Capacitor reset with FET/analogue switch
sscprick at horus.sara.nl
Sat Mar 16 16:39:12 CET 1996
In sawtooth VCO designs usually a capacitor is charged
with a current, and reset with an analogue switch of
some kind (an old discussion ;-).
I still haven't figured out all the ins and outs of the
reset process, particularly wether a FET or MOS-switch
in the form of a HC4066 ic is suitable or not. Theoretically
ir is not.
If you consider a capacitor charged to 5V and an analogue
switch with an Rds(on) of 25 ohms you can expect an initial
current of 200mA. A normal FET like a 2N4391 or a switch ic
like the 4066 normally cannot handle this current at all.
Still I've seen designs proposed where they are used without
So, what's the truth behind this? Can you get away with
using simple FETs or a 4066 as a reset switch? Do they
behave like a LED current-wise? A LED is rated ca 20mA,
but they can withstand peak-currents of up to 1A for very
short periods. Same for FETs?
rick at sara.nl http://www.sara.nl/Rick.Jansen
S&H's a module and s&h's looking good
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