Input (protection) FET or npn

Oops there is also 10k ohm to ground on the base/gate of the transistor so that it's off when no input is connected.

GH

Either should work fine. Unless you need the fastest possible 'reset', I'd increase the input resistor value (bipolar) or add a resistor in series with the capacitor to limit surge current at turn-on (t.c.

Why not go directly into the HC14? Then you could use a bigger series resistor, which would protect the resistor, too.

The FET is more robust. The main reason being that in this constellation the BE junction gets hit with the full ESD current if the spike is positive. The abs max base current peak for the 2N4401 is only 200mA.

The upper diode has no effect with the BJT in this scenario. Before that comes the BE junction has gone phut.

Hanging a 0.1uF emitter to ground is not so nice, high peak current. Won't be destructive but it isn't considered very kosher. With a FET it can be ok if the Rdson is guaranteed high enough so the abs max current won't be exceeded. Else use a small resistor in series with the drain.

I've got to have this long input pulse to make sure things have enough time to sync-up. (a very slow clock.)

George H.

OK more base resistance then as suggested above.

OK, so the FET with 10-20 ohms in the drain line.

George H.

Depends on the ESD rating you need and small resistors can be "jumped" by an ESD pulse. In the end it all depends on where this is used. I've had cases where ESD on outdoor equipment was extreme and caused a lot of grief until we fixed all areas where it could come in. For example, sneakers plus resin board decking (the artificial wood kind) plus a long enough walking distance are a recipe for an impressive zap.

Sure you can. Keep in mind for your timing calculation that the gate capacitance is subject to process tolerances and with this FET can be anywhere between 10pF and 50pF.

What is the nature of the input pulse? 5V, 50V, 500V, etc? Energy capability? 1us, 10us, 1000us, etc?

Caution: If the input pulse is very energetic and of sufficient voltage/energy, it could raise your 5V rail to device destruction.

Well, I get to spec the input pulse. 5V logic, ~10 us minimum pulse width. But this goes to students and I have to assume almost anything that can come out of a function generator might be put into it. ~+/- 20 V. And then ESD is always possible. I'll have a through hole FET, (or npn) so at some point the FET becomes a fuse... for those extrally energetic students.

George H.

I was designing electronics for one of the institutes at my university to up my student's budget a bit. On one of the first projects I asked "Is this a one-off for one of our labs or small series?" ... "Oh, yes, small series so it needs to be Ph.D.-proof". Then I knew what to do 8-)

You might want to place a big TVS across the supply and some sort of fuse, in case someone reverses polarity or turned the power supply on and the guy before had it on 24V. That could blow the lid off of the

Maybe use one of the gate-protected 2N7002s, and a biggish series gate resistor, and maybe a cap to ground for luck.