-tonywilk-

E.S.D. Protection Basics

or... An Intuitive guide to Zap Protection

1.0 Introduction

This is meant to be an insight into protecting electronics from E.S.D. (ElectroStatic Discharge) or 'static' zaps and other externally-induced nasties.

Even though you may understand some, or even a lot, of electronics and already know Ohms Law and have heard of Kirchoff - please read the introductory section - you might just see things from a slightly different perspective when dealing with ESD.

1.1 Static or Moving?

First of all... what is 'static' ?
The term was originally used to describe the phenomenon of 'static charge' - the effect of building a charge on an insulator, like when you comb your hair with a plasic comb and the 'charged' comb can attract little bits of paper. The charge is really static - not moving - because it rests on the surface of an insulator (the plastic comb). Static charge can build up on conductors too, as long as they are insulated from their surroundings - like a person shuffling along a nylon carpet.

The human body is reasonably conductive but nylon and man-made shoes are not, so the whole body can acculate a static, non-moving, electric charge. That is until that person moves close or touches some other non-charged or earthed object... then the 'static' moves!

What is often misunderstood is that a 'static discharge' is just like ordinary electric current flow, except that the initial voltage can be a lot higher than people are used to in dealing with electronics (except, perhaps, if you repair TV sets).

1.2 Static and 'Normal Electricity'

The same laws which are used to determine voltages and currents in 'ordinary' electronic circuits apply to 'static', or, in this case static discharges. Since this is meant as an introduction, the following arguments are somewhat simplified but still valid.

A static discharge is just a voltage being applied to a resistance, not a lot more, not a lot less.

Voltage, Current and Resistance
You may also be familiar with Ohms Law which states the relationship between voltage, current and resistance in a simple electric circuit:

If our statically charged person happens to be charged to, say 4000 volts (not an unreasonable figure), touches a metal filing cabinet and has a body-to-cabinet resistance of 500 ohms, then:

	V       4000
       ---  =   ----  =  20 Amps
        R        500
Luckily (for us humans) this high current only lasts for a fraction of a second since the total electric charge on a body is quite small. Semiconductor devices, on the other hand, are not so resilient, a very short duration current pulse can easily destroy many semiconductor devices.

I'll take the high Road...

Another useful law, heavily paraphrased here, is that most electric current will go along the easiest route (the lowest resistance) and some will go down harder routes.
In Fig.1 there is only one path and Ohms Law can easily be calculated.
Fig.2 shows two resistances in parallel, each can be calculated seperatly and the total current is the sum of them both.
Fig.3 is a bit more interesting (as you'll discover later), a lot of current flows in the thicker (lower resistance) 100 ohm conductor but this doesn't affect the fact that some current flows down the thin (100 times higher resistance) conductor.

Note for the more knowledgeable: I am going to stick with the word RESISTANCE in this article to avoid confusing the casual reader with the complexities of IMPEDANCE and REACTANCE  ;-)

Potential Dividers
In actual circuits, the situation is a bit more complicated by the fact that the source of the 'zap' also has a resistance. The source resistance and input resistance act to divide the applied voltage in a simple ratio of the resistances.

Fig.4 Shows a simple potential divider. The top 500ohm resistor indicates the source resistance and the lower 100ohms is our circuit. You can work out the voltage at the mid point by the ratio of the resistances or using Ohms Law. (full working at bottom of this page)

Notice that if the lower resistor in our circuit is smaller, the voltage is lower but more current would flow down it.

Fig.5 is a more complicated example (worked out below) which is intended to show that a combination of dividing the voltage (by the ratio of the source resistance and the total circuit resistance) and reducing the current (down the 10,000 ohm resistor) results in a quite small current of 0.0165 Amps (16.5mA) down one part of our circuit.

If you want to try and work this out, the details are at the bottom of this page.
For those not too fond of math... turn your imagination up a notch, look at Fig.5 and Fig.6

Look at the diagrams - they are the same circuit, but the right hand one is a bit more true to life. The 1000 volt source with 500 ohm resistance is YOU, nicely charged after a nylon-carpet-shuffle. The bottom two resistors are on your circuit board, the 100 ohms is there to reduce the voltage applied to the 10K (10,000 ohms) going into our delicate microprocessor (CPU).

To protect this CPU more we could:
a. Use a lower value than 100 ohms to reduce the voltage applied to the 10K
b. increase the resistance into the CPU to reduce the current
c. add another resistor to make the source resistance higher (this would go in the wire at the point marked by the blue arrow)

Before further discussion of actual circuits, there's another couple of points which you need to be aware of...

1.3 What's different about 'Static' ?

Having said that a static discharge (ESD) is just normal, everyday, high-voltage electricity, there are some aspects of the discharge *event* that must also be considered.

We have mentioned that a discharge can produce surprisingly high currents and that the 'zap' only lasts for a very short time. This means it causes some effects which are very different from DC current (like from a battery), the speed at which the spike of current occurs makes it more like a radio transmitter. In fact, the first experiments in wireless transmission were done using static discharges.





This is where some ESD effects really start looking like Black Magic! The very high speed, high current pulses produce their own side effects; they generate electromagnectic radiation which can induce currents in nearby wiring. It is quite easy to demonstrate this by 'killing' a microprocessor with a discharge gun (a bit of kit which generates high voltage discharges for ESD testing) set to 18,000 volts discharging into a loop of wire suspended several inches above the device.

1.4 Summing Up

A Static discharge can affect electronic equipment because of its high voltage, the high pulse currents it generates and by the induced RF field. Exactly the same effects can be generated by charging up a small capacitor to a few thousand volts and discharging it into a circuit through a hundred ohms or so -this is exactly what a discharge gun does, except that a good one from Schaffner will cost a few thousand quid! (1 UK quid is around US$1.6 ).

It isn't really a 'Black Art', and, as you'll see in the next section, you can get a long way to understanding how to protect circuits without heaps of test equipment or complicated math...

On to: 2.0 ESD Effects on Electronics



Working out potential dividers...

In Fig.4 you can work out the voltage at the mid point in two ways:
a) Work out the current using ohms law V/R = I and then work out the voltage across the 100 ohms using ohms law again V = I*R
b) Work out what proportion of the whole resistance 100 ohms represents i.e. 100 / (500+100) = 0.166 and then multiply by the source voltage.

Either way round, the answer is 166 Volts.

Fig.5 is a bit more complicated, but is a lot more useful...
Start by working out the combined resistance of the 100 ohms and 10,000 ohms using:

    Combined resistance=         1
                           ---------------
                           (1/R1) + (1/R2)
This should work out to be 99 ohms (rounded). You can then use the same method as for Fig.4 to work out the mid point voltage:

     Total series resistance =  500 + 99 =  599 ohms

                         V        1000
     Total Current   =  ---  =   ------  = 1.67 Amps
                         R         599

     Voltage at mid point =  I * R  =   1.67 * 99  = 165.3 Volts

     Current through 100ohms = 165.3/100 = 1.65 Amps

     Current through 10,000 ohms = 165.3/10,000 = 0.0165 Amps (16.5mA)

On to: 2.0 ESD Effects on Electronics

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