source and sink current

what is meant by source current and sink current. please explain..

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All the best!

sorry rohit.
I googled but I couldnt get clearly. tats y I asked. anyway thanks.

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how a conductor can source and sink different current. for ex. 8085 microprocessor buses can source 400microamps and sink 2milliamps of current.

source = produces an output current into an external load
sink = draws current from an external source

In the context of a microprocessor pin, there is an output transistor (the source switch) to VDD and an output transistor (the sink switch) to VSS. They are never turned on together because it would short out VSS and VDD causing damage. Only one at a time is turned on and they allow the logic level to go high so it can produce a current into the load or low so it can pass current to VSS (usually ground). The current ratings you refer to are how much current it can pass in each direction.

Brian.

betwixt said:

source = produces an output current into an external load
sink = draws current from an external source

In the context of a microprocessor pin, there is an output transistor (the source switch) to VDD and an output transistor (the sink switch) to VSS. They are never turned on together because it would short out VSS and VDD causing damage. Only one at a time is turned on and they allow the logic level to go high so it can produce a current into the load or low so it can pass current to VSS (usually ground). The current ratings you refer to are how much current it can pass in each direction.

Brian.

Click to expand...

thanks, if both vss and vdd are connected to seperate output transistor, then what is the problem when both are turned on together.
and also why they have different current carrying capacity in two directions.?

Maybe this can help: https://www.dataq.com/blog/faqs/whats-all-this-sink-and-source-current-stuff/

U can figure out the difference between currents (in magnitude), because differents paths have different resistance, and depend on the external sources as well.
They can be equal, doesn't matter, the difference is essentially in the direction of the uC pin (configured as input or output, if it can be bydirectional).

Regards

The pin sits at the junction of the transistor connected to VDD and the one connected to VSS. If the top one (the one connected to VDD) is turned on and the other is off, the pin is connected to VDD and current can flow out of it. If the bottom one (the one connected to VSS) is turned onand the top one is turned off, the pin is connected to VSS and current can flow in to it from circuitry outside the device. If both are turned off at the same time, the pin 'floats', it can not source or sink current and is said to be in 'tri-state' condition, in other words internally disconnected and neither sourcing or sinking current. If you turn them both on at the same time, which is not normally allowed by circuitry inside the device, current flows through both transistors and shorts out VDD to VSS through them. The current they would carry would almost certainly overheat them and destroy the device.

The amount of current they can source or sink depends on the construction of the output transistors and is decided by the manufacturer. Obviously it has to be enough for it to do it's job but making the transistor larger so they can handle more current than necessary would add to cost and also make the device work slower.

Brian.

betwixt said:

The pin sits at the junction of the transistor connected to VDD and the one connected to VSS. If the top one (the one connected to VDD) is turned on and the other is off, the pin is connected to VDD and current can flow out of it. If the bottom one (the one connected to VSS) is turned onand the top one is turned off, the pin is connected to VSS and current can flow in to it from circuitry outside the device. If both are turned off at the same time, the pin 'floats', it can not source or sink current and is said to be in 'tri-state' condition, in other words internally disconnected and neither sourcing or sinking current. If you turn them both on at the same time, which is not normally allowed by circuitry inside the device, current flows through both transistors and shorts out VDD to VSS through them. The current they would carry would almost certainly overheat them and destroy the device.



Brian.

Click to expand...

thanks for detailed explanation. could you please make sense with my example of 8085.

The simplest way to describe them is:

• source current: provide positive voltage
• sink current: provide ground

Depending on the device the output driver can be a push-pull configuration in which there are two switching devices (high side/low side) or an open collector/open drain stage in which there is only a single switching device connected as a low side switch.

The push pull driver can sink and source current equally strong because depending on the output state the appropriate switch can turn on.

On the other side with an open collector/open source configuration what you have is just a low side switch, when it is ON it sinks current (provides ground to the load), when it is off it leaves the output floating (presents a high resistance to the ground).
In this case the output would be undefined when the transistor is off so it is normally used with a pull up resistor connected from the output to the positive supply, this resistor can force a high state to the output but can't provide much current.