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Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor ( the 40 ohm resistor shown), the zener diode will conduct sufficient current to maintain a voltage drop of Vout measured across the load. In this example, the load is the LED.
Also in this example, the zener diode has a breakdown voltage of 2V. The power source is 5V and after the 40 ohm resistor we have 2.05V at the LED and at the reversed zener.
If we reduce the breakdown voltage of the zener diode to less than 2V, (try this), we see insufficient voltage passing to the LED, which has a threshold of 2V, to cause it to light. That is; the measured voltage across the LED becomes less than 2V.
If we increase the breakdown voltage of the zener diode, current is blocked through the zener and the LED burns out (try this). The purpose of the zener is eradicated.
If we now increase the load by increasing the threshold of the LED to that higher than the breakdown voltage of the zener diode (to 2.4V),(as seen in the diagram on the right) while all other parameters remain constant, we see that the current flows through the zener and the LED does not light. Thus, a higher load shuts down the load circuit. The load can only function near the breakdown voltage of the zener and below. The voltage is regulated.
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