Which is the correct way to place the diodes? Would it make any difference?

Discussion in 'General Electronics Chat' started by The Chosen 0ne, Apr 27, 2016.

  1. The Chosen 0ne

    Thread Starter New Member

    Apr 15, 2014
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    Hello, I found a circuit online for a 555 PWM controller.
    dimmer.png >> This is the original schematic.
    But if I change the direction of the diodes, would it make any difference?
    dimmer2.png >> This is the one with the direction of the diodes switched.
    When I try the two different circuit, they act different and the potentiometer also becomes slightly unresponsive with the second circuit. Why does this happen?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Check your potentiometer. It might be tuned for audio circuits (Log taper) instead of linear taper which would be best for this circuit. If it says "100k A" or A100k then it is log.
     
  3. The Chosen 0ne

    Thread Starter New Member

    Apr 15, 2014
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    Thanks for the quick response. So, changing the direction wouldn't make any difference?
    And I checked my potentiometer, its a linear trim pot. Works perfectly with the first layout.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It shouldn't in that circuit. Also, that is a messed up schematic. Make sure you put it together based on PIN numbers because the chip is not drawn correctly - the function of each pin on a real chip are not located as shown in the drawing.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I've always preferred this circuit for Pwm.

    image.jpg
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    The difference is that the duty cycle would be the greatest when the pot is full clockwise in one case and when it is fully anti-clockwise in the other case.
     
  7. The Chosen 0ne

    Thread Starter New Member

    Apr 15, 2014
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    Oh great! Yeah, I have made sure to use the actual pins to build the circuit; the pins are ordered differently on the diagram. And thanks for the second schematic, I will make sure to try it out tonight.

    Thanks for all your help guys!
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The only thing with using that for PWM, is if the frequency is "critical" in the application, adjusting the pot for duty cycle also changes the frequency of the output.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I disagree. The primary job of a schematic symbol, like all of a schematic, is to show the electrical connections and relationships, not be a preliminary pc board layout. Often you can get both out of one drawing, but the relationships must come first. How would your approach apply to an opamp or a presettable counter symbol, and how would it improve the readability, let alone understandability of a schematic?

    I have 4 different 555 symbols in my library. Each one has the pins arranged for a particular circuit, so the drawing doesn't have a bunch of crossed lines like the very common pin 2 to pin 6 connection, or tying the reset to Vcc (pin 4 to pin 8). As long as the pin numbers are correct, I say put them where the drawing looks best and tells the story in the most clear language. I've not used the comparator (triangle) 555 symbol I've seen around here (because I've not yet seen it on a commercial drawing), but I completely agree with its philosophy - at its heart a 555 is a comparator, so draw it like one.

    ak
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I just prefer when one or two (reset and power) are on the top and bottom. Why leave that real estate empty? The schematic symbol that uses two rows of four seems to fool newbies into thinking it looks like the DIP-8 so it must be that format. I try to avoid the 2 rows of 4 format because of this common error with newbies.
     
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