555 timer problem and simple smps

Discussion in 'General Electronics Chat' started by Salvador123, Oct 3, 2015.

  1. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    Hello folks,
    So I have made a small board with the tlc555 timer chip on it.the output from pin 3 as measured to ground produces a sawtooth waveform not a square wave as I measured with my osscilloscope.I have attached an image of how my timer is assembled , can some of you point out where the problem may be? Also I can't fully cross out that it might be the osscilloscope itself as it's old and maybe it can't normally reproduce the square wave on the screen I don't know.

    Also I wanted to ask what do you think of my circuit , im planning to use the the 555 timer output to drive a class A amplifier with a C class output stage using two powerful BJT'S to drive the primary of a ferrite core transformer , sort of to make a simple smps. what do you think of the circuit will it perform the way i intend? Multisim shows it should.the bipolar voltage osscillator in the multisim drawing is meant to be the output of my 555 timer, also the output transistors are BUX98A instead of the ones in the drawing because in my multisim database there were no bux98a.

    P.S. Just in case someone's worried , I have dealt with eectronics for quite some time, have repaired and built some amplifiers tv's and other gadgets , I have built a simple smps from a chip and sokme mosfets , but I just wanted to experiment with this idea.

    Thanks.
     
  2. Dodgydave

    AAC Fanatic!

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    Last edited: Oct 3, 2015
  3. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    but why it wouldn't work? it's a simple class A amplifier , the phase is split between the collector and emitter of the driving transistor which is then used to drive the outputs at 180 dregrees out of phase to one another.basically the only thing that diverges from a typycal class A npn design here is that for the output transistors I have taken away the resistors coming from the +ve line so to not cause extra unnecessary heat in them being open all the time.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I think your circuit will work as well as Dave's in theory, since at the concept level they are identical. However, neither has any deadband - a short time period in each half cycle when neither output transistor is conducting. This prevents cross-conduction, or shoot-through, a condition where both transistors are on at the same time. This happens because transistors turn on faster than they turn off. Neither circuit has a totem-pole output, so the shoot-through condition is not quite as obvious. But having both transistors pulling current through the transformer core at the same time in opposite directions is pretty much the same thing.

    ak
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You might consider a self-excited inverter design since it avoids the shoot-through problem and also excess saturation of the core.
    Here's a simple example of such a circuit.
     
  6. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    thanks folks for the interest.Well the self excited version has a problem with frequency stability I think?
    I once too tried the astable multivibrator which could then power the end stages and the BUX98a output transistors, but the astable multivibrator was kinda unpredictable when it had load attached to it.

    about the shoot through , well unless the primary inductance doesnt build up some high voltage it shouldn't be a problem the BUX98A are rated maximum at 30 amps (continious) CE and 400v CE. Mains has 230 volts and 16 amps after rectifier and capacitor smoothing there are about 325 volts and dunno how much amps?

    What do you think of the part values in the schematic ? Can I go with them ? or should something be altered or changed?

    thanks.
     
  7. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    now i havent yet attached the 555 generator board to the further amplifying stages but i have a problem the 555 in this configuration outputs not a square wave but more of a sawtooth can anyone please tell me what could be wrong , also as i asked what do you think about the values throughout the circuit for parts?
     
  8. AnalogKid

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    If you calculate the two half-cycle periods created by R11, R12, and C7, you will see that the 555 output is nowhere near a symmetrical square wave, so the on times of Q2 and Q3 will not be equal. What is the function of the Q1 circuit?

    ak
     
  9. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    Hmm, so should I then make R11 and R12 of equal value or what should i change in around the IC to make it output more of a square wave.?

    I put q1 in the circuit because i wasn't shure that the about 10Kohm and 15 volt zener power supplied 555 will have a strong enough output to directly drive a bigger transistor , so i added one more stage of simple class A.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Normally, frequency stability is not a significant concern with a DC-DC converter.
    Why do you think it is?
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
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    The circuit should work as an oscillator -> and if you put a speaker to the output pin (through a lc filter) and apply the audio source to the pin 5, it will act like a (low power) class D amplifier.

    A class D amplifier is nothing but a (power) oscillator whose duty cycle varies with the input. It can be implemented a variety of ways. The way you have chosen happens to be not very good, however.

    Look up UCD - Philips has an application note on that. And IRF has a few reference designs using their own chips.
     
  12. dannyf

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    Sep 13, 2015
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    Limited bandwidth -> maybe the oscillation is too fast for the 555? maybe the oscillator is too fast for the amplification stages? maybe the current drive is too limited? maybe you are driving a non-resistive load? ....
     
  13. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    no when i saw the sawtooth output from the 555 I was measuring with a scope and the 555 was alone driving no load at all.havent yet tried it with the rest of the transistor stages because i wasnt satisfied with the output to begin with.

    well maybe it could be due to too low current? i measured it also with a small 6.3 volt bulb attached as load and then the output was barely noticable maybe some 0.3 volts or something in amplitude.
    i am powering the IC directly from the rectified 325volt line (i know not the best idea) through a 10kohm resistor and a 15 volt zener which forms about 14.7 volts constatnly on the input of the 555, maybe the current is not enough?
     
  14. AnalogKid

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    When the output is sagging down, look at the 555 Vcc pin to see if th 15 V is holding steady. With 10 K you have only 31 mA of available current before the zener starts to drop out of regulation.

    ak
     
  15. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    perhaps i should dissconnect one series resistor making the total resistance before the zener 6.8 Kohm?
     
  16. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    I did just that , i took out one of the resistors now my resistance was about 6.6 Kohm , the zener voltage climbed to 15.0 and then slowly climbed another 0.03 and so and stayed at that. now the output from the 555 still is a sawtooth or my scope is dead but i kinda think it's a sawtooth because i have probed some astable multivibrators earlier and they showed fine waveforms.
    stupid little 555 can't even make a damn square wave, or stupid me for not knowing how to , either one of those.
     
  17. dannyf

    Well-Known Member

    Sep 13, 2015
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    Countless of 555s would dispute that, :)
     
  18. AnalogKid

    Distinguished Member

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    billions and billions...
     
  19. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    well since noone is telling me why there is no square wave I can't figure it out myself. But maybe it's a blessing I have the sawtooth type of waveform , as the rise and fall lines are more at an angle rather than vertical and this will give the transistors time to open and close (generating more heat yes but avoiding a shoot through) so , i will just try to go with it for now.

    I will limit the resistance of the 555 supply to about 3.6 Kohm, and leave the 15v zener it should then have plenty of headroom for current as thios configuration allows up to 100mA at those 15 volts before things start to get too toasty.
    I will then apply whatever " tooth" waveform i have there to the capacitor and feed it into the amplifier type something I have made there and see what happens.
    hopefully the signal will be strong enough to drive those transistors.
     
  20. Salvador123

    Thread Starter New Member

    Oct 3, 2015
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    also could someone please explain whether this arrangement (see the atached picture) would work ? the output transistors instead of connected across the whole rectified voltage line driving a capacitor instead ?
     
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