I'm trying to calculate the duty cycle of a IC 555 as an astable multivibrator. The Ra is 18K ohms, the Rb is 68K ohms, C is 0.04 and I have come up with 0.56 and that is wrong. Any ideas?

 

I'm trying to calculate the duty cycle of a IC 555 as an astable multivibrator. The Ra is 18K ohms, the Rb is 68K ohms, C is 0.04 and I have come up with 0.56 and that is wrong. Any ideas?

What equation are you using? I'd say you're wrong too.

By the way, it's meaningless to say C=0.04. What are the units?

 

Show your work for t1 and t2 ... then calculate the duty cycle

 

Sorry its .04 microfarads, just forgot to add.

 

The capacitor charges with 18k + 68k= 86k.
The capacitor discharges with only 68k.
So calculate the duty cycle.

 

Oh, ok, thanks, I'm coming up with 69.4 now, am I correct, getting closer?

 

Oh, ok, thanks, I'm coming up with 69.4 now, am I correct, getting closer?

Remember what Audioguru said? Discharge time divided by the sum of the charge and discharge times.

\(D = \frac{R_b}{(R_a + R_b) + R_b}\)

 

Finally, thank you mlog! Thank you, thank you, I just was not thinking about the problem in the right way. I tend VERY often to over think the situation, which is a bad trait. I suppose a lot of Techs over think situations though? Thanks again for the help.

 

Remember what Audioguru said? Discharge time divided by the sum of the charge and discharge times.

\(D = \frac{R_b}{(R_a + R_b) + R_b}\)

Page 6 in the datasheet has this equation. Datasheets can be very handy.

 

Thanks, I did not think to check the datasheet, my nose was stuck in my text books. Thanks again for that.