# Current Source vs. Current Sink

Discussion in 'Homework Help' started by tadpole, May 7, 2008.

1. ### tadpole Thread Starter New Member

May 5, 2008
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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. ### Audioguru New Member

Dec 20, 2007
9,411
896
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. ### tadpole Thread Starter New Member

May 5, 2008
2
0
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. ### mik3 Senior Member

Feb 4, 2008
4,846
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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.

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
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. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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That would be conventional current. Not "electron flow."

7. ### Vincenzo1309 Active Member

Dec 28, 2008
57
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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?

Regards,

8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
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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9. ### Vincenzo1309 Active Member

Dec 28, 2008
57
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Hi Ron H,

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

Regards,

10. ### Bernard AAC Fanatic!

Aug 7, 2008
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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.

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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I agree (if it's an NPN). Can you link to a datasheet that uses the opposite convention?

12. ### Bernard AAC Fanatic!

Aug 7, 2008
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Just as soon as I made the post ,started looking for confirmatoin,it was only about e week ago that i saw the sink reff, and made a "mental" note to remember where-big mistake, should have been in a log. I'll keep looking. Signetics used "supply current" for both conditions[no change of sign.]. 1972, but that was'nt the one that caught my eye.

13. ### Bernard AAC Fanatic!

Aug 7, 2008
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This was it,I think.: NATIONAL SEMICONDUCTOR. CMOS DATA BOOK, Cat# 62-1375, Pub. 1977; CD4511BC , " Output [ source ] VDD=15,I-OH=20mA, V-OH=13.95V " ;" Output [sink ] , V-DD = 15V,V-OL= 1.5V, I-OL= 7.8 mA". The output high side drives [sinks] the anode of a LED, cathode at ground. Let it just be user beware.

14. ### Ron H AAC Fanatic!

Apr 14, 2005
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657
I don't get it. Output high into a grounded LED is sourcing. Maybe I don't understand what you are saying.