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Re: [TowerTalk] OT: USB ESD protection

To: Neal Campbell <>
Subject: Re: [TowerTalk] OT: USB ESD protection
From: Jim Lux <>
Date: Mon, 12 May 2008 09:25:58 -0700
List-post: <">>
Neal Campbell wrote:
> Hi all
> We had a lightning strike occur about 1000 yards from our house which
> luckily did not affect any of the very expensive shack toys but almost
> everything in the house connected via a USB cable got fried (both USB
> ports on each end of the cable). No computers were blown (other than
> the USB ports on them and a firewire card that had similar reactions).
> Our laser printer's USB port was ruined but it also has a RJ45 lan
> connection which works just fine.
> Oddly enough, no phones, cable TV, stereo or anything connected to the
> house electrical system were affected, just USB and firewire connected
> devices.
> Does anyone know of ESD protection for USB/Firewire connected devices?


Ethernet interfaces have galvanic isolation with several kilovolt 
isolation, which helps a lot. Since the cabling is twisted pair, 
ESD/external transients tend to be common mode, not differential.

USB typically relies on series resistor/diode clamps to the rail sort of 
strategies to take an ESD hit. Ditto for 1394.  Both are vulnerable to 
common mode voltage swings if they're big.  Since most USB applications 
are cost sensitive, they design it to take the nominal discharge in the 
spec (a few pF charged to a few kV) and not a microjoule more.

As a practical matter, it is very tough to design high speed 
interconnects that are inexpensive and can be esd protected.  The 
typical strategies of diode clamps/transient suppressors, etc, all add 
significant amounts of C to ground, which causes problems with the 

There are some parts from Analog Devices and TI that provide significant 
galvanic isolation AND high speeds (we use them in safety critical 
interfaces at work.. say connecting test equipment to spacecraft), but 
they're hardly a panacea.

You're sort of stuck.. the best bet is to make sure all the USB/1394 
equipment is physically close to each other, and the chassis are all 
bonded together, so common voltages are small.  Or, use an interconnect 
that can tolerate lots of common mode voltage, e.g. ethernet, MIL-1553, 
serial fiber links, RS422/232 (for which the usual drivers are pretty 
tough, since they have to source/sink 30mA at +/-15V, and they're slow, 
so parasitic C from big clamps isn't as big an issue)

People DO sell surge suppressors for USB, but I'd be interested to know 
how well they actually work.  The USB ones talk about USB1.1 only 
(because it's SLOW).  For instance, the one from B&B has 5.1V clamps on 
the signal lines at 6.8V on the power.  I'd be willing to bet a few 
dollars that what's inside is some zener diodes.

Jim, W6RMK


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