Current Limiting Resisters

• Current Limiting Resisters
  Posted by TheHippySeal on 16 Mar, 2012 23:23
• Hi guys. How do I figure out if a device needs a current limiting resister on VCC or GND? Atmel micros don't seem need one on either, only the inputs or outputs. Is this the same for all ICs? The device I need to know at the moment is this Schmitt trigger.
  
  http://www.ti.com/lit/ds/symlink/sn74ls14.pdf
  
  In my circuit the trigger input comes from an external source, then the trigger output goes to the micro input. Should I put the resister before or after the Schmitt trigger?
  
  Cheers.
• #1 Reply
  Posted by Psi on 16 Mar, 2012 23:39
• I dont quite understand why you'd want a current limit on vcc
  
  The micro will only draw the current it needs under normal conditions. So you don't need a limiting resistor in series with vcc.
  
  If you're worried about overloading the micro controller outputs then you can current limit individual pins with a resistor so the short circuit current cannot get above the maximum.
• #2 Reply
  Posted by TheHippySeal on 16 Mar, 2012 23:48
• Thanks for the reply. So micros are like that, but what about other ICs? Particularly that Schmitt trigger.
• #3 Reply
  Posted by Psi on 17 Mar, 2012 00:10
• Quote from: TheHippySeal on 16 Mar, 2012 23:48

  Thanks for the reply. So micros are like that, but what about other ICs? Particularly that Schmitt trigger.
  

  
  I cant say i've ever seen a IC that requires a resistor in series with VCC.
  You just supply the IC with a voltage that is within its valid supply range given in the datasheet.
• #4 Reply
  Posted by TheHippySeal on 17 Mar, 2012 03:14
• Ok. Thanks for the info.
• #5 Reply
  Posted by andyg on 17 Mar, 2012 03:34
• If you really want to limit the current draw on VCC due to say a fault, then you can use a PTC Resettable Fuse at VCC's source.
• #6 Reply
  Posted by wkb on 17 Mar, 2012 07:37
• Quote from: andyg on 17 Mar, 2012 03:34

  If you really want to limit the current draw on VCC due to say a fault, then you can use a PTC Resettable Fuse at VCC's source.
  

  
  If you need fuses in the supply line of a single digital IC or MCU it is dead already anyway 
  
  Series resistors in supply lines of digital ICs I have never seen.  You would not like the end result anyway, as it limits
  the speed of the IC and it could also make the logic levels go haywire.