John Larkin said:
On Wed, 19 Oct 2005 01:09:48 GMT, "Harry Dellamano"
On Tue, 18 Oct 2005 15:17:08 GMT, "Harry Dellamano"
.
.
.
.
. VCC
. |
. +-------+
. | |
. | - 74HC14
. [R1] ^
. | | G1
. | | |\
. +-[R2]--+----+---| o---+---->
. | | | |/ |
. o | | |
. -| - === |
. o ^ |C G2 |
. | | | /| |
. +-------+ +---o |---+
. | \|
. ---
. ///
.
Now when you have a switch closure that transitions G1 input to VT- ( in
a
time >=Tmax), G2 reinforces this in a time 2xTpd very much less than any
possibility of an (R1+R2)C pull-up through VH, so that G1 input is
pulled
towards (VT-)-5V, clamped by the diode at -Vf, and the capacitor
discharges to 0V top-to-bottom through the diode->G2 output->GND->diode
loop and the position of the switch is of no consequence until G1 input
reaches 0V, at which point it assume control of the circuit state. You
have a similar action upon switch release and G1 input exceeds VT+. The
G1
input waveform would look something like this:
View in a fixed-width font such as Courier.
.
.
. +5.7V -----
. | \
. 5V | | 5V
. \ 20ms VT+ | ------
. \ /
. \ /
. \ /
. \ VT- /
. | / 20ms
. | 0V /
. | ---------
. | |
. | /
. -0.7V -----
.
.
Hey F B, you are getting close to something really good!
Now parallel C with an R3 to set any hysterias desired. Used this many
times, works like a champ.
Regards,
Harry
I suppose that, if parts are good, then more parts must be better.
John
Ok John,
If your switch closure is on the front wheel of a AB360 (Airbus) with 100s
feet of wiring thru power cables with 10KV transients then a switch receiver
with 1.0V of forward hysterias and +/-5V of transient hysterics and the
flying cap acting as a low pass filter when the output is in the high or low
states requires some parts. I have not seen a switch receiver more tolerant
of noise.
What say you JL??
Regards,
Harry
John, John you are killing me and making me look bad to FB.
Let me try to answer these probing questions.
What sort of response time do you need on a wheel switch?
<100mS is ok.
With 10KV transients (where do you get them on a plane?) I'd be more
worried about the resistors, R1 and R2, arcing over. That's an
ESD-type issue, distinct from denouncing.
True but input is transzorbed to protect resistors.
Well, that wasn't shown.
C is not ungrounded it is tied to AC ground thru G2 < 50 output R and forms a low pass input filter with R2. The low output R of G2 is vital to circuit operation.
Neither end of C is connected to ground; both ends are connected to
semiconductors, and semiconductors are what's killed by esd zaps. 50
ohms won't impress a good arc, or G2 can be the IC that's killed. Of
course, the transzorbs fix that problem.
One of my pretty firm rules is to never have semiconductors connected
to the outside world, arcs included.
Good point, the circuit shown is only conceptual. There is a 510R between G1's input and the C,R2 node.
This schematic seems to have all sorts of invisible parts. Good, that
resistor definitely shifts the damage path toward G2.
These ICs are rated +/-20ma input latching current so the two clamp diodes are not needed with large values >of R2
Guaranteed, Let's calculate the voltage change necessary at the input to cause a miss trigger to propergate from G1 to G2.
Ein = Ehy*R2*C/Tpd= (1.0*1e5*1e-7)/1e-8 = 1,000,000 volts
This is a ROM and I did take liberties but you get the idea. Actually because of G2's output 50R and R2=1e5 we only need a 2000V input change to cross the 1V hytersis threshold.
Again, assuming R2 doesn't arc.
That is exactly what my customer is talking about.
Well, if you analyze it carefully enough it should be OK. But simpler
circuits have less hazards. Without the transzorbs, this one would be
scairy.
John
