At 07:53 AM 12/31/2010, you wrote:
>Richard Prosser wrote:
> > If you have a number of inputs to be protected, then it might be
> > worthwhile setting up a secondary rail, clamped at ~0.7V below you
> > supply line.
>
>If you're going to do that, you might as well use transistors instead of
>diodes.  That way you won't be loading your Vdd-700mV rail much.
>
>Actually you wouldn't want 700mV offset, since then the clamp could easily
>be conducting somewhat within the valid range.  400-500mV seems like a
>better level.
>
>To do this with a transistor, connect the base to the 500mV offset rail,
>emitter to the signal you want to clamp, and collector to the opposite pow=
er
>rail.  For high side clamps, use PNP, NPN for low side clamps.  See
>attached.

For more protection, add to Olin's clamp a back-to-back pair of 500V
depletion-mode MOSFETs in series with "SIGNAL".  Such a circuit can
handle 277VAC connected directly to the input.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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