Current sources vs sinks

[tggzzz]

I suggest people re-read and think about Tim Fox's reply #5 (and hence his reply #1).

[Doctorandus_P]

A few years ago I built a self powered current source / sink.

It has:

• 1n4148 as a 600mV voltage reference.
• LM358 that buffers / amlifies / stabilizes the reference.
• Potentiometer on the voltage reference  ( Final current approximately between 10mA and 1.5A)
• Shunt resistor to GND
• LM358 (other half) to compare the reference with the shunt resistor voltage.
• Sziklay pair as current amplification driver
• Heatsink
• Some *&^%$#@! to get the thing stable. (That took me a few hours)

I know a silicon diode is not very stable / accurate, but the goal here was to make a self powered current source / sink. Therefore it was important to use low voltage parts. The thing from a voltage as low as 2.8V and up to the maximum voltage of the LM358 (30+V).

Because it's self powered, it just has two connections, and you can use it either as a current source (with an external power supply) or as a current sink, depending on where you put it in the circuit.

[TimFox]

OK, so would you accept this as an accurate illustration of that distinction?

(Attachment Link)

It depends on whether the resistor is above or below the load, correct?

No. Your circuit is still supplying current to a load. Whether the current is positive or negative is irrelevant. It's a "source".

OK, so can someone please show me what a sink would look like? Preferably in schematic form? (Crayon on cocktail napkin sketch perfectly acceptable.)

A quick sketch, cut-and-pasted from a description of the 7404 TTL hex inverter.  Note that this original form of TTL logic is an example of "current-sinking" logic, along with DTL.  Later "TTL" types, including 74LS, are usually DTL.

Ignore the redundant labels from the original figure, but concentrate on the horizontal connection from the output of the left circuit to the input of the right circuit.  This circuit normally operates in one of two states:  "V_hi" and "V_lo" for the voltage on that wire, with corresponding currents along the wire in opposite directions.  The two states are labeled with brackets for [hi] and {lo}.  I added designators for Q1, Q2, and Q3.  The two inverters could be on the same substrate or in two different packages.
I apologize for the bad image quality, but my crayon was dull.

Now, in the {lo} state (logic 0 at that node), Q2 is off and transistor Q1's collector is sinking current from transistor Q3's emitter.  Q3 is powered from the Vcc node on the right.  Since Q1 is saturated, it presents a low impedance to that current flow I_lo sourced by Q3, and the voltage V_lo is quite low.

However, in the [hi] state (logic 1 at that node), Q1 is off, and Q2 is a source pulling up the node to a higher voltage V_hi.  The current I_hi for this IC series is much lower than I_lo.  Q2 is probably better considered a voltage source, rather than a current source, driving a relatively high impedance.  Q2 is powered from the Vcc node on the left.

On the data sheet, the manufacturer guarantees certain voltage and current levels for the gate in isolation.
The suffixes below were changed to correspond to data sheet specs.
For the first gate's output supplying the second gate's input:
    V_oh > +2.4 V for I_oh < 0.4 mA
    V_ol < +0.4 V for I_ol < 16 mA
The requirements at the second gate's input for definite logic levels are
    V_ih > +2.0 V at 0.04 mA  and   V_il < +0.8 V at 1.6 mA
so there is some noise immunity at the high and low logic states.
From the low-state sink current of the first gate, and the low-state source current of the second gate, we quickly calculate that one output can certainly drive 10 inputs (10:1 fanout).

Incidentally, I have never seen the term "voltage sink", although "voltage source" is quite common.  If one needed that term, it might be a relatively high-impedance load fed from the source.
A CMOS gate output is usually a switched voltage source driving a capacitive load at the driven inputs.

[Analog Kid]

1. Let's at least be able to view that circuit of yours properly (I just needed to adjust the contrast & brightness of your image):



2. Too complicated, with too many extraneous factors; I still can't really wrap my head around what's a current source and what's a sink.
Can someone post a simpler explanation (w/circuit), please?
Pretend you're explaining this to a total n00b.

[Benta]

I'm with you, horrible example.
But what's so difficult about a current source (I) vs. a voltage source (U)?
U supplies voltage (and current depending on load)
I supplies current (and voltage depending on load)

The sink/source thing in digital ICs is a misnomer for output current capability, but unfortunately commonly used.

[TimFox]

An ideal current source delivers a constant current regardless of the voltage across the load.  (Output shunt resistance is infinite.)
An ideal voltage source delivers a constant voltage regardless of the current through the load.  (Output series resistance is zero.)
Real current and voltage sources do not achieve this ideal condition, but may be closer to one or the other ideal case.
Thévenin's Theorem for voltage sources and Norton's Theorem for current sources discuss the relationship between these two concepts.

[Analog Kid]

OK, let me try something else:

Say I give you the following kit of symbols:



Can you put these together to show us 1) a current source and 2) a current sink?
Feel free to add whatever other symbols are necessary (within reason!) for your examples.

And additionally, explain just exactly what an "external load" is?

TIA

[Andy Chee]

OK, let me try something else:

Say I give you the following kit of symbols:

(Attachment Link)

Can you put these together to show us 1) a current source and 2) a current sink?
Feel free to add whatever other symbols are necessary (within reason!) for your examples.

And additionally, explain just exactly what an "external load" is?

TIA

The load resistor symbol should probably be a variable resistor, rather than a fixed resistor.

As for "external load", think of it as the opposite of internal impedance 

[PGPG]

The sink/source thing in digital ICs is a misnomer for output current capability, but unfortunately commonly used.

In my opinion simply the same words happen to mean different things depending on context.

Save in context that the device will not kill a person means completely different thing than save in a context of cryptology algorithm.

(Ideal) Current source in circuit simulation is different thing than digital output working as current source (or sink). Saying that such output sources current doesn't mean it is current source from circuit analysis point of view.

I think I feel perfectly what 'source' means but only partially what 'sink' means but in digital output context I think of them as follows:
If you connect capacitor to the output (second pin grounded) than:
- when output sends a charge into it than this output sources charge (works as current source).
- when output takes charge from capacitor than this output sinks charge from capacitor (works as current sink).
I don't know if in English saying that something sinks something form something is acceptable use of word sink.

In my opinion using source/sink how they are used related to digital output is correct use. In this context these words simply means something different than when you use them in context of energy source and sink and in a similar sense.

[golden_labels]

Say I give you the following kit of symbols: (…) Can you put these together to show us 1) a current source and 2) a current sink?

Here:

And additionally, explain just exactly what an "external load" is?

Load that lies outside of the current source or sink. Which, for the topic discussed here, is usually⁽¹⁾ any load.

⁽¹⁾Not strictly true, because practical source implementations may need an internal load. But this is unnecessary detail here.

[Analog Kid]

First of all, thanks for playing along here. It's appreciated.

So I get the top example; pretty straightforward.
But the second one: what's going on there? The "sink" is, what? just a wire inside that box?
What exactly is the sink here?
What exactly does it mean being either inside that box or outside of it? What does the box itself represent?

[golden_labels]

First of all, thanks for playing along here. It's appreciated.

Throwing away reference points is intentional. Focus on the loops and where the energy comes from, not on the polarities or to which rail a component is connected.

But the second one: what's going on there? The "sink" is, what? just a wire inside that box?

Yes. An ideal sink is a short. See TimFox’s post.

What exactly is the sink here?

The entire blue box with word “Sink” in the upper part. As seen from the perspective of the circuit on the right.

What exactly does it mean being either inside that box or outside of it?

Inside the box is the source or the sink. Outside is the circuit, that considers this box to be its source or sink.

What does the box itself represent?

Either a source or sink, depending on what is written on it.

---edit
Perhaps I see, where the misunderstanding lies. Don’t look at what is inside a source or a sink. This is not what distinguishes them. The distinction comes from how the external circuit treats them.

The hydraulic analogy holds here well. Extremely well in fact, because both words (“source” and “sink”) come from the water meaning. Source is where the energy comes from. Sink is where waste energy is dumped to.

Observe: this is from the perspective of whoever/whatever uses the source or sink. Not from what is inside of that source or sink.

[TimFox]

In short, "source" and "sink" are distinguished by their function in the overall circuit.
The same subcircuit could be connected to function as either, depending on the requirements.