Help understanding ESD ratings

Question

I keep running into ESD ratings in datasheets but I have no idea what they mean, for example in LM385 datasheet it says:

What does this mean for an electronic engineer or a hobbyist like me? can someone help me understand these points please:

• What is the normal and maximum human body static voltages and what cause it to increase?
• What does the value 500V mean? is it safe to touch it with hand for example?
• How can I tell what's my ESD voltage at a given time?

Answer 1

What is the normal and maximum human body static voltages and what cause it to increase?

There really isn't a normal maximum, its dependent on the triboelectric charging between the materials your wearing (the source of the charge) and any discharge path (humidity in the air also makes a difference). See this article

What does the value 500V mean? is it safe to touch it with hand for example?

That means it was tested to 500V, which mainly concerts input terminals of the device that would be connected to the 'outside' world by a cable or something the user of a device could touch. The human body model is: "this standard is intended to simulate a person becoming charged and discharging from a bare finger to ground through the circuit under test." it involves a set capacitance and resistance and a standardized test setup.

If you want electronics to survive then get a proper ESD setup. There are numerous articles on this site and the internet that describe what is acceptable for hobbyists. An ESD mat and a wriststrap would be a good start.

How can I tell what's my ESD voltage at a given time?

You could build or buy an electrometer and measure your potential between you and ground.

Answer 2

There are devices which will measure ESD potentials. As a hobbyist you are unlikely to have any of these devices.

The notes is the datasheet are directed toward professional assembly situations where they provide information on what standards need to be met in handling/processing the devices.

For you, it means you should:

1. Read up on ESD best practices (something like this may help)
2. Make sure your workstation (you do have one right?) is properly grounded and protected against ESD buildup
3. Your tools such as soldering irons, electric screwdrivers etc are all grounded, and you wear the correct anti-static clothing and coveralls.

Answer 3

1. What is normal and maximum?

   • a few hundred volts is normal after you ground yourself and let go on a perfect insulating surface. The body voltage fluctates just with position movement change in capacitance V=Q/C assuming >100M isolation from ground. This can only be measured with similar impedance instruments.
     • After that it's anyone's guess
   • on an old color TV tube when on you can easily use tinsel to create a force from the tube tube to the flow of ions emitting from the lead glass surface enough to make the tinsel go sideways requiring more than 10kV.
2. is 500V safe? Safety depends on current and duration, not voltage. Thus series resistance and surface area or current density affects our perception level directly, not just current. usually it is measured in watt-seconds or Joules where \$Ec=\frac{1}{2}CV^2\$
   • The human body model can conduct enough current to feel a zap at 10kV easily from only 300pF of equivalent finger capacitance. But that doesn't hurt. 1/2 *300pF *(500V)^2=38 uJ which is peanuts.
   • the issue is the safety of CMOS
3. what's my ESD voltage?
   • who knows
   • if you can initiate an arc and measure the distance that started the arc,
   • then it ranges from 1kV/mm for point-to-point and 3kV/mm for a large ball surface.