Frequency Divider Issue

Discussion in 'The Projects Forum' started by Bill B, May 8, 2010.

  1. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    I built a digital clock that I am trying to power and clock from a standard wall outlet. I have been using a function generator with a 50 ohm output impedance to simulate the stepped down voltage at the secondaries of a transformer.The transformer output is 17 V p-p unloaded. I built a voltage limiter with zeners to get a 5 V p-p modified square wave input. That isn't enough peak voltage to drive a flip flop. I amplified the signal with a 741 op-amp to a level of 10 V p-p. I half wave rectified the input voltage and put a 1000 uF cap across the rectifiers to get my plus and minus voltages for the op-amp and TTL circuitry (8V). I tried feeding the amplified signal into an op-amp to divide it to 30 Hz but the output was still 60 Hz and was inverted. I can't figure out why I can't get the flip-flop to divide the frequency. One problem I have noticed is the output of the function generator gets attenuated pretty bad with all the circuitry loading it down. Any thoughts?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Hi Bill,
    You really need to post your circuit schematic. It eliminates a lot of guesswork.
     
  3. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    I will get it drawn and post it sson.
     
  4. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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  5. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    Sorry, I have the capacitor polarities backwards, but this is the schematic for the circuit in question.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Oops - I think you meant to connect C1 and C2 to ground instead of the diodes. The way you've drawn your schematic, the diodes would short the transformer output to ground.

    Try it a bit more like the attached schematic.

    [eta]
    I think you meant 1uF for your caps. That isn't large enough; your ripple will be much too high even with a light load.

    I used 10uF in my simulation; even with just 1 opamp for a load you can see how bad the ripple is.
     
    Last edited: May 9, 2010
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    It also looks like the opamp would drive the divider clock with +/- 8V rather than +8 & 0V. If the logic ground is 0V, that would damage the IC.

    You should add a series resistor and clamp diode at the divider input.
     
  8. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    I see what you are saying about the caps shorting the output. When I put this on the protoboard I wasn't working off of a schematic. The caps are actually 1000uF. Multisim labels them as 1mF for some reason. The 1000uF caps leave very little ripple voltage but seem to attenuate the input voltage pretty badly.

    rjenkins,
    although the rail voltages for the op amp are +/- 8V, and the theoretical gain should be 10, the actual gain is only 2 with a 5V p-p input. The output is 10V p-p which is perfect for driving the flip flop. Also, the Vcc value at the input of the flip-flop measures 4.9V. I fear the function generator I am using isn't producing sufficient current to drive the circuit.
     
  9. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    I think this schematic better represents what I have built. Now that I think about it, I also have 1uF by-pass caps on pins 4 and 7 of the op-amp.

    [​IMG]
     
    Last edited: May 9, 2010
  10. SgtWookie

    Expert

    Jul 17, 2007
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    OK, so try it like the attached schematic.

    Right now, you don't have enough voltage going from ground to Vcc/Vdd, and you're going well below ground. This one with the transistor on the output should take care of that problem.
     
  11. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    Thanks, I will give that a try and let you know how it works out.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Oh, make sure you have the SET and RESET inputs connected to ground.

    Otherwise, they can float and cause problems.
     
  13. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    Thanks Sgt. Wookie. That circuit worked well, at least for a little while. The positive alternation of the input voltage became severely attenuated when I tried to run it through a second flip flop and wouldn't work after that. I'm going to wire a suicide cord into the transformer and try again.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    In other words, it worked until you changed it.

    Why don't you post what you really want to do, and then we can have something to work with? Right now, we have ziltch point diddley.

    Besides that what I posted worked, and what you did afterwards didn't work.

    Obviously, your needs changed - but we know nothing about your new requirements, because you have not told us about them.

    It's your choice.

    Puzzle about it until you've figured it out, or are tired of puzzling and give up.

    Or post where you are, and where you would like to be.

    We're really pretty nice about such things here. Generally speaking. For myself, at least.
     
  15. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    OK. I'm trying to clock a digital clock with the 60 Hz outlet voltage. I'm using IC's I already have. I have a 7476 dual J-K flip flop and a 74191 Hex counter. I am trying to halve the frequency twice to 15 Hz and then divide by 15 to 1 Hz. I have been using a GW Instek function generator to power the circuit but it apparently it doesn't have the power to drive the circuit at 16 V p-p. Once I have a working circuit I plan to use a PCB mount transformer that I have rated at .5 A with 6 V rms output on the secondary which explains why I chose a 16V input for the circuit.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Bill B,
    Is it working OK now, or do you need more help?

    Trying to explain things in text leaves a lot of guesswork, and a lot of assumptions to be made by the viewer. Having a schematic available eliminates nearly all guesswork; and guesses without information are very frequently wrong.
     
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