Static electricity test.
FORD
Posts: 221
Apologies is this is not strictly a propeller question, but there are some great hardware people on this forum...
At·some stage, I will·need to submit our propeller powered equipment to authorities for some very strict tests, as it is to be used for measuring fluids where money (trade use)·is involved. One of the tests they will do is a 'static discharge' test. This involves them·discharging a charged small cap (8kv, ~10pF I think) onto any exposed metal parts of the control box to simulate static discharge·from someones finger onto the box.
Because we·need 2 metal terminals for people to touch a 1wire ID device onto, they will be 'zapping' these 2 metal parts. One terminal is ground, so that·should be OK. But the other terminal·will be·directly into an i/o on the propeller·thru a resistor, pulled up to 3.3v.
I have been told by the authorities that to simulate their test, i should use an ignitor, such as used for lighting gas stoves etc, as they simulate closely the official test.
I will be fitting a 4v zener from the i/o to ground, and another from the 3.3v supply to ground.
Would anyone have any idea if the zeners will be enough to protect the i/o and 3.3v reg from being destroyed·?
Any information, thoughts·or suggestions would be appreciated.
Otherwise, I'm just gonna 'zap it' and see. (At least it helps to boost propeller sales).
Cheers,
Chris
At·some stage, I will·need to submit our propeller powered equipment to authorities for some very strict tests, as it is to be used for measuring fluids where money (trade use)·is involved. One of the tests they will do is a 'static discharge' test. This involves them·discharging a charged small cap (8kv, ~10pF I think) onto any exposed metal parts of the control box to simulate static discharge·from someones finger onto the box.
Because we·need 2 metal terminals for people to touch a 1wire ID device onto, they will be 'zapping' these 2 metal parts. One terminal is ground, so that·should be OK. But the other terminal·will be·directly into an i/o on the propeller·thru a resistor, pulled up to 3.3v.
I have been told by the authorities that to simulate their test, i should use an ignitor, such as used for lighting gas stoves etc, as they simulate closely the official test.
I will be fitting a 4v zener from the i/o to ground, and another from the 3.3v supply to ground.
Would anyone have any idea if the zeners will be enough to protect the i/o and 3.3v reg from being destroyed·?
Any information, thoughts·or suggestions would be appreciated.
Otherwise, I'm just gonna 'zap it' and see. (At least it helps to boost propeller sales).
Cheers,
Chris
Comments
I'm sure that with some extra components, you should be able to avoid resets and, certainly, device damage. One problem with·ESD events is that arcing can occur over a centimeter at ~10kV. This means that physical distances and juxtapositioning of conductors and insulators·factor in. For example, if you had a 1M protection·resistor in series, it would just arc across the terminals and you'd be back to square one. It might be good to put a lot of grounded shielding around the iButton interface, and then·connect it back to the power supply's central ground connection via a ribbon cable (especially if the distance is greater than·a foot). The ribbon cable's parallel conductors will reduce inductance dramatically, so that the zap gets shunted, rather than passed. At least, this will take the load off the downstream components that are getting close to your 'brains'.
Parallax once (~1992) made a point-of-sale device for billiard rooms and we had to address this ESD issue. It was hard to·replicate and debug. If I recall, after a lot of experimentation and reasoning, we connected all shockable surfaces (at ground potential, already)·back to the power supply with individual leads. I believe we did away with the original ground paths.·This way, ground loops were eliminated. During an ESD event, a lot of current flows very briefly. You have to think, "What would this circuit board trace do under 10 amps of load, albeit briefly?" Not much, maybe, but the sleeper issue is that it WILL induce microwave-frequency current blasts into adjacent conductors, zapping things that are not on the schematic path.
During the Propeller chip development, we bought lots of equipment to diagnose silicon problems with. One of the coolest things you'd ever see is an 'e-beam prober'. This is a scanning electron microscope that uses·its electron beam not just for imaging, but also to·contactlessly probe chip wiring. Imagine a microscopic contactless 7GHz(!) oscilloscope, completely free of capacitive loading, and only inducing picoamps of·current flow. This machine·sold 10 years ago for $1.5M, and we bought it·for only $5k from AMD. It works great, too! In contrast to that good deal, we needed an ESD simulator (zap gun). These buggers hold 70% of their retail value after 15 years! It was impossible to find a used one at a good price. The market for these is extremely tight. Even old·junky ones were $4500! We wound up getting a new one for $6300. You can enter in the type of zap (air or contact), and set the voltage·from ·+/- 2kV to +/-16kV. It has a big, fat ribbon cable (tells you something)·to connect back to your device-under-test ground, and then changeable sharp and round tips for contact and air discharge.
When we first used the zapper on DIP Propeller test chips, they would survive 8kV zaps indefinitely. In more recent tests with LQFP Propeller chips, they were only surviving 4kV zaps. I believe the extra lead length in the DIP package (perhaps 15mm total) was providing enough inductance to effectively slow down the pulse, so that the ESD structure on the IC's I/O pad had more time to gobble up the charge without overloading and·blowing out. The LQFP, in contrast, has less than 2mm lead length before the I/O pad, making the zap much more stout and higher current, and hence, more destructive.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chip Gracey
Parallax, Inc.
Post Edited (Chip Gracey) : 4/17/2006 12:45:46 PM GMT
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Don't visit my new website...
If anyone is interested in acquiring one of these units, I'm told they're rather common and quite easy to obtain in most locales. With the proper care, they have a service life approaching 20 years.
-Phil
Love the pic too.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module"·available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module"·available from Parallax for only·$49.95 http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
Available now! Cheap 4-digit LED display with driver IC·www.hc4led.com
"I reject your reality, and substitute my own." Mythbusters
·
Phil,
Thanks for the laugh,·how long have you been waiting·for a chance to post that...
Gadgetman,
Thankyou, I will be sourcing some straight away, you have probably saved me a lot of messing around.
Chip,
Thanks for that info, its nice to know that you guys have been down this road before, and as such you have conducted those tests. We already have a 16way ribbon cable approximately 10" long running from the door of our enclosure to the main pcb inside, so hopefully that·helps us. We also have a pcb mounted behind the ibutton sensors, so we can mount one of the devices that Gadgetman has kindly mentioned right near the sensors. I will be mindful of putting large ground areas around the 1wire tracks.
Cheers, Chris