PCB Heaven
General Category => Analog discussion => Topic started by: thanosgen on June 10, 2009, 17:06:01 PM

hi i want to ask something.why in this page http://pcbheaven.com/wikipages/555_Theory/ (http://pcbheaven.com/wikipages/555_Theory/) at 6.1 the type is
Fot your calculations, take into account that:
THIGH = 0.67 x R1 x C (result in Seconds)
and
TLOW = 0.67 x R2 x C (result in Seconds)
i found 0.62 and no 0.67.can you help me how you fould this number?

Well, take for example those two:
http://www.datasheetcatalog.org/datasheet/philips/NE_SA_SE555_C_2.pdf (http://www.datasheetcatalog.org/datasheet/philips/NE_SA_SE555_C_2.pdf)
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2182.pdf (http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2182.pdf)
The first one is from fairchild that gives time constant 1.44 (1/1.44 = 0.694)
The second one is from philips, giving a time constant 1.49 (1/1.49 = 0.671)
you've got the point

i'm sorry but i did find anything in these datasheets.this circuit with diode doesn't include in pdfs.the type of duty cycle 50% is without diode.can you tell me how i find this mathematic?whitc type i start the mathematics to find the answer tto this type Thigh=0.67*R1*C ???

It's in the datasheets indeed. But you will find the forms for frequency. For example:
F = 1.49 / (Ra + Rb) x C
If you want to find the period, then you must invert the frequency. That should be 1/F. To be faster, we invert the 1.49 also, and that makes 1/1.49 = 0.67. That is how this strange number comes.
Also keep in mind that this formulas are usually very inaccurate. You should measure the output signal with a frequency meter or an oscilloscope if you want better accuracy. And also do not forget that the 555 is a nice timer chip but very very unstable in terms of frequency. It changes frequency when voltage changes, when temperature changes, when other fields surrounds it etc etc etc. If you want accuracy, you need another circuit.
You should read the 555 theory (http://pcbheaven.com/wikipages/555_Theory/) that i have written. It explains the operation easy to understand.