CMOS 7555 abstable output @ 800 KHz - trangular wave instead of square, slew rate problem?

Discussion in 'The Projects Forum' started by Marcus2012, Feb 22, 2015.

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  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hello Everyone

    I'm new to the forum so I hope its ok to post here for some advice. I was looking for some help regarding an astable output I am trying to obtain from CMOS 7555 timer if possible please. I'm simulating a basic Modified Sine Wave generator for a high frequency wave using Circuit Wizard. I've created the circuit and it seems to work ok in simulations however the output from the 7555 is not a square wave but rather triangular in the simulation. The IC is rated for 1 MHz as I understand it so It should be ok and is this just a result of the slew rate of the timer? The J-k Flip-flops still seem to recognise the negative edge without a problem in the simulation I am just wondering if this is to be expected and will still function as an astable output in reality. I have attached the circuit diagram for the 7555 component and also the simulation output.

    First graph (output device was a signal lamp to simulate ideal observation)- red wave is the 7555 output and the blue wave is the produced modified sine wave over the output device.
    Second Graph (output device was the intended inductor) - red wave is the 7555 output and the blue wave is the produced modified sine wave over the output device.



    I would appreciate any help anyone could provide please.

    Thanks [​IMG]

    Values
    C1 - 0.00001 µF
    R1 - 15 KΩ
    R2 - 82 KΩ
    Frequency - 806145.2514 Hz.
    Duty Cycle - 54.2 %

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    What is the power supply voltage. CMOS 555's tend to have a lot less drive at lower voltages, so that could be part of it. You are not showing the load either.
     
  3. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hey

    The 7555 is driven by a 7805 regulator at 5V so I could experiment with a higher supply.
    I'm not sure on the load at the moment as I have not finalised the final inductor design and I wanted to make sure I could make the wave generator first.

    Power consumption shouldn't be over 360W though but this is isolated from the wave generator circuit.
     
  4. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    I have just simulated the 7555 with a 9V supply though and it was still a triangle wave.
     
  5. Wendy

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    Mar 24, 2008
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    As an experiment, try boosting your supply voltage to 12V. Put some bypass caps across the power supply pins of the chip. 5V would cause the drive problem I'm talking about, a conventional 555 will not have this problem and will work with your design. BTW, the diode will drop 0.6V, so the power supply to the 555 is actually 4.4V (or less).
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    OK, if you have a conventional 555 give it a try. I would also look closely at what it is feeding on the output.

    Any chance of getting pictures of the circuit? Part of the problem is you are definitely in the realm of RF, it would make a pretty noisy AM signal. This means a protoboard is nothing but a large series of capacitors.

    Is this pure simulation, or is there a real circuit involved?
     
  7. #12

    Expert

    Nov 30, 2010
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    and you are presently experiencing the difference between simulating and real world construction.

    Meanwhile, a 10 uf electrolytic and a 0.1 uf ceramic should be placed right at the 555 chip from pin1 to pin 8.
     
  8. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Yeah it is pure simulation at the moment while I get my head around it all. I've attached the entire diagram below, I'm a real noob so I probably made loads of errors somewhere lol and a lot of the resistor values are just arbitrary at the moment while I finalise components. The simulation isn't going to be that reliable after the AND gates as the components have no model designations but I'm hoping the 555 simulation should be pretty accurate.

    [​IMG]

    Its based on lots of designs I've seen from varies website tutorials.

    EDIT: for this diagram I changed it to a NE555 and I have lowered the frequency to 400KHz but it didn't change anything
     
    Last edited: Feb 22, 2015
  9. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    What will this do I haven't seen it in any astable 555 setup?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    In real life you need to have something to absorb transients.

    Decoupling or Bypass Capacitors, Why?

    I strongly suspect your seeing a simulator artifact. The signal would work for a FF in any case.
     
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  11. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    I just tried it and its the same wave but your right I guess the FF will work regardless so I think I'll make it up on the breadboard and test it with a real scope.

    Thanks for the help :)

    Just to check though would slew rate affect it like this or would you still expect a clearly defined OFF period in the output wave even at RF?
     
  12. Wendy

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    Mar 24, 2008
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    There will be a slew rate, but his is exaggerated quite a bit. With such a low voltage it would be more, but I would expect on the order of 10 ns.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Marcus2012

    Turn up the bandwidth on your simulator. You do not have enough data points to catch the square wave. Your band with should be 10 to 20x of the working frequency of your signal.
     
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The block in your simulation says 555, but beside it is CM7555. If the simulation is using a standard 555 it won't run that fast.
     
  15. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Ahhh thanks this has helped a lot. Lol noob error there, now I have the nice square wave I was expecting. Now I can move on to my next question. This is going to power (hopefully) a high frequency electromagnet for about a second max. Does anyone have any incite into how I can smooth the inductive load so I don't damage anything and it works as efficiently as possible?
     
  16. ScottWang

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    Aug 23, 2012
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