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A zener diode can be used to set the Base voltage of Bipolar Junction Transistor (BJT) Current Source. That voltage, offset by approx. 0.6V, will be across the Emitter’s current setting resistor regardless the supply voltage. As long as the supply voltage is high enough to power everything in the circuit.
NPN BJT drops approx. 0.6V from the Base to Emitter. A 5.6V zener diode, will translate to there being 5V across the Emitter resistor.
If you use a 500Ω resistor, then with 5V across it, you will have 5V/500Ω = 0.01A of current flowing through it. 0.01A is usually referred to as 10mA.
Ultimately, that current also flows through the collector. The collector load, or lack there of, does not affect how much current flows as long as there is enough supply voltage to power everything.
Since the load is on the high side of the NPN BJT current source, it is technically correct to call the circuit a current sink. But, any time there is a component that sets current, it’s OK to call it a current source. Whether it is sinking or sourcing the current.
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