Hey everybody,
I have the following circuit:

It is given that at t=0 Vc0 = 200V.
At first I was asked to calculate the equation of i(t) in the circuit, so I used the KVL eq: Vc(t)+VL(t)+VR(t)=0
I solved it and found i(t) at last.
Then I calculated the maximum value of the current + the passed time until this value (tcond).

Now I am required to calculate the final voltage value on the capacitor (at tcond), I tried to use the same KVL eq: Vc(t)+VL(t)+VR(t)=0
but my answer has wrong sign, +97V instead of -97V. I checked someone's solution and saw that they used the equation:
Vc(t)=VL(t)+VR(t) and I need your help please to understand why did the signs of this equation changed.

Thank you in advance!

 

What happens at t=0? Does the switch open at that time, or does it close at that time? That fact that you are writing a KVL equation around the loop implies that it closes at t=0?

You should always ask yourself whether your answer makes sense.

Why doesn't the voltage on the cap simply stay at the +200 V that is started at?

Why wouldn't that same answer apply to voltage on it being +97 V?

Why do you think that the final voltage on the capacitor happens at the same time that the maximum current happens? Does that even make sense?

Since you haven't indicated the polarities for the voltages across the inductor or the resistor, there is no way to tell if either equation is corrector or whether they are, in fact, the same since your 'someone else' may have simply chosen different polarities that you did.

What about the voltage across the diode? Are you assuming an ideal diode?

Try sketching what the current would look like if the diode were replaced with a short. Now sketch the current for the case where the diode is present on top of that. My guess is that the mistake you are making will jump out at you almost immediately.

If not, post your sketch AND your best attempt to solve the problem and we can discuss further.

 

What happens at t=0? Does the switch open at that time, or does it close at that time? That fact that you are writing a KVL equation around the loop implies that it closes at t=0?

You should always ask yourself whether your answer makes sense.

Why doesn't the voltage on the cap simply stay at the +200 V that is started at?

Why wouldn't that same answer apply to voltage on it being +97 V?

Why do you think that the final voltage on the capacitor happens at the same time that the maximum current happens? Does that even make sense?

Since you haven't indicated the polarities for the voltages across the inductor or the resistor, there is no way to tell if either equation is corrector or whether they are, in fact, the same since your 'someone else' may have simply chosen different polarities that you did.

What about the voltage across the diode? Are you assuming an ideal diode?

Try sketching what the current would look like if the diode were replaced with a short. Now sketch the current for the case where the diode is present on top of that. My guess is that the mistake you are making will jump out at you almost immediately.

If not, post your sketch AND your best attempt to solve the problem and we can discuss further.

Thanks for your reply!
Let me please fix my previous question, sorry for English mistakes.
1. The switch closes at t=0.
2. The diode is ideal
3. I meant that I have to find the voltage on the capacitor after the conducted time (until diode blocks the current) and not the time until maximum current value, I am sorry for that.

" Since you haven't indicated the polarities for the voltages across the inductor or the resistor, there is no way to tell if either equation is corrector or whether they are, in fact, the same since your 'someone else' may have simply chosen different polarities that you did. "

Does it matter how do I choose the voltage polarities of the components?

My best attempt contains few explanations in Hebrew. I can post it but I am not sure if it will be understood enough.