I blew out the only 74HC (74HC04 NAND) part in my junk box, so I cut my
charge pump circuit down to this:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
_ _
Buffer Card Output _| |_| |_ Charge pump in servo thread = 1kHz 0-5V
---------+
|
=== C1 .01uF
| O'Scope
+-|>--+--|>--+-----+-----------
| | |
| C2 === \
| .01uF | / R1 15k
| | \
V (gnd) V V
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I played with different caps I had on hand and got the best results
with .01 uF for both C1 and C2 and 15k for R1. Even with R1 removed the
output would decay pretty rapidly, so I had to bump C1 up enough to
offset the leakage. Anything larger for C2 didn't seem to fully charge.
Then I sized R1 to get a sub 1 second drop, so would probably trip the
output at 200 to 300ms after losing the input signal. I suppose a CMOS
input and the scope should be a similar load, so the timing should not
change much if I add a Schmidt buffer to the output?
I also wired this up:
+--------- O'Scope
_ _ |
_| |_| | | C1 .01uF
-----------\/\/-----+---||--+
R1 15k |
V (gnd)
If T=RC, T=15000x.00001=.15 ?
On the scope, I got a shark fin like signal where most of the signal
change occurred by 200ms, so I guess this confirms the .15 for T above?
I didn't quite understand what I read on Wikipedia -- something like RC
is the value where the signal change is at around 63% of the signal
swing? I suppose I could use this circuit to measure caps with C=T/R
with t...@63% of signal? I'll have to play with this some more.
--
Kirk Wallace
http://www.wallacecompany.com/machine_shop/
http://www.wallacecompany.com/E45/index.html
California, USA
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