Whenever you use a Zener diode in conjunction with a limiting resistor, you must not exceed the resistor’s power-dissipation rating. If you select a resistance that’s too low, you’ll burn out the resistor. You’ll also risk exceeding the diode’s current rating, destroying it as well!
You can use the standard DC formula for power in terms of current and resis-tance to calculate the wattage dissipated by R in the circuits you’ve just finished testing. That formula is
A Zener-Diode Voltage Reducer
In this experiment, you’ll find out what happens when you connect a 5.1-volt (V) Zener diode in series with the output of a power supply that already employs a 12-V Zener diode in parallel to regulate the voltage. You’ll need all the components from Experiment AC16, along with a 3.3-kilohm (K) resistor and an extra jumper.
Think all the way back to Experiment DC15. In that set of tests, you saw how an ordinary diode, when series-connected in the forward direction, reduces the DC voltage from a battery. What do you think will happen if you connect a Zener diode in series, but in the reverse direction? I suspected that it would reduce the output of our filtered, regulated power supply by an amount equal to the rated Zener voltage. That seems reasonable, doesn’t it? Well, things aren’t quite that simple.