# sinking vs sourcing

Discussion in 'General Electronics Chat' started by alexmath, Nov 24, 2014.

1. ### alexmath Thread Starter New Member

May 2, 2014
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Hello everyone! I have trouble understanding the sinking and sourcing concepts and i hope you can make me understanding them.
There can be active low and active high inputs and also active low and active high outputs?
If the logic gate is active high that means it's sourcing the current and when the output it's low it's sinking the current?
In the picture below, why the outputs are negated? Is this meaning that the dcd is always sinking the current? Also why the nand gates are negated?
And why the order of inputs is switch? Why isn't: ABCD? Thank you so much for your help!

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2. ### Papabravo Expert

Feb 24, 2006
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There are 16 possible boolean functions of two boolean variables. NOT-AND, or NAND for short is one of those 16 functions. It is also one of the functions that is functionally complete. That means that you can use just the NAND function to realize any boolean function. It is also economically implemented in discrete and integrated circuitry. To interpret the meaning you say: "a HIGH AND a HIGH is a LOW". The deMorgan equivalent is: "a LOW or a LOW is a HIGH"

3. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,222
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hi,
For the ABCD part, is conventional to have A as the least significant bit [LSB] so DCBA.

In your circuit dont think in terms of sourcing and sinking curreny, think Logic High or Low levels. '1' or '0'

Jul 18, 2013
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This may help.
Max.

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5. ### alexmath Thread Starter New Member

May 2, 2014
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Thank you for your answers! I understand i have to think in logic terms, but all i wanted to know is that if a logic gate or something else is active low that means it can never source current, but sinks current. Is that right? It's strange for me how current is flowing the opposite direction (from output to inputs).

6. ### crutschow Expert

Mar 14, 2008
16,175
4,320
When current goes out of an output it is sourcing current.
When current goes into an output it is sinking current.

GopherT likes this.

Jul 18, 2013
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And as seen in practical terms, a switch taking an input to supply common is sinking and the input itself is sourcing.
Max.

8. ### BobTPH Active Member

Jun 5, 2013
883
152
Active low vs active high has nothing to do with sourcing and sinking of current. These designations merely tell whether a logic low or logic high is the "active" state. For instance, if a light detector has an active low output, it means that the output will be a logic low when the detector sees light. Active low and active high can apply to both inputs and outputs.

Sourcing and sinking applies only to outputs and tells whether or not current can flow out (when high) or into (when low) the output.

Bob