# Thevenin equivalent of simple circuit

I'm stuck on a problem and I don't know how to proceed.

I have a circuit that consists of a 10μF capacitor connected in series with a variable resistor (100kΩ, 200kΩ, 300kΩ). I'm supposed to draw the circuit connected to an oscilloscope that has an internal resistance of 1MΩ and an internal capacity that's negligible. The source is DC of 10V.

Here is what I got so far (circuit to the left and oscilloscope to the right):

Now I am asked to convert the circuit to it's Thevenin Equivalent, but what confuses me is that there is only 1 resistor which is unusual, and I also don't know what to do with the capacitor C1. I've looked online for well over an hour now, but I cannot find an answer.

I'd be very greatful for a push in the right direction. Cheers!

EDIT: I made an error in the picture; R1 should be 100 kΩ, not 100 Ω.

## 1 Answer

Ideally what we would do is take the thevenin across the capacitor (C1). So connect the terminals A and B to the oscilloscope equivalent and ignore the internal capacitance. By treating the series capacitor as the load, the whole network becomes a resistive network. The thevenin equivalent will become a series RC network.

Hope this hint is enough.

• Thanks for your input! I'm still a bit confused though. Are you saying I should move the C1 capacitor basically to the right of point A? – Chris May 17 '15 at 18:17
• Treat C1 as the load. So in the circuit the load is removed and the thevenin equivalent is taken across it. That way the equivalent thevenin circuit includes the thevenin resistance in series with the load, which here will be the capacitor C1. – Sada93 May 17 '15 at 18:20
• So the above circuit is almost already in it's Thevenin equivalent, right? I mean if all we need to do here is to "remove" the C1 capacitor, then the Vth would be equal to Vs and Rth would be equal to R1? That seems odd. – Chris May 17 '15 at 19:59
• Rth would almost equal R1 only because the oscilloscopes resistance is so high. But yes you are right. – Sada93 May 18 '15 at 0:22