Capacitor reset with FET/analogue switch

Rick Jansen sscprick at
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
current-limiting resistor.

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 Jansen
rick at
S&H's a module and s&h's looking good

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