Circuit lets you measure zener voltages and test LEDs

To measure a zener diode’s breakdown voltage, you need a dc voltage source whose voltage exceeds that of the zener voltage. In Figure 1, resistor R_SER provides voltage drop between V_INand V_ZEN. In any case, V_IN should exceed V_ZEN. Resistor R_SER must provide current, I_ZEN, that can keep the zener diode in reverse breakdown. That is, the current must be more than I_ZEN-I_ZENMINand less than I_ZEN-I_ZENMAX. You also need to consider the current that flows through the load. Otherwise, V_ZEN will be unregulated and less than the nominal breakdown voltage. Also, the power that the zener diode dissipates should not exceed the manufacturer’s specifications. Except for the value of I_ZEN-I_ZENMIN, all necessary data appears in zener-diode data sheets.

The circuit in Figure 2 uses one or two AA/AAA cells, which ensures testing irrespective of the value of the tested zener voltage to approximately 20 to 25V. The heart of the circuit is a ZetexLED driver, ZXLD381. It operates mainly from 1.5 or 1.2V cells, and it has a maximum input voltage of 10V. The LED driver generates constant-power pulses that charge output of the 10-μF capacitor, which requires low leakage current. The capacitor’s voltage provides constant current through R1 and tthe zener diode that connects in series. When you connect output probes to a digital multimeter’s V and COM sockets, you can directly measure the zener’s voltage when the S₂ switch is in the position Figure 2 shows.

When S₂ is in the upper position, the meter measures voltage across R₁, a 1-kΩ resistor; the meter displays a negative-voltage-drop value. R₁‘s value ensures direct reading of the meter; the voltage drop across R₁ corresponds to the zener’s current, so there is no need to switch over DVM (digital-voltmeter) ranges. R₁‘s voltage drop limits the zener diode’s voltage value that the circuit measures. If R₁‘s value is 1Ω, then the voltage drop across it is insignificant and, in millivolts, is equal to the zener diode’s current in milliamps.

 

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