# Simple RC circuit with diode question

I was trying to understand this circuit under DC conditions, but can someone explain to me why the voltage across the capacitor is 8.1 V? I can't seem to understand. This voltage however does go down to 4.2 V if I increase the simulation speed and wait for couple of seconds but how can it start of it at 8.1 V. Am I missing something?

Here is the circuit I drawn using falstad simulator.

Basically it's your simulator - it can't ever be higher than the 5V supply and 4.2 is pretty normal. Check that your initial conditions don't include some form of charge on the capacitor. Simulators can throw curved-balls sometimes and you did the right thing by asking.

By the way I think @pjc50 may have misread your voltage as being 9V not 5V - or maybe it's me - the picture isn't all that clear.

• yes sorry if it was not clear but the source was 5v . – subz Apr 23 '13 at 9:55
• @subz It is the simulator - I just tried this myself and got the same result. It seems you need to edit the capacitor's properties and uncheck "Trapezoidal approximation". – MikeJ-UK Apr 23 '13 at 10:09

That is the standard 0.7V forward diode voltage drop.

Part of the functioning of a diode is that the voltage across it has to be more than a particular value for current to flow in the forward direction. As the voltage increases, the effective resistance of the silicon junction decreases.

The value of this voltage threshold is about 0.7V for silicon P-N diodes and about 0.3V for Schottky and germanium diodes. There will always be that much voltage drop through a diode of that particular construction.

It's the simulator's way of calculating, and we call this the transient effect. Remember, the simulator is based on linear approximations. If you can actually look at the source code of the simulator, its probably dividing some value by a very small number (i.e. $\frac{1}{0.5}$). Think about your capacitor equation ($\frac{1}{jwC}$ and $C\frac{dV}{dt}$ ) and try to think what calculations the simulator is doing initially (just as if you were to do a circuit analysis the capacitor is initially considered a short then as time progresses its an open circuit. Right?).

I hope this helps you understand why we can't always trust a simulator. It just provides more data to verify our calculations.