At 03:05 AM 1/1/2011, you wrote:
>On 12/31/2010 7:39 AM, Spehro Pefhany wrote:
>
>  > 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.
>
>I'm starting to like this protection circuit....
>
>But, can you clarify which back to back arrangement we're talking
>about?  Probably because my brain is so polluted with Enhancement fets...
>
>And, I'm assuming you mean N-channel Depletion, since it seems P-channel
>depletion isn't very common...

Either could be used, but as you say, n-channel is easier to get.

Tie both gates together with both sources (all four pins together). Take
in and out from the drains (either way, it's symmetrical). The circuit
will current-limit at Idss (which is just a couple mA for something like
the LND150) up to the breakdown voltage, but it looks like a couple thousan=
d
ohms at low currents.

http://www.supertex.com/pdf/datasheets/LND150.pdf

>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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