Electronics > Beginners
74F08N current issue for ARDUINO
(1/2) > >>
vis5254:
I've a 74F08N AND GATE IC . I checked the datasheet and realised that the output voltage on HIGH condition goes to 5v and when output goes in LOW , current predominates and reach a value of 40mA . ARDUINO UNO digital pin has maximum withstanding capacity of 40mA . So what could be a better way in reducing 40mA to some 20 or 30mA ..??

Will a resistor in series will do the job ?
Moriambar:
Hi, I actually do not understand how you have connected this to the arduino ie: is it the inputs or the output of the and gate? How is the arduino pin mapped, and how is it physically connected.
The datasheet I pulled out for a 74F08N specifies an absolute maximum of 40mA when in low state, so my first guess is that something is shorted out or connected wrong...
Ian.M:
For logic, MCUs etc. as long as the supply voltage is the same and you connect a single output to N inputs*, it typically just works# - the inputs don't draw excessive current from the output.   However, you must never directly connect normal outputs% as if they are at different levels, its called contention and excessive current is likely to flow, which will probably damage them if it carries on for long (at a us timescale).   If there is a significant risk of contention, a series resistor may help - 330R is a good value for a 5V logic system as it will keep the worst case current under 15mA.


* Where N is less than the fanout limit for the logic family you are using.

# Provided you make any accommodations required to handle the differing logic levels when interconnecting different logic families e.g a bipolar TTL output will probably need a pullup resistor to drive a 5V standard CMOS input.

% The rules for wiring open collector or open drain outputs together are different - its safe, you need a pullup resistor, but the result is only high if *ALL* the outputs are high.   If any go low the low level wins.
vis5254:
I've a 7805 ic  for producing 5v .
I was trying to check if my AND would work .. I provided 5v from 7805 as vcc as well as same voltage as 2 input . I'm concerned about the current flow now .
Ian.M:
When dealing with bipolar TTL chips (basically anything 74nn(n) or 74xxnn(n) where xx doesn't contain the letter C), you need a pullup resistor to take an input to logic '1' as direct connection to +5V risks damaging the chip.  1K is a  suitable value.   OTOH to take an input to logic '0' you must wire it directly to Gnd, not through a resistor.   

If you put a separate 1K pullup on each AND input, it should output logic '1'.   Add a jumper wire to Gnd to put one of the inputs low, and the output should go to logic '0'. 
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod