# Time Constants

Discussion in 'Homework Help' started by Steve1992, Aug 15, 2006.

1. ### Steve1992 Thread Starter Senior Member

Apr 7, 2006
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I know 1RC is when the voltage across the capacitor has reached approx. 63% of Vs.

In multivibrator transistor circuits I see the time constant multiplied by 0.7.
I know 0.7V is the switching on voltage for a BJT.

Is this why 0.7 is used?
But Ive seen CMOS ics circuits use 0.7 in multivibrator/timing circuits
CMOS use FETS not BJT.

2. ### hgmjr Moderator

Jan 28, 2005
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In the case of the 74HC123 for example, each manufacturer of the multivibrator IC (A.K.A. one-shot) establishes the constant that is used to calculate the width of the output pulse of their one-shot based on the internal design of the comparator circuit. I did a quick search and found that the 74HC123 made by Texas Instruments uses 0.45 while ST Microelectronics uses a multiplier of 1.

This variation in the multiplier across different IC manufacturers has made it very tough to design with this device with any reliance on the consistency of the pulse-width for a given RC component combination. In fact the company for whom I work has placed a ban on the use of one-shots in our design for this among other reasons.

All this said, I believe one-shots do have important applications where the width of the pulse can vary widely without any adverse impact on the circuit's perfromance.

hgmjr

3. ### Papabravo Expert

Feb 24, 2006
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In 36 years of electronic design I have NEVER seen a good use for a one-shot. Just one man's opinion for what it is worth.

4. ### kinyo Member

Jun 6, 2005
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The 0.7 multiplier is actually 0.693 for the two-transistor multivibrator, the natural log of 2 (the capacitor charging just about halfway when switching occurs). It has nothing to do with 0.7V for a BJT.

And I've seen a 555 one-shot used in a reset circuit for a microprocessor.