Author Topic: Building an ESD Ground Point, Looking For Feedback  (Read 9450 times)

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Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #25 on: March 26, 2016, 02:02:43 am »
I'd suggest you buy a proper, quality, well known branded one.

Would you consider making a parachute, out of old clothes and stuff. When you have little/no experience making parachutes ?

Would you jump out with it being your only parachute (no safety second one), from 20,000 feet ?

Safety related items, are best bought (quality/branded ones).

Oh my!   |O

You do realize this is a forum full of DIY types right? And getting shocked with 120/240 isn't that big of a deal.  I've had it happen a dozen times.  And I'mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee!  Wait, where's my glasses, oh I'm wearing them.  Oh, where's my eyballs?  Rats!

Until the day you have your LAST electric shock. It is then TOO LATE, to explain to people, why safety is SO, SO important.

If you are electrocuted via something semi-permanently attached to your body. It can make your muscles inoperative, including your heart and/or breathing.

BUT looking on the bright side, at least the wife can switch off (switch provided) the unit, to make it safe, to remove the body.

Safety first!

Yes indeed. The older amongst us have seen corpses and watched people die (and nearly die) in accidents. It concentrates the mind.

When younger I lightly touched live with one hand and neutral with the other. My biceps contracted vigorously, snapping my forearm against my upper arm  - and fortunately breaking the contact. If it hadn't, I couldn't have broken the contact myself.

Seeing the device senpai has built (specifically the plug he used), i tend to agee.
Leaving the live and neutral terminals in the plug is reckless and can be potentially lethal. If the wire in the plug breaks off for whatever reason and gets in contact with the live terminal (or close enough for arcing), then this can be goodbye and farewell. Remember, this is not like getting shocked where in many cases pulling back your finger/arm or whatever severs the electrical connection. You are having a wrist strap on your arm, and there is a rather good chance that you would not be able to remove the strap due to muscle contractions.

If senpai would have used one of those plugs as shown by ChunkyPastaSauce or followed the advice of pulling out the the live and neutral terminals of the plug, then his contraption would be as safe/unsafe to his health as a commercially available solution. But the way he did it, no way...  :--

EDIT: Also, that mains plug made of some transparent plastics. Is that material actually okay for a mains plug? (I don't know, i have never seen a transparent mains plug...)
« Last Edit: March 26, 2016, 02:21:56 am by elgonzo »
 

Online rdl

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #26 on: March 26, 2016, 02:47:10 am »

...
Leaving the live and neutral terminals in the plug is reckless and can be potentially lethal. If the wire in the plug breaks off for whatever reason and gets in contact with the live terminal (or close enough for arcing), then this can be goodbye and farewell.
...

Isn't this is theoretically possible with every plug currently in use?
 

Online Ian.M

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #27 on: March 26, 2016, 03:19:27 am »
Perhaps the nay-sayers in this topic would care to say how a single fault could cause a dangerous electric shock, when there is a 1Meg series resistor limiting the maximum possible fault current to 0.24mA?

The L and N pins of the plug are required to hold it stably in the socket and to prevent unsafe incorrect insertion.  Insulating or otherwise blocking access to their terminals within the plug would be advisable.

I don't like the O.P.'s implementation as any contact between objects connected to different ESD ground terminals could put their individual 1Meg resistors in parallel, (there should have been only one resistor and the terminals wired together) decreasing the effective resistance, which would put the duty of limiting the fault current to a safe level on the 1Meg resistor allegedly fitted in the mains plug, the switch is pointless and can only increase the risk of ESD damage to components etc, I have already objected to fitting a resistor inside the plug, and I regard anchoring the ground lead by knotting it to be extremely bad practice.  However, if assembled in a competent fashion using appropriately rated resistors, there is no reason that an ESD grounding point built in a plastic project box and using a normal unmodified mains plug should be inherently unsafe.
« Last Edit: March 26, 2016, 03:23:46 am by Ian.M »
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #28 on: March 26, 2016, 03:22:48 am »

...
Leaving the live and neutral terminals in the plug is reckless and can be potentially lethal. If the wire in the plug breaks off for whatever reason and gets in contact with the live terminal (or close enough for arcing), then this can be goodbye and farewell.
...

Isn't this is theoretically possible with every plug currently in use?
It is definitely a possibility. A normally wired plug (live+neutral+earth), three wires have to share a given volume in the plug. Here, only one (somewhat thinner) wire has all the volume for itself, giving it more space to move around if it desires so. Essentially you are right that most if not all mains plugs (except the molded ones) would share a similar vulnerability, but i believe the risk of that to happen in a fully wired plug to be lower than in a plug where there is only one wire...

On the other hand, perhaps he could keep the terminals in the plug if he covers them completely with something heat-resistant... (epoxy, for example; not sure whether hot-snot would qualify). Considering what Ian.M said with regard to keeping the plug reliably in the socket and ensuring proper insertion, this might be preferable over pulling the terminals out.
« Last Edit: March 26, 2016, 03:47:30 am by elgonzo »
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #29 on: March 26, 2016, 03:41:28 am »
[...] I don't like the O.P.'s implementation as any contact between objects connected to different ESD ground terminals could put their individual 1Meg resistors in parallel, (there should have been only one resistor and the terminals wired together) decreasing the effective resistance, which would put the duty of limiting the fault current to a safe level on the 1Meg resistor allegedly fitted in the mains plug, the switch is pointless and can only increase the risk of ESD damage to components etc, I have already objected to fitting a resistor inside the plug, [...]
No. One 1MOhm resistor for every input, that is how the pros do it. For example, Warmbier 2200.W.3:
 

Online m98

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #30 on: March 26, 2016, 03:42:20 am »
It may do; I won't offer an opinion. But more importantly, how well will it do the job in a years time, and what are the failure mechanisms? Might someone else use it in a few months/years time without verifying that it is still working safely?
Let's make new mistakes. Old mistakes are boring and unlikely to win the Darwin Award :)
And where would you see the dangerous failure mechanism? If the Earth wire was accidentally connected to live, he'd still have a 1M ohm resistor in the box, and the 1M ohm resistor in the strap. So we have two redundant protection mechanisms in case that the earth wire was connected to live. Maybe he could add a little lamp in series to know when the plug has failed, or more likely, the earth wiring was somehow connected to live.
Please always try to apply common sense of physics and engineering, before picking up some fairy tales from electricians.
Touching the mains live isn't anywhere near as lethal as jumping out of an airplane without a parachute. For a reasonably healthy person it would end quite unspectacular, because you hopefully have an RCD installed on all circuits (and test it at least once a year) and aren't standing barefoot on a wet floor.

Online MK14

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #31 on: March 26, 2016, 03:47:43 am »
It may do; I won't offer an opinion. But more importantly, how well will it do the job in a years time, and what are the failure mechanisms? Might someone else use it in a few months/years time without verifying that it is still working safely?
Let's make new mistakes. Old mistakes are boring and unlikely to win the Darwin Award :)
And where would you see the dangerous failure mechanism? If the Earth wire was accidentally connected to live, he'd still have a 1M ohm resistor in the box, and the 1M ohm resistor in the strap. So we have two redundant protection mechanisms in case that the earth wire was connected to live. Maybe he could add a little lamp in series to know when the plug has failed, or more likely, the earth wiring was somehow connected to live.
Please always try to apply common sense of physics and engineering, before picking up some fairy tales from electricians.
Touching the mains live isn't anywhere near as lethal as jumping out of an airplane without a parachute. For a reasonably healthy person it would end quite unspectacular, because you hopefully have an RCD installed on all circuits (and test it at least once a year) and aren't standing barefoot on a wet floor.

I guess that there are different levels of safety, that one can achieve. So really we have different ideas, on how safe to make something like this, and how important it is to buy (or not), a safely manufactured and tested item, to perform this function.

My opinion is, that beginners should not build electronics projects, inside, live electrical plugs. It is a bad idea.

Although in this case, it is more a wire and some attempts at insulation, rather than a bigger project, inside a plug.

E.g. The glue and nonsense, he talks about putting in (as well as the wire(s)), may start to conduct and hence be a potential fire risk, at some point, as they deteriorate.

As regards ESD, they appear to be the wrong type of resistor for ESD. I.e. The type which tend to fail, if exposed to significant ESD discharges, due to the extreme pulse currents, etc.
« Last Edit: March 26, 2016, 04:11:53 am by MK14 »
 

Offline Orpheus

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #32 on: March 26, 2016, 05:11:16 am »
No. One 1MOhm resistor for every input, that is how the pros do it. For example, Warmbier 2200.W.3:


Yes. Again, the primary purpose is to protect the electronics from ESD. If you have multiple straps bonded to a single resistor, ESD from one strap could flow to another and fry an uncharged part. Strap resistance would provide some protection, but per-strap resistors seems better.
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #33 on: March 26, 2016, 05:36:06 am »
As regards ESD, they appear to be the wrong type of resistor for ESD. I.e. The type which tend to fail, if exposed to significant ESD discharges, due to the extreme pulse currents, etc.
Yes, it looks like metal thin film resistors are used in this project - one of the worst kind you can choose for this task. Thanks for highlighting it.
« Last Edit: March 26, 2016, 05:38:21 am by elgonzo »
 

Online MK14

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #34 on: March 26, 2016, 05:45:39 am »
As regards ESD, they appear to be the wrong type of resistor for ESD. I.e. The type which tend to fail, if exposed to significant ESD discharges, due to the extreme pulse currents, etc.
Yes, it looks like metal thin film resistors are used in this project - one of the worst kind you can choose for this task. Thanks for highlighting it.
Yes, apparently the laser technique (used to make them a relatively accurate value), does NOT stand up to the relatively violent "kick" of ESD discharges. I.e. The laser has created "weak" parts of the resistor.
It can even (somewhat rarely make them go short-circuit, but usually they go open-circuit) make them unsafe, both as regards safety (human), and ESD protection.
 

Online rdl

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #35 on: March 26, 2016, 07:13:22 am »
Yes, and be careful, or the boogeyman may get you tonight.
 

Offline Senpai

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #36 on: March 26, 2016, 11:00:03 am »
I haven't put a 1meg resistor in plug. Saw it wasn't practical when I went to build the thing. Decided against it as it'd too messy, too much stray wire. It's just the wire going straight to ground as it's intended to be used. Forgot to say that when I posted the pics sorry.

I know to keep an eye on the glue, regularly check it, regularly test it, etc, etc.

If the wire in the plug breaks off for whatever reason and gets in contact with the live terminal (or close enough for arcing), then this can be goodbye and farewell.
They'll have to break off and then find their way through a 1.5mm wall of plastic as well. The individual terminals have their own little boxes inside the plug. I would have rather had plastic terminals, but really see no problems with this so I'm not spending time/money to change it.

With 1 prong I'd be at risk of me recabling after a few beers (to plug something else in), plugging it in wrong, and then zapping myself later. This seems far more likely to me than the cable in the plug coming loose, managing to find its way through 1.5mm of plastic wall, and then somehow making contact with the wrong contact, which sounds incredibly unlikely.

More likely is the wire coming out and falling into the power strip, which is just as likely to happen with any of my other plugs for other appliances. Though the way the strip is mounted makes it near impossible for that to happen.

Isn't this is theoretically possible with every plug currently in use?
Pretty much. And thus, I'm not worried about it.

I regard anchoring the ground lead by knotting it to be extremely bad practice.
They're soldered, not knotted.

the switch is pointless and can only increase the risk of ESD damage to components etc
It means I don't have mains earth sockets exposed on the corner of my desk 24/7, seems pretty prudent to me?

I wouldn't turn it on while working on things. The only time it'd be switched off while working on ESD sensitive devices would be in an emergency. No different from the switches on the wall really.

Maybe he could add a little lamp in series to know when the plug has failed, or more likely, the earth wiring was somehow connected to live.
That sounds like a pretty good idea to me. Anyone else want to chime in on this, or shall we wait until I've built the thing again?

RCD installed on all circuits (and test it at least once a year) and aren't standing barefoot on a wet floor.
RCD is installed, reminders to test are in the calender. :)

Yes, it looks like metal thin film resistors are used in this project - one of the worst kind you can choose for this task. Thanks for highlighting it.
Would ceramic be better?

The special fancy ESD ones mentioned earlier in the thread went up to a whopping 10K resistance for $35NZD. 10K isn't nearly enough. Well that's all I could find without paying >$50 anyway.
« Last Edit: March 26, 2016, 11:10:19 am by Senpai »
 

Online Ian.M

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #37 on: March 26, 2016, 11:51:07 am »
You knotted the wire to stop it pulling through the hole in the project box wall.  Unfortunately that doesn't secure it against being pushed in.

Your resistors almost certainly don't have enough voltage rating for safety.  Use two in series, or put one between the incoming ground wire and the common point.  Carbon composition resistors are the cheapest option available in 1Meg that have sufficient pulse withstanding capability to ensure they survive repeated ESD events.
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #38 on: March 26, 2016, 11:53:39 am »
Yes, it looks like metal thin film resistors are used in this project - one of the worst kind you can choose for this task. Thanks for highlighting it.
Would ceramic be better?

The special fancy ESD ones mentioned earlier in the thread went up to a whopping 10K resistance for $35NZD. 10K isn't nearly enough. Well that's all I could find without paying >$50 anyway.
Certain ceramic resistors should be okay.
For example, this one on Digikey seems suitable: http://www.digikey.com/product-detail/en/ohmite/OY105KE/OY105KE-ND/823952 (datasheet: http://www.ohmite.com/cat/res_ox_oy.pdf). This resistor has a working voltage of 400V (220V mains has about 311V magnitude). Its ESD performance is not as good as foil transistors, but much better than thin/thick film resistors -- i would say it's good enough.
« Last Edit: March 26, 2016, 11:57:43 am by elgonzo »
 

Offline ChunkyPastaSauce

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #39 on: March 26, 2016, 12:06:50 pm »
Yes, it looks like metal thin film resistors are used in this project - one of the worst kind you can choose for this task. Thanks for highlighting it.
Would ceramic be better?

The special fancy ESD ones mentioned earlier in the thread went up to a whopping 10K resistance for $35NZD. 10K isn't nearly enough. Well that's all I could find without paying >$50 anyway.
Certain ceramic resistors should be okay.
For example, this one on Digikey seems suitable: http://www.digikey.com/product-detail/en/ohmite/OY105KE/OY105KE-ND/823952 (datasheet: http://www.ohmite.com/cat/res_ox_oy.pdf). This resistor has a working voltage of 400V (220V mains has about 311V magnitude). Its ESD performance is not as good as foil transistors, but much better than thin/thick film resistors -- i would say it's good enough.

The esd standards [guidelines] I've seen that specify the resistor composition, call for carbon.
« Last Edit: March 26, 2016, 01:19:09 pm by ChunkyPastaSauce »
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #40 on: March 26, 2016, 12:21:31 pm »
The esd standards I've seen that specify the resistor composition, call for carbon.
Nonsense. You need to check the datasheet of a specific resistor. Just saying that composite carbon resistors are appropriate is simply not generally true. There are plenty of carbon composite resistors which don't cut the cheese. Like this one for example: https://www.seielect.com/catalog/SEI-RC.pdf , which has a max pulse voltage rating of below 1000V. This particular carbon composite resistor is not better than a some thin film resistors (perhaps even worse).
« Last Edit: March 26, 2016, 12:30:36 pm by elgonzo »
 

Offline Senpai

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #41 on: March 26, 2016, 01:04:43 pm »
I wasn't aware there were two types of carbon resistor (film and composite) before this.

What do you think of this?
http://nz.rs-online.com/web/p/through-hole-fixed-resistors/0131700/
Datasheet:
http://docs-asia.electrocomponents.com/webdocs/0098/0900766b80098b49.pdf

Carbon film, says it's good up to 500V, 1000V max.

You knotted the wire to stop it pulling through the hole in the project box wall.  Unfortunately that doesn't secure it against being pushed in.

Ahh I see what you're saying now. Good point, I'll put another knot out the back.
« Last Edit: March 26, 2016, 01:06:41 pm by Senpai »
 

Offline ChunkyPastaSauce

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #42 on: March 26, 2016, 01:16:04 pm »
The esd standards I've seen that specify the resistor composition, call for carbon.
Nonsense. You need to check the datasheet of a specific resistor. Just saying that composite carbon resistors are appropriate is simply not generally true. There are plenty of carbon composite resistors which don't cut the cheese. Like this one for example: https://www.seielect.com/catalog/SEI-RC.pdf , which has a max pulse voltage rating of below 1000V. This particular carbon composite resistor is not better than a some thin film resistors (perhaps even worse).

Youd pick a high KV rated one, not that one.

I did go back and check the standards. Solid carbon compositions are not required by code as far as I can tell. Instead I found them in esd and protection circuit guidelines from resistor manufactures and esd control management papers. They are also frequently used in wrist straps.

 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #43 on: March 26, 2016, 01:17:06 pm »
I wasn't aware there were two types of carbon resistor (film and composite) before this.

What do you think of this?
http://nz.rs-online.com/web/p/through-hole-fixed-resistors/0131700/
Datasheet:
http://docs-asia.electrocomponents.com/webdocs/0098/0900766b80098b49.pdf

Carbon film, says it's good up to 500V, 1000V max.

I am confused. Do you actually know what an ESD discharge is?
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #44 on: March 26, 2016, 01:26:28 pm »
Youd pick a high KV rated one, not that one.

I did go back and check the standards. Solid composite carbon are not required by code as far as I can tell. Instead I found them in esd and protection circuit guidelines from resistor manufactures and esd control management papers. They are also frequently used in wrist straps.
That's a fair and correct argument. Note also that i said in my post "certain ceramic resistors". If you read the datasheet of the resistor i linked you will notice that its performance parameter make it very suitable for ESD applications. Bottom line is, both ceramic and composite carbon resistors can be used, if and only if they provide appropriate characteristics (i.e. check the data sheet).

FYI: I did not selected the ceramic resistor because senpai was asking for ceramic resistors. I did just do a quick search in Digikey for THP 1MOhm resistors (filtered by carbon composite, ceramic and foil) which are sold in single quantities and in stock, and it just so happened that the first (i.e. cheapest) matching result with (in my opinion) appropriate ESD performance was a ceramic. I am not biased towards ceramics, nor do i have something against composite carbon resistors. It is just that generalizations with regard to a particular type of resistor is meaningless if there are so many different models among one type featuring so many different characteristics. ;)
 

Offline Senpai

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #45 on: March 26, 2016, 01:31:42 pm »
I am confused. Do you actually know what an ESD discharge is?

My understanding of it probably isn't brilliant. Static charge builds up from moving around or whatever, voltage difference between you and something eventually shocks you and/or the thing/device. Idea being to give it a path to ground without breaking something?

It's something people in the IT field (me) usually don't care about so they just break things every now and again, they're usually pretty ignorant of it. "Who cares it's not my stuff the company pays to replace it" etc etc.
« Last Edit: March 26, 2016, 01:35:11 pm by Senpai »
 

Offline Senpai

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #46 on: March 26, 2016, 01:40:47 pm »
I wasn't aware there were two types of carbon resistor (film and composite) before this.

What do you think of this?
http://nz.rs-online.com/web/p/through-hole-fixed-resistors/0131700/
Datasheet:
http://docs-asia.electrocomponents.com/webdocs/0098/0900766b80098b49.pdf

Carbon film, says it's good up to 500V, 1000V max.

I am confused. Do you actually know what an ESD discharge is?
What's wrong with it? It's rated for higher voltage than the one you linked as far as I can tell? 500V vs 400V regular operation, 1000V vs 800V for short overloads.
 

Offline ChunkyPastaSauce

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #47 on: March 26, 2016, 01:47:19 pm »
Youd pick a high KV rated one, not that one.

I did go back and check the standards. Solid composite carbon are not required by code as far as I can tell. Instead I found them in esd and protection circuit guidelines from resistor manufactures and esd control management papers. They are also frequently used in wrist straps.
That's a fair and correct argument. Note also that i said in my post "certain ceramic resistors". If you read the datasheet of the resistor i linked you will notice that its performance parameter make it very suitable for ESD applications. Bottom line is, both ceramic and composite carbon resistors can be used, if and only if they provide appropriate characteristics (i.e. check the data sheet).

FYI: I did not selected the ceramic resistor because senpai was asking for ceramic resistors. I did just do a quick search in Digikey for THP 1MOhm resistors (filtered by carbon composite, ceramic and foil) which are sold in single quantities and in stock, and it just so happened that the first (i.e. cheapest) matching result with (in my opinion) appropriate ESD performance was a ceramic. I am not biased towards ceramics, nor do i have something against composite carbon resistors. It is just that generalizations with regard to a particular type of resistor is meaningless if there are so many different models among one type featuring so many different characteristics. ;)

Sounds good.

I didn't mean to imply the type you specified wouldn't work, just that I saw standards (turned out to be guidelines, not standards for anyone reading);  fyi that esd control suggested a certain type, not that it was the only type
 

Offline elgonzo

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #48 on: March 26, 2016, 02:44:23 pm »
Okay, a quick overview about the things that you need to know about ESD with regard to the choice of the resistor. (if you want to know more about ESD, how environmental factors such as humidity and other things influence ESD voltage levels, how a resistor impacts the discharge in detail, and so on.... google is never far away ;) )

Static electricity charges are often in the region of close to and beyond 10,000V. Sometimes even reaching 30,000V and more. The overall energy of an static electric charge as you usually encounter in your household is not much (just from a few microjoules to a couple hundred millijoules).

If this charge is being discharged (the ESD event) through a conducting path with low resistance, the discharge will happen in a very short period of time with a comparatively high current. It could hurt electronic components if they find themselves being a part of this conducting path. To protect such a component, the static electric charge has to be discharged before the component is becoming part of a potential conducting path to earth, and build up of more/another static charge has to be made impossible. The function of the resistor in the discharge path is two-fold. First, it will protect you (see further below) and secondly, it will limit the current flow (i.e. discharge rate) which can help protecting an electronic component which finds itself being an unwitting part of the discharge path.

When the static electric charge is dumped into the resistor it will be confronted with the voltage pulse in all its mighty glory. And the resistor needs to be able to withstand this high voltage (hopefully) without taking damage, albeit only for a short time until the static electric charge fizzles out. Resistors which are not able to handle those voltage pulses take damage. Such a damage can lead to significant change of resistance, it can lead to a shortcut or it can burn the resistor open. Depending on the specific resistor model, it might just take a few ESD events or it might take more ESD events until the damage becomes noticeable.

So, what can you do and how should you select a resistor? In the following i use the datasheet of the ceramic resistor i mentioned in one of my previous posts as example (http://www.ohmite.com/cat/res_ox_oy.pdf).

First, the resistor should be able to handle mains voltage. This has nothing to do with the ESD capabilities but is rather to your own safety. As others already explained in this thread, the resistor can protect you against an electric shock in case something goes totally haywire in your modified mains plug. It is important to know that in a 220V mains network the actual mains voltage is higher than 220V. 220V is only the RMS of the AC voltage. In a 220V network, the voltage swings from about +312V to -312V and back (240V network would have about +-340V). Thus, the resistor should be rated (i include some margin) for at least 400V.

Second, the resistor should have a max pulse voltage as high as possible and a resistance drift caused by ESD events as low as possible (while remaining financially feasible ;) ) The OY series resistor i mentioned earlier has a max pulse voltage of 20KV (which means that it might someday have to face ESD events with higher voltages >20KV which it will not like and perhaps take damage). Depending on the voltage of the ESD event, the resistor will drift a certain amount (i.e. change its resistance). The diagram on the datasheet tells you how the resistance drift rate changes over a number of ESD events (bad: it shows only for up to 10KOhm, grr... which requires you to extrapolate with the tongue at the right angle...). Both the max pulse voltage rating as well as the drift rate are important parameters that tell you how well a resistor is suited for a particular ESD application. (FYI: Foil resistors usually have higher max pulse voltage and lower drift rate; but yeah, that comes with a price to match...)

Note that the resistance of a resistor does not only drift due to ESD. It also drifts due to changes in temperature, humidity or simply due to age. Ceramics can be really bad at this. However, that the resistor is able to maintain more or less exactly 1MOhm is not really that important in your application. Whether it is 850K or 1.2MOhm does not matter that much. It would become a health risk for you if the resistor falls below 800K and your ESD box is exposed to mains voltage (due to a defect in the plug, for example).

In summary, that's what you should looking for:
- voltage rating of 400V or more
- max pulse voltage as high as possible, but at least around 20KV
- ESD event-induced resistance drift as low as possible

If you look into the datasheet you have given in your last post, you will notice that it does not tell anything with regard to max pulse voltage. That's a clear sign that you should not use it for ESD applications.

Hope that helps you when selecting a resistor from the catalog of your preferred vendor.


P.S.: You might have noticed that the resistor in my example has 2W power rating. Does the power rating matter for your application? No. ESD events you will encounter are rather low-energy events which even an 1/4W resistor could handle, well, if he meets the requirements as outlined above. The reason why i did choose this particular resistor is simply because it was the cheapest resistor i could quickly find on Digikey which fullfills the requirements (and is available in single quantities and in stock).

P.P.S.: Sorry for that long post. I couldn't help myself. ;)


EDIT: It would be a good idea to measure the resistance of your ESD resistors from time to time (perhaps one to four times per year, depending how often you use your ESD box), so that you will be aware when the resistors "lose" their resistance and start drifting towards 800 KOhm ...
« Last Edit: March 26, 2016, 03:22:57 pm by elgonzo »
 

Offline Senpai

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Re: Building an ESD Ground Point, Looking For Feedback
« Reply #49 on: March 26, 2016, 03:18:39 pm »
P.P.S.: Sorry for that long post. I couldn't help myself. ;)
No need to apologize, that was very helpful and insightful! Thanks very much!

EDIT: It would be a good idea to measure the resistance of your ESD resistors from time to time (perhaps one to four times per year, depending how often you use your ESD box), so that you will be aware when the resistors "lose" their resistance and start drifting towards 800 KOhm ...
Yep definitely will keep an eye on that.

Edit: Merging posts

Went and looked up the ones you linked on digikey. Digikeys postage is like $50 from memory though so not really worth it. RS (free post) only have them in quantities of 50 at roughly $1.80 each.

So I'm leaning towards these Vishay foil ones now:
3kV $4.55 - http://nz.rs-online.com/web/p/through-hole-fixed-resistors/8492775/
15kV $9.34 - http://nz.rs-online.com/web/p/through-hole-fixed-resistors/8492817/
Datasheet for both: http://docs-asia.electrocomponents.com/webdocs/1410/0900766b8141086d.pdf

Problem: They don't list the pulse voltage at all from what I can see.

Though the document you originally linked suggests they can handle ESD events up to 25kV: https://www.digikey.com/Web%20Export/Supplier%20Content/VishayPrecisionGroup_804/PDF/vishay-tech-resistor-sensitivity-to-esd.pdf?redirected=1

Do you think it's reasonably safe to assume they'll be okay in this application, because of that ESD paper Vishay did? It is their study and their own resistors so. Still, I don't know, so I'd rather play it safe and ask.

I'm only using one of the terminals at the moment so I suppose $10 isn't much to drop to do one of them to start with. Can slowly change out the others if/when they come into use.
« Last Edit: March 26, 2016, 03:43:59 pm by Senpai »
 


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