Current Limiting Resisters

[TheHippySeal]

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.

[Psi]

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.

[TheHippySeal]

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

[Psi]

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.

[TheHippySeal]

Ok. Thanks for the info.

[andyg]

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.

[wkb]

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.