Product manual

4-46 Elo Entuitive Touchmonitor
Transient Protection
WARNING: Lack of transient protection may produce a dangerous shock hazard. Touchscreen
damage resulting from transient discharge is not covered by the Elo warranty.
Procedures discussed in this section must be followed to maintain safety and
assure reliability of the touchscreen system.
When an AccuTouch touchscreen is installed on a monitormonitor, the
combination becomes a capacitor which stores an electric charge. The simplest
capacitors are parallel plates of conductive material separated by an insulating
material called the dielectric. In this case, the plates are the metallized coatings
of the touchscreen and the CRT face, and the dielectric consists of the various
air and glass layers between them. See Figure 4.3 on page 4-42 . The
capacitance increases with the size of the plates and closer spacing.
The potential on the inside face of the CRT can be more than 30,000 volts.
When the monitor is turned on or off, a large difference in charge builds up
between the two plates (mainly C1 in Figure 4.3 on page 4-42 ). To balance the
charge, some current must flow from the system ground to the touchscreen. If
this current does not flow, the potential on the touchscreen rises until sufficient
potential exists to cause an arc. This arc can damage the touchscreen or cause a
dangerous electrical shock. Transient protection provides a safe, controlled path
for this current by limiting the voltage buildup on the touchscreen.
Elo recommends connecting the cathodes of four transient suppression diodes to
the four glass substrate terminals of the touchscreen. Make the connection as
close to the touchscreen as possible. The anode end of all four diodes must be
attached to the CRT chassis ground through a short wire of the largest possible
diameter (16 gauge or heavier recommended). See Figure A.3 on page A-76, for
a circuit diagram.
In monitor installations where the touchscreen controller will be external to the
monitor, use the Elo transient protection cable (899389-000), included with the
touchscreen kit. You must attach the transient protection component(s) to the
touchscreen cable and connect the ground wire to the monitor’s chassis ground
before turning the monitor on. The controller need not be connected to the
touchscreen.
In monitor installations where the touchscreen controller will be installed
internal to the monitor, the serial controller’s on-board transient protection may
be used. The touchscreen must be connected to the controller and controller
grounded through one of the plated-through mounting holes before turning the
monitor on.
Some form of transient protection must be connected at all times. If you cannot
use one of the recommended solutions, contact Elo Technical Support for
alternatives.