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increase pulse voltage

Discussion in 'General Electronics Discussion' started by auboy, Apr 9, 2014.

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  1. auboy

    auboy

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    Apr 9, 2014
    Hi,

    As a newbie I'm looking for help. So currently I'm building a simple monostable multivibrator using 555 and the result is as close as I wanted. But I want the amplitude of the pulse is 100V but the signal that I get like picture shown below

    [​IMG]

    it's 9 V as 555 has max of 15V. Is there any way I can increase it? few of my friends told me that I can use BJT transistor, But since I'm a newbie in building electronic stuff so I do not know which one and I tried using 2N5550 but even when I did not trigger the multivibrator, the output is just pulsating to 150V. And all I wanted is just like one hot pulse like picture above but the amplitude is 100V.

    Is there any suggestion?

    Thank you,
    Auboy
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,220
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    Nov 17, 2011
    Welcome to the forum.

    you need a driver (or amplifier) to increase the signal level of the 555 to 100V or more. Ths circuit is rather simple:
    [​IMG]

    Observe the necessayr characteristics of the transistors with regard to the load. The schematic is an example and needs to be fine tuned for the load resitor (or load current).
     

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  3. auboy

    auboy

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    Apr 9, 2014
    Thank you so much Harald Kapp...
    This is the result of my simulation using your guidance

    [​IMG]

    as you can see there in the first signal there is signal goes to negative. why is that happen??

    I attached the the circuit perhaps I did something wrong with the circuit

    [​IMG]

    Also there is another question that I want to ask.

    how much is the fastest time period that 555 can do? Can it do 10 ns??

    And how to trigger it fast? because if you see my triggering circuit above, that is that fastest I can do? if I add the caps the triggered signal is just disappear.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I have bo immediate idea. What does the output of the 555 look like?

    The datasheet states max. output rise and fall times of 75ns (worst case). This means that a pulse cannot reliably be deisgned to be faster than 2*75ns=150ns.

    I do not understand this question. Which caps do you mean? If you increase C1, the trigger pulse will be longer and thus more reliable.
     
  5. auboy

    auboy

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    Apr 9, 2014
    okay this is the output from 555

    [​IMG]

    .
    yes I mean C1. If I change from 10nF to 1nF the triger pulse should be shorter. but turn out there was signal but it's not all the way down to zero, so the output 555 will be flat zero. What should I do with the trigger circuit if I want to make it faster so I can run for example 150 ns?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    For a shorter trigger pulse, try using a higher resistance for R2, e.g. 10k together with a small cap for C1.

    Why the simulator shows such a distorted rising edge I don't know. Using LTSPICE I don't see this distortion - and I wouldn't expect it, neither.
     
    Last edited: Apr 9, 2014
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Just a curious question Harald, but what is limiting the base current for Q2 PNP? Is it R4?
    Thanks
    Adam
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,220
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    Nov 17, 2011
    Good thought, Adam.
    The base current for Q2 is given by the base current of Q1*gain (Q1).
    Since gain(Q1) is rather undefined, it is better to add a series resistor from the collector of Q1 to the base of Q2. Another 10k Ohm should be fine.
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Thanks Harald
    Adam
     
  10. auboy

    auboy

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    Apr 9, 2014
    Is there any substitution for the transistors 2n5550 and 2n5401? I tried to find it in here but unfortunately it's already been discontinued.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    10,220
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    Nov 17, 2011
    The transistors in my schematic are just placeholders. You can use any type that has
    1. at least the same Collector-Emitter breakdown voltage as your high voltage supply - preferably a good measure more
    2. at least the same collector current rating as required by your application.
     
  12. auboy

    auboy

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    Apr 9, 2014
    One more thing Harald, if want more voltage, can use IGBT instead of Bipolar Transistor?? Of course The resistor will change as well
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    10,220
    2,201
    Nov 17, 2011
    Not with the circuit I proposed. IGBTs are controlled rather differently than bipolar transistors.
    If you want to use an IGBT for Q2, you need a gate voltage higher than the supply voltage to turn it on. A special high-side driver could create this drive voltage, but the circuit will become much more complicated.
    If at all, this makes sense for Q2 only, not for Q1 (which needs to drive a rather low current).

    Which voltage do you want to switch? You'll find 600 V PNP and NPN transistors are easy to buy.


    Read more on IGBTs here.
     
  14. auboy

    auboy

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    0
    Apr 9, 2014
    So I did the device but there is something that I did not understand. I made the device and run it through oscilloscope but the result is not good. if you see the picture below. the signal after pulsating it reach minus for quiet a long time than reaches zero? why is that happen?
     

    Attached Files:

  15. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Can we have an updated circuit diagram if there has been any changes. Where are you taking this measurement from? What do you have connected on the output is it just the scope?
     
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