555 oscillator overheating

Discussion in 'Digital Circuit Design' started by OrionV, Jul 19, 2016.

  1. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    Hello. I am trying to build an oscillator to drive 9 volts from a battery through a transformer but the 555 chip is getting really hot. I checked the connections multiple times but can't find a problem. I am using a schematic as the one here http://www.ohmslawcalculator.com/555-astable-calculator
    but am only using a 4k7 resistor on R2. Instead of R1 I have a jumper between pin #7 and Vcc. Capacitors: 10 nF, 50 V, 85 °C. Any help would be much appreciated!
    Thanks.

    IMG_20160719_132701-800x600.jpg

    IMG_20160719_132834-800x600.jpg

    Mod Note:
    Before you attach the photos, please compress the photos to a clear image and small resolution and size as 800x600 or 1024x768.
    The photos already compressed to 800x600.
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    880
    That's why your 555 is overheating. What in the world made you think you could short the 555's Discharge pin to Vcc without disastrous consequences?? And what in the world made you think you should short the 555's Discharge pin to Vcc????? Good grief.

    Put a resistor in there in place of the jumper, and the circuit will work like it's supposed to.
     
  3. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    I need around 30kHz output frequency, the resistor should be around 5kOhm. If I change the places of the resistor and the jumper will it work then or do I need to have both resistors in place?
     
  4. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    Here is a block diagram of the inside of the 555.

    555.jpg
    If you connect pin 7 (the collector) directly to Vcc. When the output of the flip flop turns High, the NPN transistor would be shorting the Vcc to Ground. That's why your IC would get hot. The resistor between Pin 7 and Vcc is not optional.

    Allen
     
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  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You need both resistors.
     
  6. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
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    Ok, if I put a 1kOhm resistor in both places would the chip still overheat?
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It shouldn't, if wired correctly.
     
  8. absf

    Senior Member

    Dec 29, 2010
    1,492
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    If you need a frequency of 30KHz, you may use the circuit attached. I have not tried it on a breadboard just simulate using Proteus. Not sure if the 555 would work well at that frequency.

    555 30KHz osc.PNG
    Allen
     
  9. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,301
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    Download a datasheet for the 555, such as this one, and read it. AFTER you've read it and made sure you understand it, THEN start designing and building. That's good practice for EVERY part you use in your designs, by the way: first, download the datasheet and read it before you design or build.

    Yes, you need both resistors.
     
  10. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    Got it, thanks. The problem was that an online calculator showed 0 ohms on R1 and I thought that it would be okay without a resistor there. Thanks for the answers.
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
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    That's the least of your problem.
     
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Word to the wise: don't ever, EVER trust online calculators; you've no way of knowing whether it was designed by an expert or by an idiot. And the fact that the calculator you linked to allows a value of 0Ω for R1 without flagging it as an error, tells me that the person who created it was either lazy or not too bright.

    Once again: DON'T EVER TRUST ONLINE CALCULATORS. Use the information on the datasheet, together with common sense, to choose component values.
     
  13. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    Alright, no calculators. My bad. I used two 1kOhm resistors and the chip stopped overheating. The new problem: it gets hot again when I attach the transformer to the output and Gnd. When connected to output and Vcc I get some AC voltage but the chip still gets very hot. Any ideas why this happens?
    Thanks.
     
  14. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    You are driving the transformer primary with a pulse that cycles between a positive voltage and ground. If the end of the transformer not being driven is connected to V+ or ground there will be an average DC voltage across the transformer. Then you end up with current that is approximately the average voltage divided by the primary resistance.

    You need to put a capacitor in series with the output of the '555.
     
  15. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    Ok, but what about the frequency? It matters the most for powering one of those small transformers from a wall charger, right? I was told that around 40kHz would be good but the output is minimal, around 3V.
     
  16. ronsoy2

    New Member

    Sep 25, 2013
    19
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    Come on OB, be more uh, "liberal." All online calcs are not junk. Of course some are risky (if they can come up with ridiculous values) but there are a lot of very nice online calculators out there that can save you a lot of time. The key is to as you point out READ THE DATA SHEET and understand what you are doing before applying power.
    On driving the transformer, the 30khz will work fine. The power output of this setup will not be particularly great so don't expect to light up a big fluorescent tube or some such with it! If we knew your final application more useful help could be given.
     
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  17. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,301
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    You're absolutely right, of course; there are some online calculators out there, especially the ones run by the major semiconductor manufacturers, that are both good and very useful. However, note that I didn't say don't ever use online calculators; I said don't ever trust them-- especially when one does not yet have enough experience to tell when the online calculator is spitting out nonsense, as in the present case.

    The same caveat applies, by the way, to using simulator programs like Spice: although Spice is a very useful (indispensable, IMO) design tool, it's nevertheless perfectly capable of coming up with the most outrageously silly simulation results from time to time. Again, experience is necessary when using Spice, to avoid being led astray.
     
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  18. Tonyr1084

    Active Member

    Sep 24, 2015
    531
    86
    I've seen transformers that were designed to operate on 60 cycles and some that were designed to work on 400 cycles. This is my personal experience and is not a definitive conclusion on transformers.

    With your lack of experience with electronics, you should know that you can't just grab a transformer and make it run at any frequency you desire. The transformer has to be of proper design in order to give you a specific result.

    So far we've learned that you can overheat a 555 in a couple ways. We (or should I say "I") have learned that you have insufficient knowledge with electronics. We also don't know the exact application of whatever it is you're trying to build. So if you want help then you need to give us all the details. What's your goal and why.

    30kHz - perhaps the 555 is not the best way to go about achieving your goal (whatever that may be). Also, different grades of 555's are available. Here again, the spec sheet is your friend. DON'T BUILD WITHOUT IT. Why not? Because with the spec sheet handy you can know what the limitations of a particular device is. In other words, you wouldn't put a watch battery in your car and expect it to be able to start the motor. The specs say it doesn't have enough OOMPH for the job. And maybe your 555 doesn't have enough for whatever your application is.

    And without knowing your application - we can help very little. So please tell us what it is you're trying to do.
     
  19. OrionV

    Thread Starter New Member

    Jul 3, 2016
    22
    2
    I want to experiment with a spark gap and the frequencies it generates. I don't have a neon transformer so that is why I need the oscillator. And I don't have sufficient knowledge in electronics because that is not what I am studying, it's just a hobby.
     
  20. ci139

    Member

    Jul 11, 2016
    341
    38
    ?? if you need high DC voltage use https://en.wikipedia.org/wiki/Voltage_multiplier -- the lower frequencies require bigger capacitors
     
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