EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: TheHippySeal on March 16, 2012, 11:23:55 pm
-
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 (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.
-
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.
-
Thanks for the reply. So micros are like that, but what about other ICs? Particularly that Schmitt trigger.
-
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.
-
Ok. Thanks for the info.
-
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 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 8)
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.