# Parallel resonance on oscilloscope

Discussion in 'General Electronics Chat' started by herbkanis, Jun 23, 2013.

1. ### herbkanis Thread Starter New Member

Oct 5, 2011
1
0
Hi everyone,
Im looking for help on how to set up an experiment to view the current flow of parallel resonance with an oscilloscope.

My resonate setup is the same as the example on http://www.falstad.com/circuit/e-res-par.html (or attached image)
Im using a 0-50 mhz sin/tri/sqr wave generator and the same components as in the example. 40.1hz, 1R, 1H, .15u

What would be the best way to view the current and voltages flowing back and forth between the coil and the capacitor?

In the example, the resonance seems to be nailed to 40.1 hz and any deviation causes the example to change noticeably. In my experiment, I put a 10k resistor in series with the coil and the capacitor and thought Id measure the voltage drop across it. However, the scope image doesnt seem to change much as I alter the frequency unless I go really far off the resonant frequency.
Also I dont see much change in the signal when I alter the inductance. (Ive made a variable inductor, .5H to .998H)

Im sure Im just unfamiliar with how to set this all up. Any help would be appreciated.

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2. ### BillB3857 Senior Member

Feb 28, 2009
2,399
348
Have you calculated the resonance points with the various values? That may provide some useful information for you. Also, if you remove the resistor that is in parallel with the tank circuit, it will improve the Q slightly.

3. ### LDC3 Active Member

Apr 27, 2013
920
160
I guess you can't read very well D), the image specifies that the resistor is 2K.

If you noticed on the web page, the current through the 100R resistor is at a minimum at the resonance frequency. You could either measure the current or the voltage drop of the 100R resistor.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
As already mentioned the values you quote don't line up with those shown in the attachment.

If you want to measure the currents flowing in the inductance and capacitance you would have to use something very much less than 10kΩ in series with either one. The reactance of a 1H inductor at 41.1 Hz is of the order of 258 Ω. The 10 kΩ would completely disrupt the original circuit resonant behavior.

The trick is to choose a series resistance just large enough to enable you to measure a voltage drop across the resistor(s) on your CRO with vertical input range set for maximum sensitivity. So your CRO input sensitivity is a factor. Also the signal generator would be set for its maximum output to give you the best chance of reading the voltages across a sensing resistor. If you could get a sensible reading on the CRO with a series sensing resistance of say 1Ω to 10Ω you would be closer to the mark.

The other decision you would have to make is where to place your current sensing resistors in relation to your measurement ground point to enable you to obtain the correct signals to allow you to infer the actual currents flowing in the respective components.