Sinking mode.

Thread Starter

Voltboy

Joined Jan 10, 2007
197
Hello.
I was reading about how to power LED's to microcontroller and I saw Sink and Source mode, I undestood the source mode very well. But the sink mode didn't made much sense to me.
How if a pin is low, the led will be on, doesn't the pin when its low is like an unconnected pin, or does it connects to ground?

Thanks :rolleyes:
 

Distort10n

Joined Dec 25, 2006
429
If the output is similar to a NPN inverter, and the LED is connected to the collector then this is an example of sinking current. The transistor is turned on, the cathode of the LED is brought close to GND, and pulls current from whatever supply it is connected to.
 

Thread Starter

Voltboy

Joined Jan 10, 2007
197
In a PIC microcontroller, if the output of a pin is low it means it is like an unconnected pin, or it means that it is closed to ground?
 

bloguetronica

Joined Apr 27, 2007
1,372
Hello.
I was reading about how to power LED's to microcontroller and I saw Sink and Source mode, I undestood the source mode very well. But the sink mode didn't made much sense to me.
How if a pin is low, the led will be on, doesn't the pin when its low is like an unconnected pin, or does it connects to ground?

Thanks :rolleyes:
When a pin is low, normally it presents 0V, relative to ground, which means that the voltage difference between that pin and Vcc will be the same of Vcc. Therefore, a LED connected to Vcc (making the IC sinking current) will light it the pin is low, but not when the pin is high.

TTL is often used to light LEDs this way, because it can sink up to 16mA, but can only source 2mA. Therefore, sourcing a LED with TTL is not a good idea.
 

thingmaker3

Joined May 16, 2005
5,084
If the TTL output is not open collector, then it will usually be some variation of a totem-pole. The upper transistor will have a collector resistor, but the lower transistor will not have an emitter resistor.
 

Thread Starter

Voltboy

Joined Jan 10, 2007
197
Ohh thanks.. now I get sinking and sourcing..
Sinking occurs when the lower pin of 'other stuff' is high and sourcing occurs when the higher pin of 'other stuff' is high?
 

thingmaker3

Joined May 16, 2005
5,084
Yes. The lower transistor sinks and the upper one sources. The upper one is in series with a resistor which will limit current.

Here's another look at sinking and sourcing:

sinksource.jpg
 
Top