sentences:  The breakdown voltage of diode D is stable over a wide current range and holds "U"out approximately constant even though the input voltage may fluctuate over a wide range.Because of the low impedance of the diode when operated like this, resistor "R" is used to limit current through the circuit.In the case of this simple reference, the current flowing in the diode is determined using Ohm's law and the known voltage drop across the resistor "R";
  
  The value of "R" must satisfy two conditions