Author Topic: Using current-limiting resistor on Vcc pin of IC  (Read 122 times)

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Offline coaxialgamer

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Using current-limiting resistor on Vcc pin of IC
« on: December 08, 2017, 07:49:39 AM »
Couldn't find a clear cut answer when looking through google.
Im using 74F ICs, and while i do know that you need to put pull down resistors on inputs, and that you should use current limiting resistors on inputs as well, according to datasheet.  Should i use a resistor on the Vcc pin to keep the IC from burning up, or does it so that on its own?
 

Offline hermit

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Re: Using current-limiting resistor on Vcc pin of IC
« Reply #1 on: December 08, 2017, 07:54:38 AM »
You should not need resistors on the power inputs.  If they were needed the datasheet would have them listed/recommended.   As long as you have the proper voltage you are good.  The chip draws the current it needs.
 
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Online suicidaleggroll

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Re: Using current-limiting resistor on Vcc pin of IC
« Reply #2 on: December 08, 2017, 08:00:23 AM »
The above is correct.  Also the "current limiting resistors" on the inputs are simply to protect against accidental over/under voltage.  The IC has built in diodes to clamp inputs to the rails (inputs are not allowed to go more than one diode drop below ground or one drop above Vcc), but if you try to force the pin higher or lower with a low impedance source, you can burn out those diodes and damage the chip.  The current limiting resistors prevent that, but unless you actually try to drive the pin above/below the rails, they won't really do anything.
« Last Edit: December 08, 2017, 10:59:58 AM by suicidaleggroll »
 
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Offline Gyro

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Re: Using current-limiting resistor on Vcc pin of IC
« Reply #3 on: December 08, 2017, 09:04:48 AM »
It used to be common practice to put series resistors in the supplies of 4000 and 74C series CMOS devices in military gear 'back in the day'. These were to protect against latch-up during unpleasant events.

These days CMOS logic devices are so resistant to going into SCR latch-up, due to excessive current on the inputs, that it's pretty much a non-issue. Series resistors to limit current on inputs that go external to the board or may exceed the supply rails are all that are that are needed. The Absolute Maximums section in the datasheet will give you the appropriate information.
Chris

"Victor Meldrew, the Crimson Avenger!"
 
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