Input (protection) FET or npn

Hi all a simple input circuit take a input pulse. And stretches it a bit. (from previous post on pulse stretching.) The input looks like this.. (bad asci art)

+5V +--------+ | R(1M) _ ---+ ^ | | | |/c +--->'HC14 pulse in---R(100)-+---| | | |\e C(.1u) _ | | ^ GND+ | GND

100 ohms in series and diodes to the rails. I can then either drive a FET (2n7000) or npn (2n4401) both work fine. The exact width of the output pulse is not critical. Which is more robust?

George H.

Reply to
George Herold
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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

Reply to
George Herold

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.

Reply to
Frank Miles

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

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

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.

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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.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Reply to
George Herold

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.

Reply to
George Herold

OK more base resistance then as suggested above.

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

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(5v/500mA) Is there any reason not to increase the gate resistance too? (50 pF Cin.... ~50 ns TC...)

George H.

Reply to
George Herold

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.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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.

Reply to
John S

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.

Reply to
George Herold

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

74HC14. I try to use CD4000 logic wherever possible because a TVS isn't a very exact spec part.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

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