Jason White wrote:
> Hello, I was wondering how robust the bases of "jellybean" BJT
> transistors (such as the 2N3904) are against ESD. I have an
> application where I need to protect inputs from a variety of
> conditions, including ESD but space (and oddly enough, the number of
> solder-joints) is at premium.
>
> I have attached a simple schematic showing the type of setup I am
> considering. Should a large ESD spike be applied the the base, what
> would likely happen? Is Q1 likely to blow out and pass the spike to
> the microcontroller?
>  =20
I'm not an ESD expert but based on rudimentary circuit analysis I think=20
that circuit would be pretty tolerant of positive spikes but negative=20
spikes would be a much bigger problem.

With a positive spike the BE junction would be forward biased and I=20
would expect it to take the bulk of the current. Even with a 15KV spike=20
the resistor should limit the current to 150ma which I would expect to=20
be below the levels where it would blow anything up.

However with a negative spike the BE and BC junctions would be both=20
reverse biased. Where the spike went would then depend on which junction=20
broke down first if the BE junction broke down first then the current=20
would  flow from ground, but if the BC junction broke down first then=20
the current would be pulled from the microcontroller input pin via the=20
microcontrollers ESD protection diodes.

Adding a diode in inverse paralell with the base-emmitter junction would=20
probablly make the circuit a lot more tolerant of ESD.
> If anybody has any advice, I would greatly appreciate it
>
> Thanks,
> Jason White
>
> For those who do not wish to view the schematic, it is as follows:
> An ESD prone input is connected through the 100k resistor R2 to the
> base of NPN type BJT transistor Q1. The collector of Q1 is connected
> to +5V through a 10k resistor R1 while the emitter is connected to
> ground. The output of this buffer (the collector of Q1) is connected
> directly to the microcontroller.
>  =20
>
> ------------------------------------------------------------------------
>

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.