555 I.C. Freq. generator

Discussion in 'General Electronics Chat' started by RRITESH KAKKAR, Aug 31, 2010.

  1. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    What is The maximum Freq. Of 555 Ic ....?:)
     
  2. RRITESH KAKKAR

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    Without Using Crystal in Circuit.....!
     
  3. SgtWookie

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    Jul 17, 2007
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    The maximum frequency is dependent upon the particular manufacturer and exact model of 555 timer. Some are low-power and low-speed. A typical limit is around 500kHz. However, some may go as high as 2MHz.

    You need to examine datasheets to determine what the maximum theoretical limit is, and then experiment to determine what your actual limits are. Your mileage will vary considerably.
     
  4. Wendy

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    Mar 24, 2008
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    Something I'll mention in passing, a 555 is an open ended device. It will do many, many things. I've breadboarded it using protoboards and perf boards, it switches incredibly fast under 10 ns fall time and under 50 ns rise time on a breadboard where these kinds of speeds are unusual.

    So when you ask "How fast will it go" it is a loaded question, it has a build in assumption that is all this chip is for, when in fact it is a general purpose part, a component. Something I wrote for the online electronics text book here.

    Chapter 8: 555 TIMER CIRCUITS

    This thread is a continuation of something posted on HELP with 555 timer.
     
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  5. RRITESH KAKKAR

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    What is Hysteretic Ossillator....?

     
  6. Wendy

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  7. RRITESH KAKKAR

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    I want to generate large current pulse of near by 5amp & 12V for this i have made ckt, but If i connect 555 ic across 12v it will work but on such high current the ic will damage permanently so pls tell me how to find solution of this problem.....!
     
  8. Wendy

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    Simple enough, you use a transistor on the output, either a MOSFET or a BJT. What is your load? The pulse has to drive something.
     
  9. RRITESH KAKKAR

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    The load is transformer generating ac ....!
     
  10. marshallf3

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    Jul 26, 2010
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    Look up the term "amplification"
     
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  11. SgtWookie

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    Another AC inverter thread. Wheee!
     
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  12. gootee

    Senior Member

    Apr 24, 2007
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    Actually, you would be using the high-current-capable MOSFET or BJT as a switch.

    Your 555 output can't supply anywhere near the current you want. But it has the "switching" shape and repetition rate that you're wanting. So what do you do? Well, a nice big MOSFET can be a great voltage-controlled high-current switch.

    When fully switched on, the resistance across the MOSFET's high-power-capable Source and Drain terminals (often called Rdson) might be in the very low milliohms. And THAT means that even with many amps through it, it won't dissipate much power! So it won't get too warm! i.e. It will not be quickly detroyed! So if you hook one of its terminals directly to your power supply and the other one to your load, BAM! You get about the same waveform shape as your 555 output, but it's now controlling almost a "dead short" (switch) directly from your power supply!

    One main performance (and reliability) issue, when using a MOSFET as a high-current switch, is that you "must" be able to make the MOSFET not spend too much time _between_ the "off state" and the very-low-resistance "on state", because between those two it has more resistance in the high-current path. And more resistance would mean (much) more "i-squared-times-r" power dissipation, which can get way too hot to touch, very quickly, and can very easily destroy the device.

    So, in reality, you have to make sure that you can bang the MOSFET's gate very hard, and very FAST, when turning it on and off. So (and I am not sure whether or not it would always be necessary, when using a 555 to drive a MOSFET), you might even want an intermediate medium-power switching device in between, such as a smaller transistor, to be controlled by your 555 output to drive the MOSFET's gate.

    And you would definitely also want to use a MOSFET with a very low gate capacitance, since any capacitance will form an RC "time constant" with the resistance in the circuit (and there always is resistance, even with no resistors), slowing down the turn-on and turn-off times, which is bad (for already-mentioned reasons).

    With such high-speed edges (you hope), you also need to pay attention to trying not to allow the formation of high-frequency "ringing", i.e. damped oscillations, especially in your gate voltage, since the higher frequencies have more energy and can tend to heat up the MOSFET, and also might incite actual sustained HF oscillation, which is much worse and more-quickly destructive.

    So you need to damp-out high frequencies. Damping implies resistance. Use a resistor in series with the MOSFET's gate, but not so much that it combines with the gate capacitance to slow down the rise and fall times too much, since, as already mentioned, that also can result in rapid overheating and device destruction.

    Sounds like fun! Before you know it, you'll be able to design a high power Class D audio amplifier that doesn't even need heat sinks!

    Cheers,

    Tom
     
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  13. RRITESH KAKKAR

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    Can supply the ic with resistance.........?
     
  14. SgtWookie

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    You cannot "supply" resistance. You can add resistance to the current path(s). However, adding resistance implies reducing current, which appears to be the opposite of what you want to do.

    The maximum current that a typical 555 bjt timer can source or sink is 200mA, but the output will be at a reduced voltage if sourcing current, or increased voltage if sinking current.
     
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  15. RRITESH KAKKAR

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    What the use of connecting capacitor in pin 5 of 555 ic, as many ckt don't connect capacitor.
     
    Last edited: Oct 17, 2010
  16. Audioguru

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    Dec 20, 2007
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    If the supply voltage of a 555 fluctuates a lot with mains ripple then a capacitor at pin 5 to ground makes its duty-cycle more stable.
     
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  17. marshallf3

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    If you haven't been following some of his (and many others') threads half the people in India are trying to figure out cheap ways to make DC:AC inverters.

    They have highly intermittent line voltage over there that, even when it's on, can vary between 200 VAC - 300 VAC and theoretically averages around 50 Hz but even that's not always the case.
     
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  18. RRITESH KAKKAR

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    I have doubt some people says that capacitor used for bypass, controlling output,etc. :confused:
     
  19. Audioguru

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    If the supply voltage is regulated and steady then pin 5 will have a steady voltage (provided by the potential divider resistors inside) even if it does not have a filter capacitor.
    The datasheet for Texas Instruments' NE555 says that a decoupling capacitor from pin 5 to ground may improve the operation of an astable oscillator.
     
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  20. RRITESH KAKKAR

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    Jun 29, 2010
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    pls, tell the name of led which are used in mobile phones.
     
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