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From: Spehro Pefhany
Subject: Re: how to reduce noise from 555 timer...???
X-Newsreader: Forte Agent 1.92/32.572
Date: Fri, 20 Dec 2002 22:13:31 GMT
NNTP-Posting-Date: Fri, 20 Dec 2002 17:13:31 EST
On Fri, 20 Dec 2002 21:49:18 GMT, the renowned Kevin McMurtrie
>Find a ~100uF capacitor that hasn't been powered up in a while. Try to
>charge it through a 1M resistor. It won't charge or it will hesitate at
>certain voltages. Charge up the capacitor with a higher current and
>leave it charged for a while. Now discharge and repeat the 1M test.
>Now it charges smoothly.
Time constant of 100 seconds, this application has a time constant
of only 0.3 seconds. I agree that aluminum e-caps must be used with
caution in long time-constant applications. We've used plenty
with time constants in the 10-20 minute range with no problems.
These days it's done digitally for a variety of reasons.
>I found a reference to it:
>They're fine for many audio applications but they will not operate an
>oscillator at a constant frequency, especially if the current is low.
>Add to that the temperature and age drifts and you definitely have the
>wrong component for a precision timer.
Who said it was a precision timer? It's an oscillator in a precision
instrument, the exact frequency is probably not important, or he
wouldn't be using a 555.
The options, staying with analog circuitry, are not so attractive if
you have to go to extremely high impedance circuits to get around
using electrolytics. The circuit may be unreliable for 20 years
instead of failing finally after 20 years of reliable operation.
"it's the network..." "The Journey is the reward"
firstname.lastname@example.org Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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