CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@sasikumar.p • 09 Sep, 2012
Why do logic gates only work on DC? Why can't we use AC in those components? I want to know the reasons behind it. Please explain in detail.
@Abhishek Rawal • 09 Sep, 2012 • 1 like gates are micro components and AC can damage or even burn/melt the CMOS transistors of logic gates.
@Jeffrey Arulraj • 10 Sep, 2012 what is DC it has zero frequency that is it

making a IC work in such conditions is much simpler than making it work in AC pulses

in general IC work only in DIGITAL PULSES and not in DC to make my point clear

the variations in DIGITAL pulses make them work not DC supply

we just power the IC with DC SUPPLY if we do it with AC the ic turns on and off n number of times within a second
@Nishant Patel • 10 Sep, 2012 its not like we can't use gates in AC.... because they are made up of CMOS,FET or BJT etc. they can function in AC as well.
but our all application is related to digital now-a-days,
so gates(uni. nand,xor;and,or,not) are made for specially for digital circuits..where decision has to be made b/w only true or false(according t diff. logic)..both have dc value(or range)..that's it.
hope it is justified.
@Guru Karthik • 10 Sep, 2012 In digital ICs there are just two levels of operation. A '1' and a '0' which is set by the presence and absence of a DC signal(supply and ground).
@rohit_jun22 • 15 Sep, 2012 you could apply AC as well but DC is required to bias or to "turn on" the internal devices such as CMOS, FET's, transistors etc.
@kiransabarish • 03 Oct, 2012 The gates are mostly ICs that can work at a maximum of 5V, not more than that. They usually see their input as HIGH or LOW signals. Mostly the voltage between 4.5-5V is taken as High, and the voltage between 0-1V will be taken as Low. So, if you use an AC (Even if its range is between -5V to 5V), then it has several states in between these two peaks. The IC cannot understand its input so, it can't work.
@Jeffrey Arulraj • 03 Oct, 2012 it is better said the frequency of signal and the time for state change is lower and so the response of ICS never are accurate in AC supply
@Ramani Aswath • 03 Oct, 2012 If you look at it purely from the logic point of view, a negative going potential is the opposite of the statement and so goes against the logic.
@Jeffrey Arulraj • 03 Oct, 2012 Do you refer the Negative half cycle of any AC signal here sir
@Ramani Aswath • 03 Oct, 2012
jeffrey samuel
Do you refer the Negative half cycle of any AC signal here sir
That is right.
However, as you and Patel said above, it is the truth table that is relevant. So '0 V' (actually low voltage) is one state and high (usually above 4 V) is the other state. The negative half cycle does not convey any logical meaning.

Analog audio circuits only use AC signals because the full wave form is needed for this.
@arunchary • 05 Oct, 2012 logic gate are developed using different technologies like ECL,TTL,CMOS etc..
accordingly the voltage levels for on and off switching will varies.all are works only on DC not on AC.very less amount of currents and voltages are required because of TRANSISTOR configuration.so basically transistors(not power transistors) are opereated in DC votages.
@Jeffrey Arulraj • 05 Oct, 2012 No transistor is trigerred by a AC signal we use a DC or square pulse to trigger a transistor

Transistors amplify the AC signals

Kindly don't confuse your self between them
@arunchary • 06 Oct, 2012 but i already mentioned not signal transistor ,though your right my pal...
@arunchary • 06 Oct, 2012 one more thing bjt is used to give phase reversal output so when we are giving positive (logic 1) means negative peak ( logic 0) will come its in TTL like that each family is varis with component configuration...
@Jeffrey Arulraj • 06 Oct, 2012 No that is not the case

In a CE amplifiers the input Analog signal is given as a op with an 180 degree phase shift

And this phase reversal does not have any role in the reversal of the Supply voltage given at the collector terminal (Vcc)

And so there is a slight error in your statement
@arunchary • 08 Oct, 2012 ok.
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