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555 oscillator queer behavour

Discussion in 'Electronic Basics' started by panos v, Dec 11, 2004.

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  1. panos v

    panos v Guest

    I have a question concerning the function of the 555 oscillator in
    astable mode.
    In particular the external elements i have connected to the 555 are
    those depicted at the
    datasheets of all the 555 ics (LM555, NE555 etc). The 2 resistors are above
    1K Ohm (Ra=1K & Rb=6.8K),
    the C capacitor's value is greater than 0,0005uF (C=0,001uF) whereas a
    0,1uF capacitor is connected
    between the "Control Voltage" pin and the ground.
    The above connection with the aforementioned values for the external
    elements adjust the 555 so that
    it theoretically produces a frequency of 98KHz, more or less. The problems
    lies in the fact that the
    experimental frequency, produced by the ic, is lower than the theoretical
    by around 30KHz.
    The discrepancy of theoretical and experimental value appears also in other
    "valid-practical" values of
    Ra,Rb & C with different deviations. I have also tried the LinCMOS version
    of 555 (TLC555) with the same results.
    Another odd phenomenon, equally unwanted with previous one, is the
    shifting of the produced frequency whenever
    the value of power supply of the 555 is changed, always ofcourse within the
    allowed limits (4.5->16Volts).
    The datasheets of the various 555 mention clearly that the HIGH and LOW
    times, and hence the frequency, are
    independent of the power supply.


    Any help is wanted

    panagiotis
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: 555 oscillator queer behavour
    I'd try bringing the value of Ra up to 2.2K, Rb to 15K, and C to 470pF. The
    equation will still give you just about the same frequency numbers, and you
    won't be working the poor discharge transuistor quite so hard.

    See if it works. That would cause both of your "Series of Unfortunate Events".

    Chris
     
  3. According to the NS data sheet:

    t1 = 0.693 (RA + RB) C
    t1 = 0.693 (1k + 6k8) * .001e-6 = 5.4e-6 seconds

    t2 = 0.693 (RB) C
    t2= 0.693 (6k8) .001e-6 = 4.7e-6 seconds

    frequency = 1/(5.4e-6 + 4.7e-6) = 99kHz.

    So we agree on what is expected. Have you tried several different
    capacitors? This one may be mismarked.
    Is the supply well regulated? Do you have a bypass capacitor across
    the supply pins of the chip? For *stable* supplies, the voltage
    should have a very minor effect on frequency. Bumpy supply voltages
    have a pronounced effect on the cycle.
     
  4. I am not the expert even with simple things like 555 timers, but
    I have used them a lot (perhaps proving I am not engineer)...
    Following info on my datasheets, that second cap (as opposed to
    the timing cap) is often 10 nF. I cannot say this is the problem.
    However, with regard to the timing capacitor, I have found large
    differences between different types of caps and also variation
    between different manufacturers of same types of caps with same
    value. Also, most of us in here (including me) are hobbyists and
    use surplus store components, which includes resistors. I rarely
    buy or use 5% or even 10% tolerance resistors. The unmarked ones
    are claimed to be 20% tolerance. I find they are better than this
    at room temperate, but maybe they are not stable in a live circuit.
    And then there are differences between 555 timers from different
    vendors and most of us are not using the stringently toleranced
    military grade ones that are rated for higher temperature stability.
    I note that the 555 can get hot in some circuits. Many, including me,
    are guilty of trying to get this poor little IC to drive more than
    the couple of hundred mW it was intended to sink or source.

    I once added up all these variations that I actually encountered
    using my usual surplus shop components and could easily account
    for nearly twofold discrepancies in frequency. I also suspect that
    the info in the datasheet is really and truly valid for tightly
    toleranced components.

    Another permutation was the steadily increasing frequency which
    disappeared once I changed the timing cap for another with exactly
    same value from same vendor. I think these tiny caps can get damaged
    during soldering and then misbehave.


    Dominic
     
  5. John Fields

    John Fields Guest

    ---
    With decent 5% carbon film resistors with predictable characteristics
    being about as common as dirt, you really ought to consider
    "upgrading". :)
    ---
    ---
    According to Signetics' original data sheet for the 555, typical
    spec's for the commercial NE (commercial temperature range) device
    are:

    MONOSTABLE ASTABLE
    ---------------------+-------------+-----------
    INITIAL ACCURACY 1% 2.25%
    TEMP SENSITIVITY 50ppm/°C 150ppm/°C
    VCC SENSITIVITY 0.1%/V 0.3%/V


    This is with Ra or Rb vaying from 2k to 100k, C equal to 0.1µF, and
    Vcc either 5V or 15V.

    Certainly nothing here should result in a twofold change in output
    frequency.
    ---
    ---
    600mW absolute maximum according to Signetics, and with output drive
    of up to 200mA, it's a little gutsier than it might seem to be at
    first glance.
    ---
    ---
    The info in the datasheet has nothing to do with the tolerances of the
    components, it only has to do with the capabilities of the 555 itself.
    If you want to determine the total variation in whatever of the
    output, then of course you have to determine the contributions of the
    several components and determine how they'll affect the output.
    ---
    ---
    Yes; there's usually a soldering time/temperature spec associated with
    caps which should be followed if you expect reliable and predictable
    operation. Bottom line though, and no insult intended, I suggest that
    if you want to build stuff which is going to work the way you want it
    to, stop buying junk. :)
     
  6. See also the thread resulting from OP's cross post in s.e.d. I asked a
    few other questions there, in

    I'm hoping he'll reply, as I'm curious about the cause.
     
  7. I have a question concerning the function of the 555 oscillator in
    Poster got deviation from expected frequency.


    I do not care to upgrade because I have no need. A discrepancy
    between theoretical and actual frequency and duty cycle always
    occurs and this can be fixed by swapping timing caps and use of
    one or more potentiometers at Ra and Rb. I have indeed used the
    high precision components and still needed to tune the 555 to
    get the exact outcome I want.

    There are two issues developing here. One is that the poster got
    a large deviation. This is not likely due to the vendor of the 555,
    but smaller scale differences do exist between vendors. It could be
    due to a bad timing cap resulting from mismarking, inaccurate
    construction or damage during soldering. The timing cap is my usual
    first suspect.


    Yes, we all agree.


    "Junk", as you refer to it, is the stuff of life for many hobbyists. I
    have learned much from junk. I have fixed a lot of junk. I have built
    junk, and I have encountered many people who have praised my junk. Indeed,
    I even maintain "The Astronomy Junkyard"....

    http://www.megspace.com/science/stp/

    Heck, the universe revolves around junk!

    Dominic
     
  8. John Fields

    John Fields Guest

    ---
    Yes, I know. That's what he said.
    ---
    ---
    My post wasn't directed to you, but rather to the OP, who was
    complaining about exactly the sort of thing which might be remedied by
    using components with tighter tolerances.
    ---
    ---
    I disagree. The theoretical response of the device always lies within
    a _band_ defined by the errors inherent in the device itself and its
    peripheral components and, such being the case, the _actual_ response
    will always lie somewhere within the realm of that band.

    If it doesn't, _that_ is when the discrepancy occurs.
    ---
    ---
    Rather a sloppy way of "designing", don't you agree?
    ---
    ---
    Yes, surrounding a device inherently capable of only 1% accuracy with
    0.05% resistors and capacitors to try to achieve an initial accuracy
    of 0.1% has its problems.
     
  9. John, others,

    Depends on the application. If I wanted high precision timing,
    I would use a crystal and would say that not using this is a
    sloppy design to begin with. The lack of use of a crystal by the OP
    suggests that the design is permitted to be a little "sloppy".
    If precision timing is not needed, the use of the crystal
    would constitute a sloppy thinking process, because it just adds
    extra components and cost and maybe even trouble-shooting and
    additional failure modes to the design. At least where I live
    there is a drammatic price and time difference in buying precision
    components (online usually) and picking the lower precision
    components off the shelf at the local surplus shop. It sounds like
    the OP will be quite content, and learn much (just like me), with
    common lower precision components and a little tuning with a trim
    pot.

    I note that many multimeters have cap testers. No idea how accurate
    they are, but since the timing caps seem to have higher inaccuracy
    than the rest of the typical astable 555 circuit, I would think checking
    the capacitance would be informative in predicting the frequency. In
    worst case, one could get relative values to compare caps, which is
    still helpful.

    BTW... when I write "theoretical", I mean specific equations for
    frequency and duty cycle using stated values on components Ra, Rb
    and C. These equations are only approximations regardless of how precise
    components you add to the 555. A case in point, for instance, a couple
    somewhat different constants for the frequency constant in the
    numerator for the astable calculation exist (1.49 and 1.44). These values
    are mentioned in books about the astable mode and are discussed
    independently of vendor, yet they differ by about 3.4%. Until now, I have
    never heard anyone claim otherwise and have expected to have easily 5%
    deviation from calculated frequency and/or duty cycle. Maybe you know
    something I do not?

    Actually, I have been curious about where that constant comes from to
    begin with.

    Dominic
     
  10. John Fields

    John Fields Guest

    ---
    I disagree. Whatever the application, and whatever the precision
    required, having to go back and redo work because of careless errors
    made is sloppy workmanship.
    ---
    ---
    And by that logic, not using an OCXO would constitute a sloppy crystal
    oscillator design, and so on up the chain...
    ---
    ---
    Lack of crystal? You must be joking; the poor guy doesn't even know
    where a 2:1 error in output frequency is coming from, and you've got
    him sitting down deciding how much slop he can live with, before the
    fact!
    ---
    ---
    How on earth did this crystal manage to work its way into the
    discussion? It's certainly not relevant to the discussion, which is
    about where a 555 timing error is coming from.
    ---
    ---
    Yeah, great.
    ---
    ---
    Read the multimeter spec's...
    ---
    ---
    They're _not_ approximations, they're exact.

    To determine how closely the device will conform to the equations
    however, the device specifications must be read.
    ---
    ---
    I suspect that's probably true.
    ---
    ---
    The short answer is, "Because of the voltage divider".

    The long answer follows:

    If you look at the "front end" of a 555, you'll find a voltage divider
    and two voltage comparators hooked up like this:


    Vcc
    |
    [R]
    |
    +----|-\
    | | >
    TH>-----------|+/
    |
    [R]
    __ |
    TR>-----------|+\
    | | >
    +----|-/
    |
    [R]
    |
    GND

    The first thing to notice is that since the resistors are all equal,
    the voltages on the inputs of the comparators connected to the string
    will always be 2/3 Vcc and 1/3 Vcc. That is, they will be
    ratiometric. On top of that, since the resistors are all made of the
    same material and are very nearly isothermal, temperature changes
    affecting one resistor will affect all of them, with the result that
    the voltages on the inputs of the comparators connected to them will
    stay constant for changes in temperature.

    Now, looking at a simplified diagram of a 555 hooked up as an astable
    multivibrator:


    Vcc Vcc
    | |
    [R] [Ra]
    | U1A |
    +----|-\ +---------------->OUT
    | | >---+ | |
    TH>--+---------|+/ | +-----+ | +-----+
    | | +--|R Q|--+ | |
    | [R] | _| D |
    __ | | U1B +--|S Q|---G Q1 |
    TR>--+-+-------|-\ | +-----+ S [Rb]
    | | | >---+ | |
    | +----|+/ GND |
    | | |
    +-----------------------------------+--Vth
    | |
    [R] [C]
    | |
    GND GND


    Assume that the circuit has just been powered up and that C is at 0V
    and is just beginning to charge up toward Vcc through Ra and Rb.


    Then, since

    T = k (Ra+Rb) C (1)


    and, since

    Vcc - Vth1
    k = ln ------------ (2)
    Vcc - Vth2

    where Vth1 is Vth at turn-on and Vth2 is Vth at 2/3Vcc,


    if C needs to charge to 2/3 Vcc to get U1A+ to go more positive than
    U1A-, we can say:

    3 - 0
    k = ln ------- = ln 3 = 1.1
    3 - 2

    and, therefore:

    T = k (Ra_Rb) C = 1.1RC


    Once U1A+ goes more positive than U1A-, the output of the 555 will go
    low, Q1 will turn on, and C will begin to discharge through R2.
    However, since C only charged to 2/3 Vcc before Q1 turned on and
    started discharging C, it will only have to discharge to 1/3 Vcc
    before it goes more negative than U1A. Since this a voltage ratio of
    2:1 we can write

    2/3Vcc
    k = ln --------- = ln 2 = 0.693
    1/3Vcc


    so the discharge time will be

    T = k (Rb) C = 0.693 Rb C


    This time, when the cap discharges to 1/3Vcc and starts charging
    again, the same situation will prevail. That is, there will be 1/3Vcc
    across the cap and it will have to charge to 2/3Vcc before the cycle
    will start anew, so it will charge to twice the voltage that was
    across it when it started to charge, so the time constant will be the
    same as when it was discharging, but now it will be charging through
    Ra _and_ Rb, so the time to get to 2/3Vcc will be longer than the time
    it took to get from 2/3 Vcc to 1/3 Vcc. That's the reason for the
    asymmetrical duty cycle.

    Finally, since frequency is the reciprocal of time, we can write

    1 1
    f = ------ = ---------- = 1.44 Rc
    k RC 0.693 RC


    and that's where f = 1.44 RC comes from.


    The other values you've seen may have been generated considering
    leakage and bias currents.
     
  11. John Fields

    John Fields Guest

    ---
    Aaaarrghhh!!!

    Charge time = t1 = 0.693(Ra + Rb)C

    Discharge time = t2 =0.693 (Rb) C

    Total period = T = t1 + t2 = 0.693 (Ra + 2Rb) C

    1 1.44
    Frequency = --- = --------------
    T (Ra + 2Rb) C
     
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