Author Topic: EEVblog #678 - What is a PCB Spark Gap?  (Read 42457 times)

0 Members and 1 Guest are viewing this topic.

Offline alper.y

  • Contributor
  • Posts: 13
  • Country: tr
    • Personal
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #25 on: October 31, 2014, 09:21:44 am »
I didn't get that why there shouldn't be any soldermask over spark gap? If both sides are exposed to air and there is a soldermask between pads, isn't it sufficient to create a current path? Does carbonized pcb mean that some current passes through board material itself?
 

Offline max666

  • Frequent Contributor
  • **
  • Posts: 367
  • Country: at
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #26 on: October 31, 2014, 09:22:34 am »
The expression 3000pd + 1350 is quoted very often, but does anyone know where the 1350 is derived from? The expression is obviously a rather rough and ready one, as it suggests a zero width gap will not break down until 1350V. I have some evidence which appears to contradict this.
A zero width gap isn't really a gap, is it? That reminds me of a quote by Apple Inc.: “A tap is a zero-length swipe.”

But to be serious for a moment, that formula isn't intended to be used for very small gaps (and probably not for very wide gaps either). The real thing surely is nonlinear and this formula is only an linear approximation.
 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 8605
  • Country: gb
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #27 on: October 31, 2014, 11:31:12 am »
I didn't get that why there shouldn't be any soldermask over spark gap? If both sides are exposed to air and there is a soldermask between pads, isn't it sufficient to create a current path? Does carbonized pcb mean that some current passes through board material itself?
If you have solder mask between the arc electrodes everything is OK until an arc occurs. Then you get fried solder mask, which is not good. FR4 is far harder for the arc to damage, so its OK to let the arc run across its surface.
 

Offline LektroiD

  • Frequent Contributor
  • **
  • Posts: 258
  • Country: gb
  • If it didn't explode, I'm happy.
    • Music here
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #28 on: October 31, 2014, 12:45:38 pm »
I use 15-22r resistors at the input. too much stress and the resistors go up in magic smoke (I'd rather smoke a resistor than a high-end opamp or vactrol).

It's great news that you've started to use Patreon too, I hope you get plenty of continued support, your videos are more than worth it. I too set up an account there as a content provider, yet when it comes to payments, they are asking for my paypal password (to receive money?)... Hmmm...!
 

Offline JackOfVA

  • Supporter
  • ****
  • Posts: 350
  • Country: us
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #29 on: October 31, 2014, 12:56:38 pm »
Isn't there a significant difference between breakdown voltage as measured between two rounded objects (spheres traditionally, but in this case a smooth rounded PCB trace and a ground plane strip) versus two sharp points? 

The electric field potential at the sharp point is divergent and will experience breakdown at about one-third the voltage that will be seen between two smooth balls.

Hence, instead of roughly 3 million volts/meter breakdown for dry air at STP two points will break down closer to 1 million v/m.

And, there are some other non-linearities associated with breakdown e.g., Paschen effect in breakdown voltage versus pressure. There's also a difference in breakdown voltage with polarity for point emitters, but not so much with spherical or smooth emitters.

Nice overview of the subject can be found at http://cas.web.cern.ch/cas/Slovakia-2012/Lectures/FairclothHighVoltage.pdf


 

Offline Circuitous

  • Supporter
  • ****
  • Posts: 237
  • Country: us
    • Corgi-Tronics
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #30 on: October 31, 2014, 01:06:12 pm »
Dave,  nice video and explanation.
I would really like to see one on input protection... mains input,  DC power input,  and logic level input/output.

Offline sunnyhighway

  • Frequent Contributor
  • **
  • Posts: 276
  • Country: nl
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #31 on: October 31, 2014, 01:25:39 pm »
I too set up an account there as a content provider, yet when it comes to payments, they are asking for my paypal password (to receive money?)... Hmmm...!

That's just the typical american style of doing business. They always want to know more about you than what's necessary to do business with you. Gotta fill the demographics database you know. Worth a lot of money. Instead of having you answer 12 pages of irrelevant questions to fill that stupid demographic database they can now access all that data themselves without  boring you with all those stupid questions by simply using your password. They will call it "making things easier for the customer, but for some weird reason I don't feel inclined to share that opinion.
 

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 37664
  • Country: au
    • EEVblog
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #32 on: October 31, 2014, 01:28:03 pm »
It's great news that you've started to use Patreon too, I hope you get plenty of continued support, your videos are more than worth it. I too set up an account there as a content provider, yet when it comes to payments, they are asking for my paypal password (to receive money?)... Hmmm...!

They shouldn't ask for your PayPal password!  :o
 

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 37664
  • Country: au
    • EEVblog
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #33 on: October 31, 2014, 01:33:04 pm »
Dave, very interesting video! I noticed your example recommending the rounded trace sticking in the ground fill, but isn't the triangular design better, because a sharp edge promotes spark forming? It would erode quicker, sure, but the initial protection would be better.

There is no such thing a a perfectly round edge, especially in etched PCB traces.
Macroscopically looking round edges and straight sides actually have many microscopic points. You can see this in the video when the spark makes it's way around the arc.
The user gets to weight up the pros and cons. Personally I'd rather go for reliability over some essentially unknown decrease in the arc voltage.
 

Offline Royce

  • Contributor
  • Posts: 49
  • Country: us
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #34 on: October 31, 2014, 01:43:56 pm »
Dave,  nice video and explanation.
I would really like to see one on input protection... mains input,  DC power input,  and logic level input/output.

Joining with the couple of others in the thread. I'd really like to see the video as well. The protection, how one would test it, and how one would expect it to be tested by some sort of agency.
 

Offline StefanHamminga

  • Contributor
  • Posts: 36
  • Country: nl
    • PRJCT.NET
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #35 on: October 31, 2014, 03:08:20 pm »
There is no such thing a a perfectly round edge, especially in etched PCB traces.
Macroscopically looking round edges and straight sides actually have many microscopic points. You can see this in the video when the spark makes it's way around the arc.
The user gets to weight up the pros and cons. Personally I'd rather go for reliability over some essentially unknown decrease in the arc voltage.
I was thinking more of 'what's maximum protection I can throw in there' for a simple prototype board, rather than reliability in production quality boards. Out of curiosity I took a quick snap of an OSH Park PCB I had laying around (0.5mm pitch, 0.1524mm ground fill clearance at the thermal pad). The rounded edges look pretty clean to me. Not trying to be a smartass, just curious to find out what the opinions are on finding the balance between low breakdown voltage and lifetime of the contact set.

What about the solder on an ENIG board? At one hand the plating doesn't have the insulating properties of the solder oxidation, but it doesn't have its (thermal) mass either.
My initial gut feeling would have me try something like the included KiCad screenshot (I can post the footprints for anyone interested).
 

Offline LektroiD

  • Frequent Contributor
  • **
  • Posts: 258
  • Country: gb
  • If it didn't explode, I'm happy.
    • Music here
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #36 on: October 31, 2014, 04:41:29 pm »
It's great news that you've started to use Patreon too, I hope you get plenty of continued support, your videos are more than worth it. I too set up an account there as a content provider, yet when it comes to payments, they are asking for my paypal password (to receive money?)... Hmmm...!

They shouldn't ask for your PayPal password!  :o

My mistake, that was for payments, I was in the wrong part of the site. I've sorted it now ;)
 

Offline zapta

  • Super Contributor
  • ***
  • Posts: 6189
  • Country: us
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #37 on: October 31, 2014, 05:36:45 pm »
Would be interesting to discharge a 2.5KV capacitor through the spark gap and see what is the end voltage of the capacitor. I presume that once it starts conducting at the breakout voltage it keeps conducting at lower voltages as well.
 

Offline f4eru

  • Super Contributor
  • ***
  • Posts: 1086
  • Country: 00
    • Chargehanger
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #38 on: October 31, 2014, 06:13:40 pm »
A tip to mention :  put this structure on the component side (TOP in this case), so you don't risk to short it or to contaminate it with flux when soldering.

Offline 2shy

  • Newbie
  • Posts: 8
  • Country: pl
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #39 on: October 31, 2014, 07:14:31 pm »
There's a nice app note on designing for ESD and EMC from NXP:
http://sa.nxp-lpc.com/docs/an10897.pdf

On the 22nd page they wrote a little bit about spark gaps, including the formula that Dave showed in the video. Unfortunately I couldn't find the formula regarding the air humidity.

BTW, it's my first post here, so hello to everyone :)
 

Offline StefanHamminga

  • Contributor
  • Posts: 36
  • Country: nl
    • PRJCT.NET
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #40 on: October 31, 2014, 07:53:59 pm »
I wonder if the reduced static charge buildup due to the higher conductivity of humid air has something to do with not factoring in humidity in this rule of thumb?
 

Offline sebmadgwick

  • Regular Contributor
  • *
  • Posts: 127
  • Country: gb
    • YouTube Channel
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #41 on: November 01, 2014, 08:47:32 pm »
Is this also applicable to battery powered devices?  If so, then why would the spark arc to battery ground given that this is just a arbitrary voltage level, presumably just as arbitrary as the IC pin?
 

Offline eV1Te

  • Regular Contributor
  • *
  • Posts: 186
  • Country: se
  • Your trusted friend in science!
    • richardandersson.net
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #42 on: November 02, 2014, 04:08:13 am »
Someone mentioned the altitude rating for some equipment, the breakdown voltage is directly proportional to the air-pressure at normal conditions, so at 5000 m ASL the breakdown voltage is about half, (since the pressure is about half) which might affect equipment that uses higher voltages. Electronics in weather balloons etc. where highly susceptible to breakdown when people were still using vacuum tubes.

Interestingly for very small spark-gaps or at low pressures, the breakdown voltage increases with reduced distances or pressure, because the electrons do not have time to "find" an atom to collide with.
Take a look at this Paschen Curve for example, the X-axis is = spark-gap distance times pressure (bar*cm) (half distance requires twice the voltage), at a distance of 10 um at sea-level the breakdown voltage would starts to increase. So if you would increase the altitude of the device, then the breakdown voltage would actually increase instead of decrease. This only works down to ca. 1-5 um, then field-emission starts to happen.

 

Offline alper.y

  • Contributor
  • Posts: 13
  • Country: tr
    • Personal
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #43 on: November 02, 2014, 06:45:16 pm »
I didn't get that why there shouldn't be any soldermask over spark gap? If both sides are exposed to air and there is a soldermask between pads, isn't it sufficient to create a current path? Does carbonized pcb mean that some current passes through board material itself?
If you have solder mask between the arc electrodes everything is OK until an arc occurs. Then you get fried solder mask, which is not good. FR4 is far harder for the arc to damage, so its OK to let the arc run across its surface.
Thanks.

So this difference shouldn't affect breakdown voltage, right?
 

Offline SeanB

  • Super Contributor
  • ***
  • Posts: 16272
  • Country: za
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #44 on: November 02, 2014, 07:03:13 pm »
It does affect it, as the solder mask is an insulator, so it raises the breakdown voltage of the gap. When it does break down the solder mask decomposes and leaves a carbon film which reduces the insulation resistance, so that it will either break down at a lower voltage or will conduct continuously.
 

Offline alper.y

  • Contributor
  • Posts: 13
  • Country: tr
    • Personal
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #45 on: November 03, 2014, 07:23:00 pm »
It does affect it, as the solder mask is an insulator, so it raises the breakdown voltage of the gap. When it does break down the solder mask decomposes and leaves a carbon film which reduces the insulation resistance, so that it will either break down at a lower voltage or will conduct continuously.
I didn't understand fully. Since both pads are exposed to air, even if there is an extra insulation layer between them on the board, I don't expect rise in breakdown voltage. But almost all sources suggest not putting solder mask between pads. I am not sure that it is suggested considering breakdown voltage or other issues like carbon film traces as you mentioned.

Thanks  :)
 

Offline alank2

  • Super Contributor
  • ***
  • Posts: 2183
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #46 on: November 14, 2014, 01:07:11 am »
So the idea that you want to protect certain nets by giving them a way to arc over to board ground, right?

Q#1 - Would this mean that you would preferrably have a TVS on the main vcc net and ground that could soak it up?  Or does the arc itself lose some of the power?  Or both?

Q#2 - Is there a benefit to putting a spark gap on a net you want to protect that already had a TVS protecting it?  Is there a benefit to having both?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 21609
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #47 on: November 14, 2014, 07:35:58 am »
1. The current from the transient has to go somewhere.  If your power supply already has a very low impedance, even at surge frequencies (ESD has a rise time in the nanoseconds) and for large currents (an 8kV ESD spike is some 16A peak), you don't need to worry.

There's also the matter of surges, for circuits that are connected to very long wires that may be subject to induced lightning transients, or full blown strikes (which includes telephone and mains lines, but Dave's recent home security module teardown illustrates another example).  These surges are much longer duration and lower impedance, and so contain far more energy.  Even a very low impedance power supply is unlikely to stomach this much energy (a small 8/20us pulse with a peak of 10A will charge a 100uF capacitor by maybe 2V), so it needs to be dissipated with a TVS instead.

2. There could be, but note that a spark gap won't fire unless the voltage across it gets high enough to trigger.  If this never happens, the spark gap was wasted time, effort, space and (if using GDTs) money.

A very robust system might be constructed thusly:
- Pin to the outside world
- Spark gap
- Series impedance (air cored inductance, resistance: as much of each as is tolerable)
- MOV or TVS (the bigger the better, given limits of capacitance)
- More series impedance (including ferrite beads if needed for EMC)
- TVS or clamp diodes
- Filter cap(s) or band limiting parts (for bandwidth reduction (e.g., radio antenna input, scope front end, etc. etc.) or EMC)
- Series resistance
- device (transistors, ICs, etc.).

Very fast and large pulses cause enough voltage drop across all the series impedances to cause breakdown of the spark gap.  This occurs for anything from ESD (fast, short) to sufficiently powerful surges (slow, but lots of peak current and voltage).

What's left after the spark gap ranges from short blips of manageable amplitude (the absorbed ESD spark might go from 8kV, 16A and 50ns down to 2kV, 16A and 5ns, followed by a "drool" of maybe 100V and 100ns) to nothing at all (a sufficiently small surge will never spark, and goes through the first series impedance unaffected).

The second stage is capable of absorbing lots of energy (MOVs are a cheap source of energy capacity, but high in capacitance; avalanche TVSs of comparable ratings are very expensive; thyristor type TVSs have good ratings but can latch on and blow fuses), but at a relatively high voltage drop (plan on a peak voltage up to three times the MOV rating).

The third stage reduces the excess voltage of the second stage, bringing it down to manageable levels for the device.

The final stage is always the device itself being protected.  No practical surge protection system will fully control the transient, and some will always appear at the device.  If the device can ride out some voltage, but must carry zero current (example: unprotected MOSFET gates), the transient protection must reduce the transient to no more than that voltage.  If it can shunt some current, it also becomes part of the protection system (e.g., input clamp diodes, CMOS outputs).  Some circuits don't mind much either way (a discrete audio amplifier built from BJTs and resistors will draw some current when driven beyond normal range, but rarely anything destructive by the time the protection system is doing its job; such circuits, historically speaking, have rarely if ever bothered with ESD protection at all, anyway).

Finally, the purpose of properly designed protection is to limit those transients to the device ratings, as sufficient for the equipment specifications (how much a beating it can take, and whether it must remain functional during or after such events).

- Consumer junk rarely uses TVSs at all: they can use chips with built-in ESD protection, and the attached power supply handles surges.
- Professional stuff usually has adequate protection (one MOV or TVS, some series impedance as applicable).
- Two stage protection is occasionally seen in special purposes.  Example: CRT monitors, where internal breakdown in the CRT can deliver over 20kV at impedances much lower than an ESD test, and where the protected circuit (video amplifier) must have absolutely minimal loading capacitance.
- Three and four stage protection are the gold standard, but rarely if ever used simply because it's not necessary.  Some examples might possibly include military gear that has to survive rough handling, machine model ESD, radar bombardment and jamming, and nuclear EMP, where operation must continue without any reaction or interruption during such events.  But I haven't seen such equipment up close, so I don't know what they use.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline mfeinstein

  • Contributor
  • Posts: 36
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #48 on: November 16, 2014, 07:48:17 pm »
Hey Dave,

Two things I think are important to comment about this technique:

1- This won't be recommended in high-speed lines, since this "stubs" could cause some signal integrity issues like reflections and distortions. ( stubs on high-speed traces )

2- Be careful with these exposed pads just sitting so close with the exposed ground, your soldering process could short them out, since there won't be a solder mask to avoid the solder bridge. ( solder bridges )
 

Offline pugglewuggle

  • Contributor
  • Posts: 10
Re: EEVblog #678 - What is a PCB Spark Gap?
« Reply #49 on: November 24, 2014, 05:46:57 pm »
Dave,

I'd really like to see a video or series of videos on input protection just as you suggested. It'd be great to see this for both analog and digital (high speed and low speed, busses, etc). Maybe some tidbits on protecting against impulse/ESD vs slow ramping overvoltage vs long periods of overvoltage.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf