Please advise on pull up/down resistors and use of them.

Discussion in 'General Electronics Chat' started by AgeingHippy, Nov 28, 2010.

  1. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
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    Hi all

    I am currently working on a 3D printer project http://www.reprap.org which I am finding every interesting. It is exposing me to a great deal of disciplines I have never worked with. One being electronics.

    I am currently trying to design a stripboard mount for a pololu stepper driver and although I have found a number of solutions none are quite what I want.

    Anyway - to my specific issue. The board provides 3 pins which are used to tell the chip the level of microstepping required. All the way from full stepping down to 1/18'th of a step.

    The MS2 and MS3 pins have internal pulldown resistors and I will be adding a pulldown resistor to MS1 to ensure it does not float when I wish to do full stepping.

    I am connecting DIP switches to these 3 pins so I can manually set the microstepping level by pulling the pins high.

    I am confused though, since I imagine the 5v side would have a resistor to prevent too much current flowing through the pins in the event of a fault condition. If I place a resistor on the 5v side and throw the dip switch we have the pin connected to GND via the pulldown resistor and to 5v with another resistor. I cannot understand what the pin will actually be at. High or Low? I want it to go high if I throw the DIP switch.

    Hope you guys understand my question and can point me in the right direction.

    thanks
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    As I understood, you want to put a resistor in series with the DIP switch. To ensure the input will receive a logic high when the DIP switch is closed, use a resistor with at least 10 times smaller value than the pull down resistors.
     
  3. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
    16
    0
    magic

    Thanks Mik3

    So the pololu docs states 100k resistors are used for the pulldown so I will use a 10k resistor for the DIP 5v side.

    Another question - there are 3 DIP switches. would it be adequate to connect the 5vside of all 3 DIP switches to 5v using only 1 10k resistor rather than a resistor for each switch?

    Thanks
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
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    Doubt it, then there would be one logic line instead of many. Resistors are cheap, around here 3¢ each. What part of the world are you? It help recommend certain parts.
     
  5. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
    16
    0
    Hi Bill

    I am in York, in the UK.

    I have plenty resistors but using one will not result in 1 logic signal rather than 3 as there will be a DIP switch between the pins and the resistor, each pin with it's own DIP switch that can be thrown independently if you understand.

    Something like this is what I am thinking...

    Code ( (Unknown Language)):
    1.  
    2.            |--DIP---MS1
    3. 5v--10kΩ---|--DIP---MS2
    4.            |--DIP---MS3
    Cheers
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    2,534
    A pullup or pulldown resistor can be on the output of a chippie, but it is also for the input of a gate (or whatever) so they aren't floating even if they aren't selected.

    Without knowing your exact application it is hard to say.

    Here is a rough concept drawing...

    [​IMG]
     
  7. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
    16
    0
    Hi Bill

    The 3 pins, MS1, MS2 and MS3 are all inputs which are used to set the level of microstepping of the attached motor. MS1, 2 and 3 are pins on the stepper driver.

    Cheers

    PS Bill - you are only 4 posts away from hitting 10,000 !!! WOW - fairly active then :)
     
    Last edited: Nov 28, 2010
  8. Wendy

    Moderator

    Mar 24, 2008
    20,764
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    If they are inputs, then they probably should each have a resistor as shown.

    Thanks, been active here for 3 years. If you hang around at all you'll find yourself picking up a lot of electronic info, and other stuff besides. We're a pretty friendly bunch around here, and when the occasional troll pops in they are either set straight or banned (which is rare).

    Feel free to ask questions, there are no dumb ones. The text book on the top of this page is pretty good too. I help write and correct a little.

    I especially like do nothing circuits, the kind that blink and mesmerize. :D
     
  9. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
    16
    0
    Thanks Bill

    I will stick a resistor on each, although I do feel 1 should be sufficient.

    I must say just as I think I am starting to understand a small part of how things work I discover my understanding is totally wrong :cool: but there you go - keeps me on my toes and off the streets lol

    I am sure I will be coming back with many questions - most probably pretty dumb. I have a lot to learn and am only just starting on this incredible journey.

    later
     
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