Author Topic: Static discharge: is it all relative?  (Read 1746 times)

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Offline davidlawson

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Static discharge: is it all relative?
« on: June 04, 2015, 06:46:07 am »
Hi there!

I have a question regarding static discharge. My current understanding is that voltages are all relative – if you 'zap' someone when you touch them, it's because they had a different static charge to you.

How does that work with circuits then? What if a PCB has a higher static charge than you, and you touch it while wearing a grounded wrist strap? If voltages are all relative, why do we assume it's safe to handle electronics when we just have no potential difference between ourselves and 'ground'?
 

Offline Dago

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Re: Static discharge: is it all relative?
« Reply #1 on: June 04, 2015, 07:18:59 am »
Hi there!

I have a question regarding static discharge. My current understanding is that voltages are all relative – if you 'zap' someone when you touch them, it's because they had a different static charge to you.

How does that work with circuits then? What if a PCB has a higher static charge than you, and you touch it while wearing a grounded wrist strap? If voltages are all relative, why do we assume it's safe to handle electronics when we just have no potential difference between ourselves and 'ground'?

It is no safer to touch a charged PCB with a grounded hand than vice versa. The idea with ESD protection is that both are at the same potential. Meaning that the board is not hanging in the air charged, but laying on the ESD mat at the same potential.
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Offline helius

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Re: Static discharge: is it all relative?
« Reply #2 on: June 04, 2015, 07:19:19 am »

Quote from: davidlawson on Today at 04:46:07 PM
Hi there!

I have a question regarding static discharge. My current understanding is that voltages are all relative – if you 'zap' someone when you touch them, it's because they had a different static charge to you.

This requires that both you and the other person are conductive. If you charge up an insulator, by rubbing a rubber balloon against a cat, for example, and then touch the balloon--no current will flow and no shock, because the charge in the rubber stays where it is. When you and the other person with a different charge come close together, your conductivity allows those charges to move. The electric field between the different charges causes them to concentrate in the part of your body that's nearest the person, like your finger. This concentration of charge gets greater the nearer you get to them, as the electric field gets stronger still. In effect you have formed a capacitor between you and them. But before you actually touch them the field overcomes the breakdown voltage of air, and the spark travels across the gap.

Voltages seem like they're relative, because the definition of a voltage is the integral of the electric field between two points. Charge isn't relative: an object is electrically either positive, negative, or neutral, without relation to any other object. Static charges are produced by triboelectricity, the greater affinity of some materials for electrons against other materials.

How does that work with circuits then? What if a PCB has a higher static charge than you, and you touch it while wearing a grounded wrist strap? If voltages are all relative, why do we assume it's safe to handle electronics when we just have no potential difference between ourselves and 'ground'?

This is an example of an unsafe ESD environment, as the short path to ground can cause high currents to flow in a device when static charges are present. To prevent ESD events the PCB must be kept in a static-safe (equipotential) environment, and many more precautions are needed besides a wrist strap. The use of a correct wrist strap with a 1 megaohm resistor would limit the discharge current in this scenario, however.
 

Offline davidlawson

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Re: Static discharge: is it all relative?
« Reply #3 on: June 04, 2015, 07:45:15 am »
Ah I see, so the aim is to maintain a zero potential difference between the PCB and ground (it seems incorrect to say the PCB "has the same voltage" as ground) so that when you go to handle the PCB while grounded yourself, there is no current flow.

It seems like the 1 megaohm resistor would limit discharge between yourself and ground, but not necessarily between you and the PCB. If you knew that the PCB wasn't grounded, I'm guessing you would need to connect the PCB to ground via a large resistor for a while before handling it? Or is that what ESD mats effectively do?
 

Offline rs20

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Re: Static discharge: is it all relative?
« Reply #4 on: June 04, 2015, 07:56:22 am »
Voltages seem like they're relative, because the definition of a voltage is the integral of the electric field between two points. Charge isn't relative: an object is electrically either positive, negative, or neutral, without relation to any other object. Static charges are produced by triboelectricity, the greater affinity of some materials for electrons against other materials.

Not really relevant, if you have two objects both with an identical positive static charge, then they can't spark against each other. So for all intents and purposes, we're talking about relative things here. The only thing convenient about neutral is that it is easily reached by connecting to earth (optionally through a large resistor), so it's a convenient way to ensure zero relative voltage. If you leave the wrist strap out, OR if you fail to ground the PCB, there is a possibility of ESD damage. Which is exactly what the OP was saying (and to be fair, you said that eventually too).

It seems like the 1 megaohm resistor would limit discharge between yourself and ground, but not necessarily between you and the PCB. If you knew that the PCB wasn't grounded, I'm guessing you would need to connect the PCB to ground via a large resistor for a while before handling it? Or is that what ESD mats effectively do?

Correct on all points :-+, the resistor in the wrist strap does not protect against ESD in any useful way (well, it reduces your effective capacitance, since it's only the self-capacitance of your body, not that of the entire earth, but since the self-capacitance of the PCB in question is likely much smaller, this is a negligible difference and certainly nothing you'd rely on).
 


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