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Rescaling resistors on a 555-timers

I'm having an astable 555-timer with R1 = 8k2, R2 = 100, C = 1.5uF.

Using these formulas:

T1 = 0.693 * (R1 + R2) * C [ ms ] T2 = 0.693 * R2 * C [ ms ] F = 1.44 / [ (R1 + 2*R2) * C ] [ kHz ]

for my R1, R2, C, will yield these results:

T1 = 8.62785 (ms) T2 = 0.10395 (ms) F = 0.1142857 (kHz)

So far so good.

The problem is now that I need to lower C to 1uF and still try to retain T1, T2 and F as much as possible. For 100% accuracy, I simply reevaluate the equation and extract.

First R2:

R2 = T2 / (0.693*C) R2 = 150k

then R1:

T1 / (0.693*C) = R1 + R2 R1 = T1 / (0.693*C) - R2 R1 = 12.3M

Verifying with the equation for F:

F = 1.44 / [ (12.3M + 2*150k) * 1u) F = 1.44 / (12.6M * 1u) F = 1.44 / 12.6 F = 0.1142857

This means that the new values for R1 and R2 are correct, since all of the variables T1, T2 and F are correct.

However, a resistor of 12.3 megaohms sounds pretty silly, so going for

100% accuracy on T1 and T2 is probably not a good idea. What I'm doing is not rocket science, but a small pulse generator. The problem is that I don't have any 1.5uF-caps at home, and buying one of them will cost me $12. That's not an option, and that's why I have to rescale the resistors to fit my needs.

As a last resort, I can ofcourse use 1uF in parallell with two 1uF in series to get 1.5 uF total, but I would prefer - if possible - just changing the resistor values.

Unfortunately, this type of equation system is too difficult to me, so I would appreciate a hand.

--
Sincerely,                      |                http://bos.hack.org/cv/
Rikard Bosnjakovic              |         Code chef - will cook for food
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Reply to
Rikard Bosnjakovic
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I think you have the wrong units - should be seconds and Hertz,

There's an easier way to do the math, if you divide the capacitance by 3/2 then multiply the resistance by the same amount.

12M is kind of high to use with a 555 -- hmm somhow you've gone from a reasonable 8K2 upto 12M3 --- there's an error in your arithmetic... All your answers are out by a factor of 1000

also How precise is the capacitor - it's pointless using 1% precision resistors if the capacitor is temperature sensitive and only made to 10% precision.

Ihe 12.3M should be 12.3K use a 12K resistor - that should be close enough, the other resistor 150 ohms - a standard size,

Bye. Jasen

Reply to
Jasen Betts

"Rikard Bosnjakovic" schreef in bericht news:BQtnf.40154$ snipped-for-privacy@newsb.telia.net...

Assuming R2=100k, and your formulas for T1, T2 and F are correct, I achieve the next results:

T1 = 112ms T2 = 104ms f = 4.6Hz

So recheck your calculations.

There's no rocketscience in rescaling the R's. As you divide your capacitor by 1.5, you'll have to multiply the resistors by the same amount. Just substitute in the formulas. So R1=12k3 and R2=150k. Most commonly used resistors have 5% tolerance. Capacitors even more. So R1=12k will be the right choice.

petrus bitbyter

Reply to
petrus bitbyter

Thats why if you need precise values variable resistors are used so that you can tweak the R value to get the precise pulse width you need.

Dan

--Dan Hollands

1120 S Creek Dr Webster NY 14580 585-872-2606 snipped-for-privacy@rochester.rr.com
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Reply to
Dan Hollands

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