![]() As we draw more load current 99% of the current originates with Q1. Yes you need the 100uF capacitor to assure power supply ripple doesn't cause problems. The reason I went to a 5.6 volt Zener is to compensate for the 0.6V drop across the B-E junction of Q1. 5 a NPN transistor with a Hfe or DC gain of 100 in effect "multiplies" 1mA from the Zener-resistor circuit to 100mA. To get around the power limitations we use a series-pass transistor. That is why we use transistors in conjunction with Zener diodes. This setup is almost worthless as a power supply in itself except at low currents. 4 RL is now 100 ohms and has taken so much current from Z1 we no longer have any voltage regulation at all. While total current though RS remains the same, part of the current for Z1 (Iz) goes to RL and we are on the edge of no voltage regulation. 3 we dropped RL from 200 Ohms to 150 Ohms increasing IL. But what happens of we increase the load from RL? Note that to work properly we must maintain a minimum value of Iz. 2 we have a properly operating Zener diode regulation circuit at Z1 = 5.1 volts with a 10-volt supply. The next question becomes, just how much current can this circuit provide to a load? Let's look at the problem. ![]() If RS is too large then we lack the minimum current Iz to maintain voltage regulation. If RS is too small excessive current will overheat Z1. If Vin increases to say 18 volts, then the Zener current Iz increases maintaining the voltage across RL.Īt all times the voltage drop across Z1 plus RS always equals the supply voltage Vin, while the voltage across RL thus IL is constant. If Vin drops to 14 volts the Zener current Iz drops to maintain the voltage on RL. Z1 at 10.2 volts maintains a steady voltage across RL as the Vin varies over a particular range. Our total current (Is) flows through RS and divides through Z1 (24mA) and RL (51mA). ![]() Z1 and RS are in series while a 200 Ohm load resistor RL is parallel to Z1. Fig.1 illustrates a basic Zener diode connection. Zener diodes are always used in there reverse bias mode designed to breakdown at a particular voltage. They will be used in conjunction with common bi-polar transistors to boost the output current and can used for real-world voltage regulators by students and hobbyists.Ī Zener diode is a two terminal solid state device that when forward biased will conduct and act like any other silicon diode. This will explore the basic operation of Zener diodes and their use as voltage regulators. Transistor-Zener Diode Regulator Circuits
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