Help with 555 Timer astable

Discussion in 'General Electronics Chat' started by amruth11, Oct 27, 2011.

  1. amruth11

    Thread Starter Member

    May 24, 2011
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    Hi all, i would like to create various frequencies using a 55 timer upto 1Mhz for my electromagnetic induction power transfer project. But i learnt that the 555 goes unstable over a freq. of 360Khz. I need help from u guys in how to overcome the 360Khz problem. I would also like to know if i added 2 pots in place of the resistors where R1= a 100k pot and R2= a 1M pot and can it be done. Thx in advance. :D
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You have several options here.

    1. use a function generator instead.

    2. use a faster 555, such as a TLC555, a CMOS version sold by Radio Shack. It has weak drive, but does go higher in frequency.

    3. look for a different type of circuit that does what you want, and leave the 555 behind.
     
  3. amruth11

    Thread Starter Member

    May 24, 2011
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    Hi, thx for the help. I cant use a function generator cause its to costly and i would like to build one rather than buying, i looked into the CMOS u mentioned but i didn't see any big difference but if u know something plz do tell me, and for the alternative i got this from a book about 555 and this what it says:

    HIGH FREQUENCY OSCILLATORS
    360kHz is the absolute maximum as the 555 starts to malfunction with irregular bursts of pulses above this frequency. To improve the performance of the oscillator, a 270R and 1n can be added as shown in the second circuit:

    The dia. has been attached. Sry for the bad quality pic.
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    This MOD may help it's performance at 360kHz, but it's still 1/3 of the 1MHz you are looking for.

    Look at page 8 of this datasheet. Typical freq. can be around 2.1MHz
    TLC555 CMOS Timer
     
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  5. amruth11

    Thread Starter Member

    May 24, 2011
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    Thx for the valuable info. Now i will just use a CMOS insted of a normal 555. I would still like to know weather i will still have to add a 1k in series if i am using R1 and R2 as variable resistors, will the 1k still be needed.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
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    This is really a fun device.

    Try this super simple circuit--you will not find it app notes--only one resistor to adjust--using pin 7 as an open collector output buffers it well while the complementary (CMOS) output drives the timing capacitor. Half-cycle timing is approx 0.5RC. Tie pin 4 high--forgot that on the schem.

    Page 9 of the previously mentioned data sheet further expands on the limitations by specifying the propagation delays--0.1uS and 0.2uS for positive and negative transitions--you can see that 0.3uS or 30% of the cycle is used up just in propagation delays at 1mHZ. Try a number of devices without making any adjustments--the one that runs at the highest frequency has the lowest prop delays.

    Yes, you can get reliable 1mHZ operation from the TLC555, but temperature stability will be poor as the prop delays are subject to temp--operate at a fixed temp.
     
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  7. Wendy

    Moderator

    Mar 24, 2008
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    A TLC most definately is not open collector. It's drive is 10X stronger one direction than the other (it can sink more than it can drive), but it is a CMOS device. The amount of current it can output is dependent on power supply voltage. At 15V it can handle 100ma, at 3 volts it may not be able to handle 1ma.

    What you are showing is a hysteretic oscillator, which I describe in this article.

    555 Hysteretic Oscillator

    A conventional 555 will have symmetry problems, while a CMOS device should produce an extremely symmetrical square wave. This is because a CMOS 555 is true rail to rail on the output, it can reach the power supply voltages, while a conventional 555 can only hit around Vcc - 1.3VDC.
     
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  8. amruth11

    Thread Starter Member

    May 24, 2011
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    THx guys for all the info. now i can do my own circuit depending on the need and the the info given by u people. I will post the circuit later. :D
     
  9. amruth11

    Thread Starter Member

    May 24, 2011
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    Hi all this is the circuit i. The highlight on the C1 is the place where i am going to place 2 female type pins one for the + and - and its going to be kept open cause i will have to change the cap every time i change the freq. and i am using a program to determine the R1, R2 and C1. Plz tell me the improvements i can do and if there is any prob. with the ckt plz tell me wat should be done. :D
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    A single variable resistor here is a very bad idea. If you put 0 ohms there the 555 will burn up, as pin 7 switches to ground with a transistor there.

    [​IMG]
     
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  11. amruth11

    Thread Starter Member

    May 24, 2011
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    Thx for the advice, do u think i can add another 1k before that and then will it be ok.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Yep, the other variable is OK with out a minimum resistance, though the circuit will quit functioning. Have you seen this page of my blog?

    Bill's Index

    My Cookbook

    My general blog has quite a few articles on 555's, though not quite what you're looking for.
     
  13. amruth11

    Thread Starter Member

    May 24, 2011
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    0
    Hi bill, i saw ur blog and i think i have to experiment a few things and see which one suits me well:
    1. I am going to try adding 1k before both the variable and test it out.
    2. 1k before R1 but not R2, but i think that will have problems and will quit functioning.
    3. If non of these work, i am going to select the freq. i need and use normal resistors.

    If u have any other ideas or any other circuit which is close to mine, inform me plz. :D
     
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