Many times we have found itself in the necessity to use a Zener diode and cannot to know the voltage of operation. Many times we cannot read the characteristics or the type that is written on her. In this case, but in different we can also use the circuit of Fig.1. The T1, D1 and C1 they constitute a system of half-wave rectification. If push the switch S2 a DC current passes in from current limiter Q1 and the diode Zener DZ which we check. Q1 stabilizes the current in the price of 10mA independent from the zener voltage that we have connected in the connector J1. In the connector J2 we connect a digital voltmeter. There we see the voltage in the zener DZ. The circuit is reliable for measurements down to 25Volts. You have clue of low voltage 0.8Volts likely the diode DZ, is connected reverse. Change the polarity of diode DZ.
Q1=2N5363 [N-Channel JFET] S1=ON/OFF SW For 230Vac 1A DZ=Diode Zener Under Test
D1=1N4002 S2=Push Button Switch [Normal Open] D.V.M=Digital Volt Meter
C1=470uF 50V T1=230Vac//12Vac 300mA Transformer
J1-2=2pin conn. with 3.96mm pin step