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dhruv.narwat
dhruv.narwat • Aug 29, 2008

problem with 74LS244 buffer

hi everyone
I have made a circuit using AT89C4051 and some of the inputs and outputs i have buffered using LS244.i needed to have some delay for buffer,so i used a simple RC delay circuit with value C=10uF and R=10K and connected the output across resistance to enable pins of buffer.
now the problem arises here.according to simple equation time constant(Tc)=RC=100ms , the buffer should get enabled only after 100ms BUT the voltage at enable pin comes down to 1.15V and stays there..after some time when i keep on checking with probe then it comes down to 0.5V i m not able to understand it. please give me some suggestions

also its affecting voltage level of other buffer port pins???
shadeslayer
shadeslayer • Aug 29, 2008
you must be knowing the concept of how the multimeter works,, so you are using RC circuit and capacitor stores the charge and it will gradually go down due to leakage ,,which depends on your resistance value..now when you try to test voltage using multimeter then cap may be discharging using your multi meter,, that can be the reason of that ..
dhruv.narwat
dhruv.narwat • Aug 31, 2008
but why it stays at 1.15V and doesnt come to almost zero volt without checking with multimeter.
reachrkata
reachrkata • Sep 1, 2008
😕
I think for me to get a better idea (perhaps for the others too) can you just share with us the schematic if the buffer part with enable signal.

-Karthik
just2rock
just2rock • Sep 2, 2008
Druv,
regarding buffer voltage going down is only because of capacitor discharge theory,& reason why stagnant at 1.15V is due to buffer/cap characteristics.
For efficient Delay manupulation Multivibrators(mono/bistable) is the best choice.It will be better if you can show me schematic of the same.
REgards,
J2R

dhruv.narwat
hi everyone
I have made a circuit using AT89C4051 and some of the inputs and outputs i have buffered using LS244.i needed to have some delay for buffer,so i used a simple RC delay circuit with value C=10uF and R=10K and connected the output across resistance to enable pins of buffer.
now the problem arises here.according to simple equation time constant(Tc)=RC=100ms , the buffer should get enabled only after 100ms BUT the voltage at enable pin comes down to 1.15V and stays there..after some time when i keep on checking with probe then it comes down to 0.5V i m not able to understand it. please give me some suggestions

also its affecting voltage level of other buffer port pins???

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