Re: CD4060 oscillator, max resistor value

On Fri, 03 Aug 2007 13:32:15 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Hello Folks,

Got a design here that contains a few 4060 long timers with RC
oscillators. Battery operated so consumption counts. Some datasheets
state that the maximum value for any resistor in that area is 1M. Since
Rs (feedback resistor) must be at least twice Rt (timing resistor) this
leads to rather lowish timing resistor values. Since this is a logic
gate oscillator that requirement causes quite some current at voltages
around 10V.

I built a number ( 35-ish) of long-duration timers (20-24 hours) using
the Philips 4541, which has a very similar oscillator. After mulling
over the exact problem in the data*** you note, I did some
experiments with both Rs and Rt at 1M and Ct at about 1.4uF. (Actually
2 x Siemens poly 0.68 in parallel) It worked fine, and never caused
any problems to my knowledge in the 10+ years they have been in
service. I did note the time duration shifted by a little more than
the specs suggest over the range 0-50 Deg C, but precision was not
called for in the application.

Barry
.

Quantcast