Fig. 1

Current Limiter Allows Safe Testing of Zener Diodes, LEDs

by Lewis Loflin

In Fig. 1 I am testing the breakdown voltage of a Zener diode. By using a constant current source from an LM334 or LM317 I am assured of a stable measurement and protection for the Zener diode.

This also works for testing light emitting diodes.


Fig. 2

Fig. 2 is the circuit I used. The current value through the Zener is 0.064 / Rset. 3.3 Ohms produces ~19mA. 2.7 Ohms ~24mA, 1 Ohm 64mA.

Simply connect the Zener diode or LED and measure the voltage across the device.


Fig. 3

Fig. 3 works the same way as the LM334 circuit. The current I = 1.25V / R. If R = 47 Ohms then I = 26.6mA.





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