Shift register 74HC165 current limiting?

Thread Starter

poddy

Joined Mar 31, 2005
4
Hi all,

I'm totally new to this hobby, so please forgive me if my question is too stupid...

If I wan't to use a shift register similarly as below how can i make sure that the current through the chip isn't too large? Is there some internal resistance for the input pins? If I connect a +5V battery to the +5V and flip the switch then, without a resistance, the current would be very large, no?
I hope my question makes sense and you can help me out.

Thanks!
- HP
upload_2019-4-3_13-24-24.png
 

ericgibbs

Joined Jan 29, 2010
8,733
hi poddy,
The 74HC165 will only draw a few micro amps from the 5V supply,
The 470R in series with the LED is also used to limit the current drawn thru the 74HC
E
 

dl324

Joined Mar 30, 2015
8,891
Is there some internal resistance for the input pins?
It would be out of the norm for logic IC's to have current limiting resistors.

The information you want is typically in the datasheet.
upload_2019-4-3_11-54-57.png
For 74HC165, the recommended maximum output current depends on supply voltage.
If I connect a +5V battery to the +5V and flip the switch then, without a resistance, the current would be very large, no?
Large is a subjective term. Large enough to destroy the LED? Probably not. Large enough to damage the shift register? Possibly.

74HC outputs will withstand brief shorts. This is from a 74HC guide:
upload_2019-4-3_12-4-57.png
 
Top