# Today In School

Discussion in 'General Electronics Chat' started by hondabones, Feb 5, 2010.

1. ### hondabones Thread Starter Active Member

Sep 29, 2009
123
1
I absolutely love this forum... yet I never get on here and post or anything. (I plan to make more time... knowledge is power) I am currently going to school for electronics. I am a third quarter student. We are studying AC Electronics right now. Today we learned about resonate frequencies.

Now to get to the purpose of this thread...

For lab our prof has been having us build circuit after circuit after circuits of RL series, and RC series circuits and checking our math on the oscilloscope. The guy behind me built his circuit using 33k Ω resistor, 33 mH inductor, and 70 kHz for the frequency. He could not get the sine wave to look right. I found that he didn't have the scope calibrated correctly, so I helped him out. This didn't help. The amplitude was still off by nearly 1.6 V from the projected 3.66 V. So after 10 -15 minutes of newbie frustration the Prof comes over and discovers that my calibration of the scope was not done on the correct setting. (the function generator amp knob needed to be set on 1 V increments with a 4 V peak, this is to insure the best view for our circuits on the scope.) He re-calibrated the entire circuit reads 4 V like it is supposed to. He then attaches the probe to the resistor to get the sine wave and the amplitude gets higher... that's right, I said it went up! To anybody who knows about this stuff we learn that this reading should be less (3.66 V in this case). This of course baffled us... including the Prof. We did all the necessary trouble shooting and found out that the frequency for this particular circuit was very close to the resonate frequency and that was the reason for the "impossible circuit"

Sorry for the long post... I figured some of you guys would like to here such a random case of coincidence. We learned of resonate frequency and by randomness we built one. (FYI... 66 kHz is the resonate frequency according to the scope for this circuit)

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,039
287
If the capacitance of the scope probe happens to resonate with the inductor, you will see a dramatic increase in voltage at that point.

eric

3. ### hondabones Thread Starter Active Member

Sep 29, 2009
123
1
We believe that the winding capacitance of the inductor had something to do with it.

Jim

Last edited: Feb 5, 2010
4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Every parasitic inductance and capacitance was involved.