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I have created this simulation ( see below) of a transistor and zener diode voltage regulator. The output is supposed to be a steady 5 V. But it is like the top bit s cut, to an approx 4.4 V value, then it plunges to the minimum voltage value. I really dont understand why and how to fix it my simulation of transistor zener diode circuit

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  • $\begingroup$ Welcome to engineering SE. Can you please share the part number for the Zener diode used in the simulation. Also what is the name of the simulation tool? Also 0.6V difference is voltage drop across the transistor. Also any reason why you are using a AC voltage source. Please update your question with above requested information. $\endgroup$ – Mahendra Gunawardena May 3 '19 at 1:06
  • $\begingroup$ 5V Zener - 0.7V VBE of series pass transistor = 4.4V. As Mehrandra states below, use a larger Zener. $\endgroup$ – StainlessSteelRat May 3 '19 at 17:22
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The simulation has an AC source feeding the Zener Diode - Series pass transistor circuit. Replacing the AC source with the DC source should eliminate voltage plung situation. Alternatively inserting a rectification circuit between AC source and the Zener regulation circuit will also solve this issue.

The 4.4V output voltage situation can be solved by selecting a 5.6V Zener diode such as a 1N4734A from Vishay

Below is a example zener diode - series pass transistor 5V output circuit

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Below is a list of parts from other vendors

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Alternatively below is list of 1N47 series zener diodes from Vishay

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References

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