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#1
05-07-2008, 10:47 PM
 tadpole New Member Join Date: May 2008 Posts: 2
Current Source vs. Current Sink

Consider an open-collector output connected to a totem-pole input. Do I understand correctly that the open-collector output is a current sink whether its logic value is HI or LO?
#2
05-08-2008, 12:07 AM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

An open collector output can sink a current to 0V but it cannot source a current. It needs to have a pullup resistor or transistor added for its output to source a current and be high.
#3
05-08-2008, 12:43 AM
 tadpole New Member Join Date: May 2008 Posts: 2

Quote:
 Originally Posted by Audioguru An open collector output can sink a current to 0V but it cannot source a current. It needs to have a pullup resistor or transistor added for its output to source a current and be high.
So if a pull-up resistor of appropriate value is properly connected between the output and the supply voltage, the output logic is HI, and the output is driving a TTL input, is the output said to be sourcing current to the TTL input?

I understand how the connections are made (and how to calculate an appropriate value for the pull-up resistor), but the definition of "source" and "sink" are somewhat unclear to me. I thought a gate output was called a source if the electron flow was going into it, and a sink if the electron flow was coming out of it. I understand that for a properly connected open-collector output, the electron flow comes out of the output whether the output logic is HI or LO (although the current is very small if the output is HI).
#4
05-08-2008, 12:48 AM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

If we say an output source current it means the current flows out of the chip. If way say the output sinks it means the current flows in the chip. By the way you dont need to have a pullup resistor to drive a TTL input because a floating TTL input is taken as high.
#5
05-08-2008, 01:29 AM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

Most circuits use a positive power supply. If an output goes high (positive) then it sources.
If an output goes low then it sinks.

It is best to drive a TTL input high. TTL outputs force the next input high and do not allow inputs to float. Floating inputs go high very slowly. Floating inputs pickup interference.
#6
05-08-2008, 01:32 AM
 thingmaker3 Super Moderator Join Date: May 2005 Location: Rural, Oregon GMT -8 Posts: 5,072 Blog Entries: 6

Quote:
 Originally Posted by mik3 If we say an output source current it means the current flows out of the chip. If way say the output sinks it means the current flows in the chip. By the way you dont need to have a pullup resistor to drive a TTL input because a floating TTL input is taken as high.
That would be conventional current. Not "electron flow."
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#7
04-27-2009, 01:05 AM
 Vincenzo1309 Member Join Date: Dec 2008 Posts: 57

Hi

I think it is important to know about the source current, as we can determine how much current actually flows out of the chip. So we can determine how to amplify the current to drive our load, like using a transistor etc.

But why do we have to know what current actually flows into the chip (sink current)? Can it help us in any way?

Regards,
#8
04-27-2009, 03:20 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Does this help?
On first inspection, it may look like the emitter of Q1 is reversed biased when the input is high, so no current should flow. It is reverse biased, but the collector of Q1 is lower in voltage than the emitter, so their roles are temporarily reversed (inverted). The inverted transistor will have low beta, but current will still flow through the emitter (now the collector) junction.
When the emitter of Q1 is low, Q1 acts as a normal transistor.

EDIT: See the Wikipedia article on transistor-transistor logic if you want to see a typical TTL gate schematic.
Attached Images
 open collector source-sink.PNG (15.6 KB, 106 views)

Last edited by Ron H; 04-27-2009 at 03:30 AM.
#9
04-27-2009, 03:24 AM
 Vincenzo1309 Member Join Date: Dec 2008 Posts: 57

Hi Ron H,

Thanks a lot! It sure helps!
Have a nice day!

Regards,
#10
05-08-2009, 03:33 AM
 Bernard Senior Member Join Date: Aug 2008 Location: Tucson AZ USA Posts: 3,160

Just take source and sink with a grain of salt[ what ever that means ] ,because IC data sheets do not all agree with each other on the subject; maybe check date of publication.To me an open collector sinks.

 Tags current, sink, source

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