Electrostatic devices Flashcards
how is static electricity and stored?
Static electricity is generated and stored on the surface of non-conductive
materials and discharges to the first available ground source. Items such as
human hands, air, and glass store high positive charges, whereas plastics store
large charges of negative electricity.
what is an “Electrostatic-Sensitive Devices” (ESD)
A device that is susceptible to static electricity, therefore precautions are required when working with them to avoid damages on the devices
What can done to protect ESD
the device and
everything that it comes into contact with must be brought to ground potential by
providing conducting surfaces and discharge paths.
servicing the ESD at an electrostatic-
free workstation where the surface and the tools are electrically bonded to a common ground.
Wrist strap- electrically bonded to the work surface
through a resistance (1 –2 Megaohms).
What precautions are taken when transporting EDSs
1.specialised packaging is essential for
individual devices, PCB modules and the complete LRU. The packaging for
devices and PCB modules are bags are made from
a material which is “Quasi-conductive” (a material whose surface or volume
resistivities are too high to be conductive, but conductive enough to “bleed off”
charges in no more than a few milliseconds).
2. shorting the connecting leads or pins of
devices by means of wire, spring clips, metal foil or by inserting the leads or pins
into a conductive foam material.
3. For PCB modules having edge connectors,
specially formed strips called “Shunts” are placed over the connectors to keep
them all at the same potential and also protect them against physical damage.
what precautions are taken on the aircraft for replacing LRUs containing ESDs
a). All electrical power from the system should be removed by pulling
the system circuit breaker(s).
b). If the power is not removed during LRU removal or installation,
transient voltages may cause permanent damage.
c). After the removal of an LRU from its rack, a conductive shorting
dust cap must be installed on each of its electrical connectors.
Under no circumstances must the electrical pins in the connectors
be touched by hand.
d). The conductive dust caps from the unit to be installed can be used
on the unit being removed.
e). The removed unit is then transported with the conductive dust caps
fitted.
Procedure to replace circuit boards/ cards which contain ESDs.
a). The body of the operator must be grounded by using the wrist strap
provided, connected to the appropriate ground jack.
b). The card is removed using the top and bottom, or left and right,
extractors on the card. Touching the connectors, leads or edge
connectors of the card must be avoided.
c). The removed card is placed in the conductive bag, which is then
secured, in accordance with the manufacturer’s approved
procedure.
Note: Should the bag need to be secured with a tie, cotton twine
should be used, since this is ‘neutral’ as far as static electricity is
concerned.
d). The replacement card is then removed from its conductive bag and
installed following the precautions listed above.
What is used to warn personnel to not touch connector pins of PCBs or LRUs
n additional decal is often affixed near the connector
what is the function of the MOS in an electric circuit
a voltage-controlled resistor in which the MOS equivalent resistance between the drain and source is varied by a
voltage applied to the gate electrode
what is done to avoid damage to the MOS/CMOS
incorporate protective circuitry on the gate electrode input pins (usually some type of resistor-diode network) to provide an alternative path for transient voltage such as electrostatic discharges. It is not the voltage discharge to ground but the potential
difference between the pins on the device which causes the damage. With the elimination of such potential difference the damaging effects of an electrostatic discharge can be prevented.
