Cheap protection...

[Ludis Langens]

"Mike (Perth, Western Australia)" wrote:
> 
> >>'Scuse me for butting in, but a CD4049 (Inverting) or CD4050
> >>(Non-inverting) may work better than a 74C14.  The CD's (Powered by
> >>5V) accept 15V at their inputs.
> 
> The only 'slight' problem with that is if the Vcc rail to the device is
> lower then the input spike (or whatever) then it forward biases the onchip
> parasitic diode - depending on the type of CMOS you are using it could
> cause a latchup anywhere on the chip...

Check the 4049 and 4050 specs again - they are designed to handle input
voltages much higher than the supply rail.  These two chips are made
without a diode between the input and the VCC pin.

> If however the input resistor values are high enough then the resulting
> current is lower, less likelihood of latchup. Also best to add a cap from
> cmos device signal input to ground for these reasons:-

GM uses the 4049 in their older ECMs to buffer "switch" type inputs. 
They also use a 100K series resistor, a signal diode to VCC, and a small
filter capacitor (perhaps 1000pF).  The switch inputs also have a 1200
ohm resistor to either ground or the 12V battery voltage.  The 1200 ohm
is before the 100K series resistor.  Look at:
  http://www.cruzers.com/~ludis/1227170sheet3.gif

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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