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how do you calculate V\$_{R3}\$ in the circuit below?

enter image description here

V\$_{in}\$, all the resistor values and the type of the diode are known.

By simulating this circuit one can see that V\$_{D}\$ is increasing with higher values for R\$_{3}\$. How is that even possible?

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  • \$\begingroup\$ Did you know there is a circuit editor? You can use it when you post questions. \$\endgroup\$
    – Voltage Spike ♦
    Commented Apr 5, 2016 at 21:42
  • \$\begingroup\$ Yes, i saw the circuit edtor. But the picture was already made at this point. Thanks for the remark, next time i will try the editor. \$\endgroup\$
    – drak0
    Commented Apr 7, 2016 at 19:05

1 Answer 1

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If you are using a diode that follows the Shockley equation, you can iterate to find a solution. Assume a plausible Vf for the diode (say 0.7V for a silicon diode), calculate the currents and voltages, then find Vf for that current and substitute it back it. Repeat until the answer has converged to the desired accuracy. In most cases the diode voltage won't change much with R3 so it will converge rather quickly.

For rough calculation and relatively high voltage \$V_{IN} (\frac{R_3}{R_1+R_3})\$ compared to \$V_F\$ you may get a pretty accurate answer without any iterations.

Before you do this, convert the resistors and voltage source into their Thevenin equivalent - one resistor and a voltage source.

If you are required to come up with a closed-form solution, there is a method described in Wikipedia using the Lambert W function.

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