A Hidden Threat To Handhelds

Logic Bomb writes: "An article from the San Francisco Chronicle focuses on a lawsuit against Palm, but talks about a larger issue: static and handheld computers. Basically, as computing equipment becomes smaller and more likely to be carried around, major damage from static becomes a serious threat. As the blurb at the end of the article says, it takes 3500 volts for a human to feel a shock, but only 200 to potentially scramble a microchip." We already mentioned the lawsuit, but this has more information about the supposed risks to your motherboard.

err

[crazney]

it takes 3500 volts for a human to feel a shock, but only 200 to potentially scramble a microchip.

excuse me? it depends on the current flowing and stuff.. for example, I got shocked by 240v two days ago, and i bloody well fealt it.

getting shocked....

[benny_lama]

Partly wrong....you can become an electrical conductor at any voltage if the conditions are right. Besides, it takes a combination of the right voltage pushing a high enough current to hurt or kill you. 100mA conducted through the body is enough current to kill the average person. When you get zapped by static electricity, there is a potential there of about 13kV....but there is a very small amount of current....that is way you don't get hurt.

Of course, the story was misquoted

[Ctrl-Z]

It doesn't take 3500 volts for a human to feel a shock, naturally. It takes 3500 volts for a human to feel a static discharge, which is what the story asserted.

Re:It's a class B computing device... no big deal

[choco]

You are confusing ESD (electrostatic discharge) with EMC (electromagnetic compatibility). They are very seperate issues.

is a serious problem. It IS possible to design interfaces which offer useful resistance. But it is suprisingly hard to design and build in practice and it causes problems throughout the electronics industry. To build a Port (USB, serial, whatever) which can resist electrostic discharges requires that you use most of the following :

Protective devices which can dissipate the energy. The risetime from Static discharges is very fast and overwhelms all but the best protective devices.

Drivers/receivers which are hardened against static (the major semi manufacturers who do such chips do now make some - but they tend to cost more

Careful mechanical design to further reduce the problem - arrange that the "grounds" always touch first - preferably through a few hundred thousand ohms of resistance.

Optical isolation (although many people fail to understand the limitations of this technique - the stray capacitance between the isolated section and everything else is almost always high enough to allow static damage to happen.

More importantly manufacturers need to test their designs properly using realistic test models. Much equipment - including from the big name manufacturers pays little or no attention to this issue. Presumably for cost reasons - although if the right measures are "designed in" from the start the premium is going to be pretty small. It's interesting to compare the serial interface from a top branded PC with a functionally-identical interface from some serious telecomms kit.

I'm an engineer - not a lawyer. But I do know that I'd hate to have to do the finger pointing in the "Palm v motherboards" issue. If forced to comment I'd say that both sides should share the responsibility.

Static Electricity

[chrysrobyn]

I believe that the poster is unfamiliar with today's microelectronics. Yes, static electricity has gotten a bigger deal as the geometries have gotten smaller. However, do you remember the days of the PC clones with static strips nearby? If you didn't touch the static strip and were walking on carpet, you could fry the entire computer (my friend toasted two motherboards that way, by way of the keyboard). How often do you hear about this kind of thing today? (Expected answer: every now and then) How does this compare to the days that electronics were much less pervasive? (Expected answer: It happens less often)
The reason behind this is that chip manufacturers have been working on modelling the kinds of static electricity that humans produce (human body models) and machines produce (machine models), and designing I/Os to accomodate the new parameters.
Yes, some companies take their chances, ignoring static electricity (and there are some performance benefits to doing that), but these are risks that most of us can weigh.

Re:It's a class B computing device... no big deal

[Matt_Bennett]

I would consider it a componant of the radiation the device emits...


But the device isn't emitting it- the person is the source of the static discharge. The static charge is absoulutely *not* generated by the operation of the device- If it was, then I agree that it would have to be taken into account.

For static discharge, the FCC class rating of the device is a non-issue. It is a concern safety-wise, but the FCC rating is about emissions, not about safety. There are other, separate, certifications about safety.

I've taken part in EMI testing for class B certification- static discharge is not a part of it.

Re:3500 volts for a human to feel a shock?

[cybercuzco]

Ok, they clearly say in the writup that they are referring to *STATIC* electricity. The stuff coming out of your wall is not static electricity. Granted, its made of the same stuff, but wall socket electricity has a much higher amperage than static electricity does. You can shock yourself with 10000V of static electricity just by rubing some slippers across a carpet, but since the amperage is only a few hundred thousandths of an amp the total power is low, and really the total power of a shock is where all the danger is. The warning "Danger, High Voltage" is a misnomer it should really be "Danger, High Power" Electric circuits, however, are much more sensitive to even low power electric shocks, thusly 200V and a few microamps can fry a circuit, while you dont even feel it.

Electrical Misconceptions

[xtal]

Yes and no. It takes about 0.183A (IIRC) to cause your heart to go into an irregular pattern, resulting in a heart attack. Higher current loads through the heart are different; They cause it to stop, and (likely) start beating again. This is the principle used to start your heart again after it's stopped beating.

Much has to do with the resistance in ohms of your skin when you have the electrical shock applied; Are you doing something stupid like working on a grounded metal roof in wet bare feet with power tools (case study in class, that one), etc etc etc.

Many variables are at play here; Power is dangerous and something to be resepected at any level. I zapped myself real good with 25kV once, never again .. :)

Re:Having to deal with this...

[tzanger]

The real problem here is that nearly every integrated mobo has the serial port contained in the northbridge/southbridge chipset, so discharge to the port means discharge to a critical IC in the computer as well.

I doubt that it fried anything bigger than a TTL<-->RS232 converter IC. The reason? The designs of the large ICs don't work well when designing conversion circuitry. Sure, the UART will be part of the chipset but the signals on the I/O will be TTL or CMOS level outputs.

Those signals will then hit an IC like the MAX232 (An RS232 converter IC from Maxim Electronics) which contains the charge pumps and converts that +5/GND (or +3/GND) signal to +12/-12V required to meet the RS232 spec and back. Chipmakers like Maxim also make static-protected versions as well. (In Maxim's case, they usually designate ESD-protected devices with an E suffix.) These chips are good for a 11kV direct zap using the human body model.

No, I don't work for Maxim. Burr-Brown (Now part of TI), National Semiconductor (now spun off to Fairchild) and a host of others make these chips. I'm just most familliar with Maxim's.

If the motherboard fried, they used substandard (IMO) converter ICs. I've hit my laptop and several PCs very hard with ESD and I've yet to have a problem. The biggest problem is that ESD is a slow killer. Rarely does it fry something outright. Usually it just weakens the oxide layer on the semiconductor FETs and causes early death and spurious operation.

Palm should be no more liable for this than every company which manufactures serial, parallel, USB, FireWire and really ANY external device. If the guy bought a shit motherboard, deal with it. It's not Palm's fault.

10000V can wake you up!

[manon]

Things are more complex than just the amount of voltage you get through your body. One can die from 400V and survive from a shock of 10000V. How come? The current is very important and so is how well you are grounded.

(The funny thing is, the 220V (110V US) we use daily is less dangerous than the 24V in your phone when ringing.)

How much current is dangerous? Well, 5 milliamperes can be felt, 10 will be felt and hurts, 15 will really hurt, 30 will freeze you on to the current source. And we are just talking about milliamperes people.

Re:ESD isn't a joke - but everyone thinks it is

[baptiste]

if you connect your strap to a case and DONT plug it in, that is fine. the strap is there to allow you and the case to be at the same potential. As long as this is the case, you will NOT zap anything.

Wrong - when you pick up that motherboard sitting on teh table and zap it (cause you may not be at the same potential) you still hate it. Ground a strap to GROUND is important. Why? Because it disappates the charge from your body. Storing boards in ESD bags? They disappate any stored charge on teh board when you picj it up. Grounding your PC case - again, disappates any charge in the case.

So yes, if you strap yourself to an ungrounded case, you won't zap the case cause you are at the same potential, but you can still zap external compnents you pick up. Yes its rare and something is betetr than nothing - I agree. But for proper protection every thing MUST be grounded. But at the bare minimum, always touch the metal of the case before messing with a PC - that helps but you STILL can zap something in a dry environment.

Re:ESD isn't a joke - but everyone thinks it is

[DataSquid]

Storing boards in ESD bags? They disappate any stored charge on teh board when you picj it up.

Not quite. If I remember my ESD training I got at one place I worked for, most ESD bags shield and don't dissipate.

Testing for ESD

[scharkalvin]

Thirty years ago I worked for Digital Equipment Corp, as a systems testing engineer. Our department did hardware and software testing of all systems configurations that went out the door, I worked on the LSI-11 based stuff. One of the tests we had to do was for ESD, or static discharge as it was there called. The test consited of drawing an arc from any exposed surface of the equipment, to pass it had to withstand an arc of at least 5000 volts (IIRC). The tester was a high voltage power supply that could be adjusted to as high as 15kv (it was limited to a few MA). The probe was an old VTVM high voltage probe with the tip replaced by a metal sphere about 6-8" in diameter. The tester would crank up the voltage and pass the sphere around the outside of the computer to draw an arc from various locations as the computer was running a systems diagnostic. There had to be no systems failures. Usually we didn't fry anything (I can't remember ever destroying anything), but sometimes the diagnostic would fault or the computer would re-boot when the arc was drawn. I wonder if any of todays PC's could pass such a test? (Not running windows!, probably get the BSOD when you draw an arc!).