# Tank circuit

Discussion in 'General Electronics Chat' started by zogorean, Feb 5, 2013.

1. ### zogorean Thread Starter New Member

Jan 12, 2013
11
0
hi all.

Can anyone help with a formula or schematic for a simple tank circuit (LC circuit) including a capacitor?

I am trying to create a tank circuit and just seem to fail. I have a 4700uF capacitor, and cannot figure out what size my coil should be. I have made a simple circui with a coil in series with a charged cap, but when i monitor the whole thing on a oscilloscope, nothing shows appart from the fact that my voltage drops to 0v and thats it.

Any help is appreciated.
Cheers, Zogo

Oct 18, 2012
3,527
679
3. ### zogorean Thread Starter New Member

Jan 12, 2013
11
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lol, yes its the new L circuit...

Ok, so now I have an AC tranformer outputing about 11v, a few caps and a few coils. I also have an oscilloscope. But Im a biiiig newb at this.

I connect the cap, a coil and my transformer in series. Puting a magnet on the coil vibrates the coil like mad. Thinking 50hz as my transformer outputs 50hz.

But where should I connect my oscilloscope probe to measure the circuit and most important, how will I know that the circuit is in ressonance? i mean should the frequency chnage? There is a calculator for the frequency, cap size and solonoid size, but according to my calculations, my circuit should ressonate at 400hz, and I still measure 50hz.

What am I, the newb, doing wrong here?

4. ### tshuck Well-Known Member

Oct 18, 2012
3,527
679
Where to measure depends on what you are making... what are you attempting here?

The circuit will be in resonance when the amplitude of your signal is at a maximum, over the frequency spectrum.
http://en.wikipedia.org/wiki/Resonance

The 50Hz might not be attenuated enough so that the 400Hz is even noticeable...

5. ### Audioguru Expert

Dec 20, 2007
10,728
1,201
A series LC is a low impedance at resonance. If it resonates at 400Hz and you feed it 50Hz then your LC is not resonating and is not doing anything useful.

Your 4700uf capacitor value is much too high for 400Hz and it is a polarized electrolytic type. But you are destroying it by feeding it AC.

I had a 240VAC fan. It would not run on my 120VAC. So I resonated it at my 60Hz with a series 6.8uF film capacitor and it ran fine with about 195VAC across it.

jaygatsby likes this.
6. ### zogorean Thread Starter New Member

Jan 12, 2013
11
0
yes what am I doing?

First of all, thanks for all your very swift replies.

I am trying to create a LC circuit and want to measure it with my oscilloscope, that I got for christmas. I have a whole bunch of old vcrs, dvd players and radios, so parts would be easy for me to rip from these. I want to understand the LC circuit and ressonance in general.

Is there a simple circuit that you can recomend me with that has been tried, that I can try to reproduce? This would be the easiest for me at this moment, in order for me to better understand this phenomenon.

7. ### jaygatsby New Member

Nov 23, 2011
184
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You're a pro.

How did you calculate 195VAC?

8. ### tshuck Well-Known Member

Oct 18, 2012
3,527
679
Oh, what? Hey, look at that....
..and..... that.

9. ### Audioguru Expert

Dec 20, 2007
10,728
1,201
An inductor in series with a capacitor is series resonant. It is a very low impedance at resonance like a short circuit. It will smoke a transformer if it resonates at the frequency from the transformer.

To study LC resonance you need a variable frequency generator and a resistor. The resistor limits the current.

10. ### Audioguru Expert

Dec 20, 2007
10,728
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I didn't calculate anything. I didn't measure the inductance and resistance of the fan. I simply tried a few capacitor values until the fan ran as fast as possible. Then I measured its voltage.

11. ### zogorean Thread Starter New Member

Jan 12, 2013
11
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Hey thanks for that, tshuck.

12. ### chuckey Well-Known Member

Jun 4, 2007
75
10
That fan trick, I tried to use a 110V fan on 230V, worked out cap size, put it series. Still got 230V across fan the inductance of the fan and the reactance of the cap resonated :-(
On a similar topic, I was building a single to 3 phase converter, so I needed a 2 seconds delay for the start cap. Built a timer circuit which drove a triac powering up a relay. Plugged it in, relay pulled in, after 2 seconds, bit of a click, relay still in. I fiddled around, lost patience and used a relay out, ready made timer. same thing, except that after 2 seconds I could hear the driving relay dropping out, but main relay stayed in. On further fiddling about it seems that the .1 MF cap I had across the driving relay contacts (snubbing the coil of the main relay) resonates with the relay, so once it had pulled in the resonance of the .1MF, held it in. I changed the cap to .03 and it all works.
Frank

Jan 3, 2012
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14. ### Audioguru Expert

Dec 20, 2007
10,728
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If it resonated then the fan would have too much voltage and would be destroyed.

The fan has inductance that limits its current. A series capacitor that resonates with it cancels its inductance so its current is much higher.
When the current is much higher then the voltage across its resistance is also much higher and can be measured.