Hertz) (a) and during horizontal scan (higher frequency ~35kHz) (b) , for WINDOWS screen display

(a) (b)

Figure 4. Oscilloscope tracing of the voltage across the resistor shown in figure 2, during CRT vertical scan (lower frequency 60 Hertz) (a) and during horizontal scan (higher frequency ~35kHz) (b) , for DOS screen display
activate a liquid crystal device, which has the distinct feature of operating with very low current and voltages. A typical liquid crystal device, such as the twisted nematic structure commonly employed for watches and calculators, requires that the voltage should exceed a certain threshold value, called the Frederick’s voltage, a value that is typically about a volt or two.

Such a device connected across the resistor mentioned before provides a simple ground failure indicator. For example, if the bond wire is not properly connected to the screen, the necessary voltage does not exist across the resistor and the LCD is not activated. Also if the bond wire is not properly connected to ground, then one end of the resistor is floating and again the voltage across the resistor vanishes. The LCD could display a simple message like "SCREEN OK" when the CRT is turned on and proper connections are made. Other optional messages could include a "SCREEN FAILURE" message or simply an "OK" or "NOT OK" message. See Figure 5 below for a typical message of this kind. The "OK" would be displayed when the system is powered. If the screen experiences a ground fault, the "OK" symbol will change to an "OK" with a "X" across it signifying that the screen has failed.

A LCD display was designed to demonstrate that it could detect the field induced current from the computer monitor. A LCD display functions best with a low-frequency, low voltage, (a-c) signal since the operation of these devices using DC mode results in space charge built up within the liquid crystal device causing the external field to be nullified. The field-induced current from the computer filter is ideal for a LCD application. We have shown that almost any design of LCD cell will work providing it is designed with a low threshold voltage of less than 1.0 volts and a low capacitance, usually less that 100 pf. A non-threshold type LCD will work with both lower voltages and higher capacitance. It is suggested that in the final design that the LCD be mounted on or near the front of the monitor screen and have a message for the operator such as "SCREEN OK" when the computer monitor is powered-up indicating that the monitor filter is properly grounded.

We have demonstrated several of these configurations, by making prototype liquid crystal devices. The essential features of these devices are as follows: A message should appear across the liquid crystal, which can be eliminated, when a voltage is applied across the liquid crystal cell. This requires, two things to happen, (a) the structure of the liquid crystal where the message has to appear has to be different then the surrounding area, and (b) the structure where the message appears should be alterable using an applied voltage. This is best illustrated by an example. Consider that we want to construct a liquid crystal device that displays the message ‘Screen Failure" when it is not powered and the message disappears when it is powered.

The device can be constructed, by making the structure of the liquid crystal to have a twisted nematic configuration where the letters have to appear and a uniform parallel director configuration where there are no characters. This requires that the surface alignment layers be prepared as shown in Figure 6, below. One of the surfaces has the characters imprinted in terms of the modified surface alignment layer, where as the other surface has liquid crystal molecules aligned uniformly all over the surface. When such a structure is assembled into a liquid crystal cell using conventional liquid crystal filling techniques, a liquid crystal cell is produced which has twisted nematic structure where the characters are present and uniform structure where there are no characters. Since the cell surfaces have a conductive coating in the inner surfaces of the cell, the application of the

Figure 6 Orientation of the liquid crystal molecules on the two plates. The orientation of only a small region around the character S is shown on the top plate.

electric field causes the liquid crystal molecules to align with the direction of the field and the cell when observed between cross polarizers appears black. Without the power it appears with the character, since the polarization of the light has been rotated by 90 where the characters are imprinted in the

Figure 7 Photograph of the liquid crystal device taken between cross polarizers (right) and parallel polarizers (left) in absence of the field (top) and presence of the field (bottom). The bottom right picture is not shown because it is completely dark.

alignment layer. Whether the message appears black on white or white on black depends on the way in which the polarizers are oriented. The photograph of the actual operating device is shown above in figure 7.

Another device that we have fabricated is based on essentially the same principle, but the message OK is written or printed on the device itself, while the "X" to indicate failure becomes a part of the electrically controlled liquid crystal device. In this configuration the X appears across the word OK without power while it is eliminated when the liquid crystal is powered, as shown below in figure 8.

Figure 8. Photograph of a liquid crystal device taken between parallel polarizers. The picture on the left is without power while the one on the right is when the device is powered.

Conclusion

In conclusion, we have shown a cost effective visual indicator for detecting ground connection failure of external CRT filter screens. An LCD visual indicator can be incorporated into any type of CRT monitor system including monitors with "built-in" glare screens that are bonded to ground.

References

J. P. Franey and R.G. Renninger, Field Induced ESD from CRTs: Its Cause and Cure, 1994 EOS/ESD Symposium Proceedings, EOS-16, pp 42-47.

 

WB01511_.gif (114 bytes) BACK TO ARTICLES PAGE