Calculating duty cycle? - Page 1
Forum section: electronics / beginners

author: etstudent
author_rank: Regular Contributor
timestamp: October 12, 2012, 07:27:23 pm
content: I'm not coming up with the right answer here o a question in my book, and it seems easy. What am i missing here?Question is this. If Ra is 18K ohms, Rb is 68K ohms and the C, capacitor is 0.04 microfarads, calculate D, or Duty cycle. I am coming up with 0.56 but that is wrong according to my instructor. Any help? Any suggestions? The other choices are 0.44, 0.79, and 0.92 if that helps any. Thanks for any help.

author: Astroplio
author_rank: Regular Contributor
timestamp: October 12, 2012, 08:07:07 pm
content: Is this 555 related?

author: lavo-1
author_rank: Regular Contributor
timestamp: October 12, 2012, 08:08:55 pm
content: I,m with you as I get 0.558 as R1+R2/R1+2R2 = Tm/Tm+TsI have been wrong before though!

author: etstudent
author_rank: Regular Contributor
timestamp: October 12, 2012, 08:26:02 pm
content: I apologize, yes its a 555 timer used as a astable multivibrator. I knew I forgot one piece of the puzzle, sorry.

author: Noize
author_rank: Regular Contributor
timestamp: October 12, 2012, 08:55:36 pm
content: I got 0.558 as well.

author: etstudent
author_rank: Regular Contributor
timestamp: October 12, 2012, 09:18:04 pm
content: I don't get it, because I came up with the same thing as you .558, so what is the deal. Does it have to do with the 555 astable multivibrator operation?

author: Noize
author_rank: Regular Contributor
timestamp: October 12, 2012, 10:25:08 pm
content: From the formula you get values from 0 to 1. But duty cycle is expressed as a percentage so maybe 55.8% is the correct answer?

author: etstudent
author_rank: Regular Contributor
timestamp: October 12, 2012, 10:28:45 pm
content: I'm not sure, but that is not the correct answer. Oh well maybe someone will chime in with a new idea.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 12, 2012, 11:11:35 pm
content: I'm quite sure that 0,558 or 55.8% is the correct answer to YOUR posted question, but I would like to see a copy/paste duplicated of your test question word for word, as it may be you have not understood the questions true meaning. This is not a pop at you, just that I am sure we would all admit to having read a question in a paper and misinterpreted its true meaning.

author: etstudent
author_rank: Regular Contributor
timestamp: October 12, 2012, 11:52:38 pm
content: Thanks, in my first post I copied the question work for word. Is it that you have to combine Ra and Rb first then that number with Rb which is 68K? That might be the issue, I'm getting 69.4 in that case. I'm not home, so I don't have my text books, let you know later. Again thank you for the help.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 13, 2012, 12:54:37 am
content: Well this has proved to be a good question as I have found two differing calculations for Duty cycle in a 555 astable circuit. I have used the formula I quoted earlier as R1+R2/R1+2R2, but I had a look online and found it also calculated as R2/R1+2R2,so this, using your values gave 0.44./This answer however is too low a duty cycle to be possible from the two resistor and capacitor circuit you have quoted, not without the help od a diode across R2.I did however find a online 555 calculator(cheating I know) which gave the answer of 0.558http://www.horrorseek.com/home/halloween/wolfstone/TechBase/com555_555TimerCalc.htmlSo I am waiting for a much brighter person than me to confirm.

author: twbranch
author_rank: Contributor
timestamp: October 13, 2012, 01:08:53 am
content: I think maybe the instructor is asking for something else. I get 56%!

author: etstudent
author_rank: Regular Contributor
timestamp: October 13, 2012, 01:14:35 am
content: Wow this turned out to be quite the tough question. I will say this 0.44 is a choice. so maybe your onto something. Thanks for the help again.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 13, 2012, 01:36:31 am
content: I dug the R2/R1+2R2 formula from the data sheet for the 555 chip from Texas Instruments, but I am sure this formula only applies to duty cycles above 50%. This answer of 0.44 or 44% may be numerically correct, it is however an impractical circuit due to charging on the capacitor through R1 and R2 and then discharging through R2. I am going to try this circuit in Spice tomorrow on my desktop and see what I get.

author: Noize
author_rank: Regular Contributor
timestamp: October 13, 2012, 01:53:45 am
content: Referring to page 11 of the NE555 datasheethttp://www.ti.com/lit/ds/symlink/ne555.pdfOutput waveform duty cycle = 0.558Output driver duty cycle = 0.441

author: Noize
author_rank: Regular Contributor
timestamp: October 13, 2012, 01:56:05 am
content: So when you next see your instructor ask is it output driver duty cycle or output waveform duty cycle?Also print out page 11 from the data sheet make it into a paper aeroplane and throw it at him.

author: etstudent
author_rank: Regular Contributor
timestamp: October 13, 2012, 02:13:20 am
content: I already sent him off an email, but he won't get it till Monday obviously. I think you are onto something, with output waveform duty cycle. Thank you again for the help.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 13, 2012, 04:06:30 am
content: I have just run a LtSpice simulation of an Astable 555 circuit with your values and the duty cycle is indeed 55.8%. This I think proves that the original answer you got was correct. Now if your Tutor was after Driver duty cycle (which I have never had the need to measure) then it should have really been specifically in the question.

author: etstudent
author_rank: Regular Contributor
timestamp: October 13, 2012, 12:06:34 pm
content: Thank you, it was indeed 44.1 which means it was the Driver duty cycle. Confusing question, but we got it. Can you show me, or tell me how you got to that?

author: Noize
author_rank: Regular Contributor
timestamp: October 13, 2012, 06:06:55 pm
content: Quote from: etstudent on October 13, 2012, 12:06:34 pmLook at figure 12 and the formula on top of page 11 in the datasheet for the 555 timer!http://www.ti.com/lit/ds/symlink/ne555.pdf

author: etstudent
author_rank: Regular Contributor
timestamp: October 13, 2012, 07:51:06 pm
content: Thank you, got it now. Now that we solved that, is that not a odd question? Is that formula used much in the field? Thanks again for the help, appreciate it much.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 13, 2012, 08:23:29 pm
content: Not sure what output driver duty cycle is to be truly honest. Duty cycle to me is the percentage of a waveform that is at its highest or on state. This Driver duty cycle seems to equate to the percentage of the wave form that is at its lowest of off state. As I tried this circuit example in LtSpice and used the simulated oscilloscope to measure the duty cycle as I would on my DSO, I am more than certain that the true duty cycle is 55.8%. If someone could clarify the meaning and use of Output Driver Duty Cycle, I would be very interested.

author: etstudent
author_rank: Regular Contributor
timestamp: October 14, 2012, 05:07:11 pm
content: Well we finally got it, so that is all that matters. Onto a new text book, and some more experiments. Thanks for all the help.