# Finding voltage in RLC circuit $$\C = 1 \mu F\$$

$$\L = 2.2 mH\$$

$$\r = 1.7 \Omega\$$

$$\f = 4000 Hz\$$

$$\V_{IN} = 2.6 V\$$

I have this RLC circuit where I need to find $$\V_{OUT}\$$. I have tried it by finding the $$\Q\$$ factor and then multiplying it with $$\V_{IN}\$$ which gave me the value $$\84.56427 V\$$. However, when putting that value in as an answer in the system I'm using, I don't get the right answer, even if I round it up to $$\84.6 V\$$ or $$\85 V\$$.

What am I doing wrong?

• is Vin a DC source?
– Big6
May 5, 2016 at 22:39
• So what do you estimate to be the correct answer? Also, importantly, what is your question? May 5, 2016 at 23:16
• Remember units for calculations (Farads, Henry, Ohm) , you're probably forgetting to multiply the values by their factors of 10. May 6, 2016 at 9:42

You have a fixed frequency (4kHz), series RLC circuit with a constant peak Vin of 2.6V, so you can think of it as a voltage divider.

Work out your reactances at 4kHz and Vout will be proportional to Vin.

$$X_C = \frac{1}{2\pi fC}\Omega = \frac{1}{2\pi(4000)(10^{-6})}\Omega$$ $$X_L = {2\pi fL}\Omega = {2\pi(4000)(2.2\times10^{-3})}\Omega$$ $$V_{OUT} = V_{IN}\frac{(X_L+R)}{(X_L+X_C+R)}$$

Hope this helps!

• Hi, Thank you for answering. I got the value 1.11459 V, but that doesn't seem right either.. May 6, 2016 at 11:37
• I mean 1.53108, either way it isn't right May 6, 2016 at 12:07
• I understood what I did wrong, Thank you for your help! :) May 6, 2016 at 14:14
• Gday mate, my equations are correct I crunched the numbers and got 1.1146. Why doesn't that seem correct? You have to think about it without any numbers. This is a passive circuit without any switching of any kind. A fixed frequency, we can only assume its a sine wave. The output cant be higher than the input in this situation. It has to be a ratio of the input and your input is only 2.6V. May 6, 2016 at 14:20
• @Vetenskap good on you for admitting your mistake. I personally think the best quality in an engineer is knowing when (and why) you are wrong. Its taken me many years of being wrong to know when I am right. Don't have any fear in making mistakes just as long as you admit when you are wrong. May 6, 2016 at 14:27