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Let's say you've got a standard astable multivibrator (AMV) 555 circuit that includes all the wiring, two resistors (R1, R2) and a capacitor (C1). I wonder what is the purpose of the capacitor and how does it affect the frequency?

For some capacitors the frequency is not "clean" - it fluctuates and for some it's very stable, why is that so?

PeterJ
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roman
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2 Answers2

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You should provide a link to the circuit diagram. The component names may vary.

Assume R1 is the charge resistor and R2 is the discharge resistor.

A capacitor is like a water reservoir. Current flows into it and it fills - but with charge rather than water. As the current flows in the "head" increases - in this case the measure of fullness is the capacitor voltage. So

Current flows into the capacitor via R1 and R2 in series. The voltage on the capacitor rises. The two resistors and the capacitor between them set how long it takes the capacitor to fill so they affect that part of the oscillator cycle.

At a certain voltage level the 555 "decides" there is enough voltage on the cap so it turns on a discharge transistor inside it. The capacitor now discharges via R2. The cap and R set the time taken for the discharge part of the oscillator cycle.

So the capacitor slows down the rate of rise and the rate of fall of voltage. Without the capacitor the circuit would try to oscillate very fast and would not be controllable or predictable.

With a capacitor the time to charge is ABOUT t= (R1+R2) x C.
The time to discharge is ABOUT t = R2 x C.

Looks for circuit - oh good, R names are the same as I used ;-)

enter image description here

NE555 data sheet here

If undamaged and correct polarity caps are used the oscillation should be stable as long as the data sheet limits are met. It MAY be that some capacitor values are outside the allowable range.

"Charge flow"

While it may be stated with some justification that charge does not flow into a capacitor but rather that it flows from one "side" to the other, the statement is confusing as all get out for beginners and is not in fact strictly correct either. And in some cases is almost completely incorrect. Almost.

ie - the claim that capacitors do not fill with charge should only be made in pubs or closed meetings of the verbal pedants society OR decent references to the overall relevant material should be given.

It will confuse beginners immensely to make such statements in isolation.
Consider eg the classic V=q/C and how it is usually used and explained.
Consider mutual capacitance and the fact that an isolated sphere of Radius R has C=kR and that charge does indeed flow into it to charge it
. (The earth has a capacitance of about 700 microFarad).
And more.

An extremely good treatment is given by the Wikipedia Capacitance entry. Understand half of what's there and you are well on the way to starting to start to become a Grand Master [tm].

Russell McMahon
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  • The voltage over the capacitor oscillates between 1/3 and 2/3 of the supply voltage (there's an internal voltage divider that feeds two comparators). – starblue Sep 26 '11 at 16:15
  • @starblue - ? & yes. That's why I said that charge and discharge times are about the same as the time constants involved. ie the cap is swinging across a range of about 1/3 of supply so using t=RC is in the order of correct. – Russell McMahon Sep 26 '11 at 16:44
  • Capacitors don't "fill with charge". A charged capacitor has the same net charge as an uncharged capacitor. – endolith Sep 26 '11 at 17:18
  • @endolith: So is it incorrect to talk about charging a battery, since a dead battery and a charged battery have the same net charge? – David Schwartz Sep 26 '11 at 17:47
  • @Russell - great answer. really helped :)) many many thanks!! – roman Sep 26 '11 at 18:10
  • I'd also would like to understand why for some values the frequency would oscillate between several values (mostly 2-3 for most capacitors I used and for some A LOT of values) – roman Sep 26 '11 at 18:12
  • @DavidSchwartz: No. "Charge" as a verb has an entirely different meaning from "electric charge". You can charge a fuel tank or a water bucket, but that doesn't involve any change in electric charge either. http://electronics.stackexchange.com/q/14462/142 The phrase used here was "it fills with charge", which is wrong. Electric charge is *removed* from one side while being added to the other. http://amasci.com/emotor/cap1.html – endolith Sep 26 '11 at 19:21
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    endolith: So it would be wrong to say that a charged battery had a charge? Seriously?! – David Schwartz Sep 26 '11 at 19:37
  • @endolith et al - see addition to my answer re "charge" – Russell McMahon Sep 27 '11 at 04:59
  • @DavidSchwartz: A battery could be charged and have a net charge, or discharged and have a net charge, or charged and be neutrally charged, or discharged and neutrally charged. I didn't invent the damn word. – endolith Sep 27 '11 at 05:39
  • @RussellMcMahon: Charge *does* flow into a capacitor. It just doesn't "fill" the capacitor, since it's flowing out the other side at the same rate. Don't ever say that a capacitor fills with charge, especially to a beginner. – endolith Sep 27 '11 at 05:42
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    Sorry, that's just absolute nitpicky silliness. The capacitor fills with charge as each plate acquires a charge relative to the other one. The accumulated charge results in a growing electric field. – David Schwartz Sep 27 '11 at 05:51
  • @endolith - If you wrote your last response after I edited my post I suggest you read my post again. And again and again if necessary.eg A waterslide has a top holding tank and water is pumped from the pool to the tank and then discharged down the slide as required. Water (charge) does NOT flow into the waterslide considered as a whole - it just gets pumped from one "side" to the other./ If we consider the top tank or a capacitor positive plate with the -ve plate **held referenced to zero = to ground** the similarities should give cause for thought. If not please read my answer ... – Russell McMahon Sep 27 '11 at 06:52
  • "**ALL** models are wrong. Some models are useful" ~~= George Box. Water models are often good for visualisation but never provide the whole story. The originally cited declaiming webpage and my waterslide model are both flawed but help understand what's happening. Charge pumped around the capacitor loop adds energy to the storage system. Ground referencing some point in the "model" as a reference is just as valid as considering the capacitor "balanced". The "charge does not increase perspective has plate voltages balanced wrt gnd - which we conveniently ignore when desired." – Russell McMahon Sep 27 '11 at 07:21
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First note that it does not help to refer to R1, R2, C1, without reference a circuit. The standard 555 circuit has 2 capacitors, I guess you are refrerring to the one at pin 6. The other one (at pin 5) is for stability, it has nothing to do with the frequency.

If the datasheet does not say it all, this chip is so famous it has a wiki page all for itself! http://en.wikipedia.org/wiki/555_timer_IC states: In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C. Cut-n-paste of the formula does not work, but you can check it here (or in the datasheet).

Wouter van Ooijen
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  • Thanks for the response - but you didn't answer what is it used for on the electrical level. I know it's used for the frequency setting - but how does it really effect the frequency and why it's unstable for some values? Thanks :) (and yes, it's the basic astable circuit without the optional capacitor) – roman Sep 26 '11 at 18:08
  • http://www.fairchildsemi.com/ds/LM/LM555.pdf p6: the capacitor is charged through the two resistors. For a higher value capacitor, it will take longer for the voltage on the capacitor to reach the trigger level. – Wouter van Ooijen Sep 26 '11 at 21:15