555 high frequency oscillator question

Discussion in 'General Electronics Chat' started by magnet18, Mar 21, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Hi all,
    I'm trying to create a frequency with the 555 at about 20-30kHz, but when I try at high frequencies, i just get a constant high output

    I'm following this schematic
    [​IMG]
    with a 10nF cap, a 1K resistor for R1, and a 2K resistor for R2, which should be 28kHz; any ideas what I'm doing wrong? can the 555 not handle these high frequencies? because it was working with a 10K pot for R1 and R2 with a 10nF making about a 4kHz arc, I can't think why it wouldn't be working :(
    (yes, I have a decoupling cap on pin 8)
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    That frequency is quite modest for the '555' timer.

    How are you monitoring the output?

    Re-check you wiring &/or try a replacement chip if you have a spare.

    Perhaps also try with a much larger timing cap value to make sure all is working OK otherwise.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A standard 555 is rated for 1Mhz, the CMOS TLC555 (TI) is rated for 2Mhz. I've run them routinely faster than 30KHz on a protoboard. This suggests one of the parts is bad, be it capacitor or 555.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Check to make sure you didn't short pins 2 and 1. That would have the same effect as a shorted cap.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    Check the wiring first. Human error is the most frequent cause of error by more than an order of magnitude.

    Make R2 bigger and C proportionately smaller. See if that works. It may also explain why your pot worked.

    Edit: Here's that pdf. See p. 5-6 in addition.

    John
    View attachment 555AN.pdf
     
    Last edited: Mar 22, 2011
    magnet18 likes this.
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I suggest that your R1 should be a minimum of 100 Ohms per volt of Vcc, so if your Vcc is 10v, then 1k Ohms should be OK.

    Pin 7's sink current is somewhat limited; I don't really know WHAT the limit is offhand, but I prefer to not test it. If you try to sink more than about 15mA via pin 7 for a bjt 555, the saturation voltage could become excessive, which could cause the timer to get "stuck". Note that CMOS 555's have less sink current capacity; the TLC555 has a max saturation voltage of 0.4 @ 10mA, which is why I suggest 100 Ohms per volt; it works for both bjt and CMOS 555's.

    But, if your resistor values are getting down below a couple K Ohms, you really should look at reducing the size of the timing cap.
     
    magnet18 likes this.
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The sink current capability of 555 Pin7 is not mentioned in the data sheet(or I couldn't locate it). The pin7 is always shown connected to a NPN collector with the emitter grounded.

    It really does not matter for capacitor less than a few microfarads.

    However, very long timing range of 10 of minutes requires electrolytic capacitors of over hundred uF being used, the discharge current via pin7 is significant and can do damage to the chip in the long term. Normally, there will be some resistance between the capacitor and pin7 but this can happen when the person wants a very short pulse with a very long timing period, by having no resistor between pin7 and timing capacitor.

    I heard people have their 555 destroyed for no particular reason in this situation.

    I would suggest then to include a series resistor of ohmic value twice that of the supply voltage. e.g. in series a 22Ω resistor between capacitor and pin7 to limit the current to 500mA.

    Or alternatively, someone can provide information otherwise that it is safe not to use a series resistor.
     
    Last edited: Mar 22, 2011
    magnet18 likes this.
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    All right then,
    Without going through a lot of analysis, there seems to be around 4.7k to 6.3k of resistance between Vcc and the base of pin 7's open collector output, along with a 3.3k resistor to ground.

    If pin 7's transistor is too far out of saturation, power dissipation will get excessive pretty quickly. Let's just say for the moment it's 6.3k from Vcc to the base; so if Vcc=10v, the Ib = (10v-0.7v)/6,300 = 1.47mA, and Ib*10 = 14.7mA collector current to be in saturation. This actually pretty closely agrees with the datasheet, where saturation voltage is described with pin 7 current at 15mA.

    So, I suggest that 15mA is a realistic limit to keep pin 7's saturation voltage low. If the transistor comes too far out of saturation, power dissipation in the open-collector output will become excessive.

    In the schematic in the original post, if Vcc=10, then the 1k resistor will allow up to (Vcc - Vcc/3) /1k = 6.666mA to flow from Vcc into pin 7 (remember, it only has to drop to the lower limit; 1/3 and 2/3 Vcc) and the discharge from the cap, Vcc * 2/3 / R2 = 3.333mA for a total of 9.999mA maximum sink current for pin 7.

    To me, this is acceptable.

    [eta]
    In a simulation using a component model 555, 10v Vcc, 182 Ohms for R1 resulted in Vsat of 1v and 50mA current; but this is not a "real" 555. Mileage would likely vary quite a bit between various 555's.
     
    magnet18 likes this.
  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Thanks very much guys :)
    I always seem to manage to find the limits of things and mess them up :rolleyes:
    I'm using 12V, so I'll up my resistance values substantially and drop the cap and see what happens. Hopefully this solves the problems.
     
    Last edited: Mar 22, 2011
  10. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Well... um... this is embarrassing... but I just realized I had a TL071 in the socket... :(
    Thank you very much for the information though, I intend to change the resistance values to prevent mishap.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The most obvious things can sometimes be the most elusive. I'm sure that you were absolutely certain that you were plugging in a 555 timer.

    The resistance and capacitance values are OK.
     
  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Yep, I distinctly remember checking the tube, realizing it was TL071's, and putting it down and getting the 555's, now I check, and it's the 555's on the shelf and the TL071's in all my failed circuits.
    Sometimes life is just funny

    I thought it was- 100ohms per Vcc Volt would be 1.2K minimum, and the R2 value should be at least 3K, right?

    [EDIT]
    oh, just realized I never told you that my Vcc is 12V
    I intend to change them just to be safe.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I've given you my "best guesses" and reasonings for doing so.

    I still suggest that 10mA is a good target max for sink current from pin 7, and (basically) how to calculate it.
     
  14. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Thanks again, I went with 10K for R1 and 22K for R2 with a 1nF cap, giving me a nice 25kHz signal, ripe for the modulating :D
    It was just a breadboard circuit so it was no big deal to change the values :)
    but, if ever in need, I have bookmarked this thread for future reference (and downloaded that pdf, I intend to give it a good read)
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Want something interesting, some of the 555's I have the numbers rubbed clean off. Must have come from the manufacturers that way. :rolleyes:
     
  16. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    You have learned a valuable lesson.

    If you know anyone in the medical profession, a nurse or physician, ask how they prevent making the same type of mistake when dispensing drugs or doing other similar activities. Identifying something by position/location/sequence may be fine for some computer applications, but it is a very poor method for human activities. At a minimum, always read the label when you pick it up, and again just before using it.

    John

    Edit: Another lesson to learn about experimentation is to change one variable at a time. Sometimes, in our rush to get a quick and dirty answer, that rule is violated, but it is still a good rule. In this instance, you changed the chip and the timing elements at the time time. I think it would be instructional to go back to the original values you had just to see how they work with the correct chip. You may well end up using the new values you found to work, but why leave the other questions unanswered?
     
    Last edited: Mar 23, 2011
  17. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    That is generally a good policy, but what about when there might be a chain reaction, for examle, the mosfet fails, and takes the 555 with it, and trying a new fet with the old 555 just blows it, but trying the new 555 with the old fet results in it blowing also?

    But yes, in general I agree that it's a good idea to change one thing at a time.
     
  18. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    There's a difference between trouble shooting and developing. Obviously in the former, if the mosfets failed, one would not change the oscillator without changing the mosfets or at least confirming they had not been damaged and visa versa.

    I can tell you that many graduate students will try methods A, B, C, and D to do something, then throw up their hands, go to the professor, and say "nothing works." That is not a good way to impress the professor nor to get anywhere with the research.

    Let me give a more concrete example. Let's say you want to synthesize a particular compound. (I picked this example, because you are interested in chemistry.) You study the literature, syntheses of similar compounds, and so forth. You then come up with several possible routes to the synthesis. After considering each of them, you pick the one best, most likely to succeed route. Chances are probably 1 in a 100 or less it will work as you designed it. What should your next step be?

    Well, it was your best idea, and it failed. What do you think the chances of your second best idea working without a glitch are? You can see where that logic is headed. Instead, you investigate the failure, try to find out why or what happened, and modify your thought process accordingly. You may decide that the first method is still the best and modify it accordingly. Or, you may find that the first method doesn't have a chance and the second method has the same fatal flaw, so you try the third method, etc. None of that is strictly one one variable, one experiment. But the approach is a logical progression, not just trying multiple independent approaches. RB Woodward (Nobel Prize, chemistry) used to create teams. Each team had different approaches for one step. When that step was done, the teams would switch to the next step. It was cut-throat competitive and efficient.

    Back to you oscillator. The Philips ap. note makes certain recommendations. SgtWookie and others, based on a lot of experience, came to slightly different recommendations. The fatal flaw in your first try was the misidentification of the chip. You did get it to work by correcting the chip and changing the timing components. If it were sitting on my bench, I would be so darn curious, I couldn't sleep without trying the original configuration with the right chip. After all, it was your design. Don't you want to know whether you were right?

    John
     
  19. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,794
    948
    A ritual I have developed doing QC work that must be perfect.

    Go through your procedure in a dry run form. Set out, or pick up and play act with every part and piece of equipment.

    When you finish you either 'do' it. OR correct some oversight or missed step or tool. :) usually you do the latter.
     
Loading...