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  #1  
Old 05-07-2008, 10:47 PM
tadpole tadpole is offline
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Default 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?
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Old 05-08-2008, 12:07 AM
Audioguru Audioguru is offline
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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.
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Old 05-08-2008, 12:43 AM
tadpole tadpole is offline
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Originally Posted by Audioguru View Post
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).
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Old 05-08-2008, 12:48 AM
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mik3 mik3 is offline
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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.
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Old 05-08-2008, 01:29 AM
Audioguru Audioguru is offline
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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.
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Old 05-08-2008, 01:32 AM
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Quote:
Originally Posted by mik3 View Post
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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Old 04-27-2009, 01:05 AM
Vincenzo1309 Vincenzo1309 is offline
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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?

Kindly advise.

Regards,
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  #8  
Old 04-27-2009, 03:20 AM
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Ron H Ron H is offline
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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.
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File Type: png open collector source-sink.PNG (15.6 KB, 106 views)

Last edited by Ron H; 04-27-2009 at 03:30 AM.
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  #9  
Old 04-27-2009, 03:24 AM
Vincenzo1309 Vincenzo1309 is offline
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Hi Ron H,

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

Regards,
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  #10  
Old 05-08-2009, 03:33 AM
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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.
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