Teledyne HOW Static Charges are Created, Model 9110T NOx Analyzer Primer on ESD

Model 9110T NOx Analyzer Primer on ESD

14. A PRIMER ON ELECTRO-STATIC DISCHARGE

TAI considers the prevention of damage caused by the discharge of static electricity to be extremely important part of making sure that your analyzer continues to provide reliable service for a long time. This section describes how static electricity occurs, why it is so dangerous to electronic components and assemblies as well as how to prevent that damage from occurring.

14.1. HOW STATIC CHARGES ARE CREATED

Modern electronic devices such as the types used in the various electronic assemblies of your analyzer, are very small, require very little power and operate very quickly. Unfortunately, the same characteristics that allow them to do these things also make them very susceptible to damage from the discharge of static electricity. Controlling electrostatic discharge begins with understanding how electro- static charges occur in the first place.

Static electricity is the result of something called triboelectric charging which happens whenever the atoms of the surface layers of two materials rub against each other. As the atoms of the two surfaces move together and separate, some electrons from one surface are retained by the other.

Materials	Materials
Makes	Separate
Contact

+ + + +

PROTONS = 3	PROTONS = 3	PROTONS = 3	PROTONS = 3
		ELECTRONS = 2	ELECTRONS = 4
ELECTRONS = 3	ELECTRONS = 3
		NET CHARGE = -1	NET CHARGE = +1
NET CHARGE = 0	NET CHARGE = 0

Figure 14-1: Triboelectric Charging

If one of the surfaces is a poor conductor or even a good conductor that is not grounded, the resulting positive or negative charge cannot bleed off and becomes trapped in place, or static. The most common example of triboelectric charging happens when someone wearing leather or rubber soled shoes walks across a nylon carpet or linoleum tiled floor. With each step, electrons change places and the resulting electro-static charge builds up, quickly reaching significant levels. Pushing an epoxy printed circuit board across a workbench, using a plastic handled screwdriver or even the constant jostling of StyrofoamTM pellets during shipment can also build hefty static charges.

Table 14-1: Static Generation Voltages for Typical Activities

MEANS OF GENERATION	65-90% RH	10-25% RH
Walking across nylon carpet	1,500V	35,000V
Walking across vinyl tile	250V	12,000V
Worker at bench	100V	6,000V
Poly bag picked up from bench	1,200V	20,000V
Moving around in a chair padded	1,500V	18,000V
with urethane foam

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