Im desinging a curcuit that calls for oscillation and PWM so i decided to use the LM556, however the frequency (of oscillation) formula(s) i have, have left me scratching my head.
One book says the formula is:
1.44/(Ra+2Rb)/Ct
another book says its:
1.44/(RA+2Rb)Ct
Which is it?
do i divide or multiply
and the the values i use as Ra and Rb, are they in kilo-ohms or ohms?, neighter book gave much detail and my figures dont come any where near the ones in the book, not even if i moved the decimal place.<p>basically i want a pulse of 1Hz,and another curcuit for 50-3000Hz <p>Is there a better formula out there?<p>Thanks in advance to anyone who helps me out.
i need help with a Frequency formula
Re: i need help with a Frequency formula
For Astable operation.<p>Time High<p>Thigh=0.693(R1+R2)C1<p>Time Low<p>Tlow=0.693(R2)C1<p>Period<p>Pt=Thigh+Tlow = 0.693(R1+ 2R2)C1<p>Frequency<p>f=1.44/(R1+ 2R2)C1<p>Duty Cycle<p>D= R2/R1+ (2R2)<p>hope that helps<p>bodgy<p>[ October 13, 2002: Message edited by: bodgy ]</p>
On a clear disk you can seek forever.
Re: i need help with a Frequency formula
For additional clarity: F = 1.44/((Ra+2*Rb)*Ct)
Re: i need help with a Frequency formula
Hello ELM...<p>Probably the reason you see two formulas is because of the
variation in the interpretation of the algebraic order of
operations.<p>The first formula:
1.44/(Ra+2Rb)/Ct
is the most concise form of the two for all one line and
would be taken as:
1.44 divided by (Ra+2Rb)
and then the result of that division gets divided by Ct.<p>The second formula:
1.44/(RA+2Rb)Ct
is not quite as clear as the first, and
would be taken as:
1.44 divided by the product of:
(Ra+2Rb)*Ct
or in another more clear notation:
1.44/((RA+2Rb)*Ct)<p>You will notice that the two formulas yield the same
answer when the above interpretations are applied.<p>This occurs once in a while in various other written
formulas also, but usually the writer has the sense to
make it clear.<p>The reason this happens is because sometimes when going from
true type printed material to a text form which is more
suitable for the computer text files the second part of
the formula is completely shown in the denominator, while
the first part is shown in the numerator, and they appear
in two distinctly separate lines on the page divided by
a graphical line that indicates algebraic division, yet when
typed into a text file the preference is to type on all one
line where some ambiguity in the algebraic order of operations
comes up (particularly in the denominator terms).
This can also happen when a graphical algebraic formula
is transferred to another type of print media, so whenever
you see a formula with the characters on all one line you
have to watch out for this.
One thing that might tip you off is that there is no "*"
multiplication sign in front of the "Ct" in the second form
indicating that it is from a true algebraic grouping of
terms, rather then a 'left to right' ordering of operations.
This again renders both formulas exactly the same.<p>In simpler terms:
1.44 divided by (Ra+2Rb) and then the result
of that division gets divided by Ct.<p>If for example Ra=1000 and Rb=1000 and
Ct=0.1uf, then:<p>1.44/(1000+2000)/0.1E-6 = 4800<p>Good luck with your circuits,
Al
variation in the interpretation of the algebraic order of
operations.<p>The first formula:
1.44/(Ra+2Rb)/Ct
is the most concise form of the two for all one line and
would be taken as:
1.44 divided by (Ra+2Rb)
and then the result of that division gets divided by Ct.<p>The second formula:
1.44/(RA+2Rb)Ct
is not quite as clear as the first, and
would be taken as:
1.44 divided by the product of:
(Ra+2Rb)*Ct
or in another more clear notation:
1.44/((RA+2Rb)*Ct)<p>You will notice that the two formulas yield the same
answer when the above interpretations are applied.<p>This occurs once in a while in various other written
formulas also, but usually the writer has the sense to
make it clear.<p>The reason this happens is because sometimes when going from
true type printed material to a text form which is more
suitable for the computer text files the second part of
the formula is completely shown in the denominator, while
the first part is shown in the numerator, and they appear
in two distinctly separate lines on the page divided by
a graphical line that indicates algebraic division, yet when
typed into a text file the preference is to type on all one
line where some ambiguity in the algebraic order of operations
comes up (particularly in the denominator terms).
This can also happen when a graphical algebraic formula
is transferred to another type of print media, so whenever
you see a formula with the characters on all one line you
have to watch out for this.
One thing that might tip you off is that there is no "*"
multiplication sign in front of the "Ct" in the second form
indicating that it is from a true algebraic grouping of
terms, rather then a 'left to right' ordering of operations.
This again renders both formulas exactly the same.<p>In simpler terms:
1.44 divided by (Ra+2Rb) and then the result
of that division gets divided by Ct.<p>If for example Ra=1000 and Rb=1000 and
Ct=0.1uf, then:<p>1.44/(1000+2000)/0.1E-6 = 4800<p>Good luck with your circuits,
Al
LEDs vs Bulbs, LEDs are winning.
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