Caps and resonators

1. Sep 27, 2009

Idea04

For a series LC circuit, can a bipolar electrolytic capacitor be used. Also how do you tap the resonant frequency from a series LC circuit.

2. Sep 27, 2009

vk6kro

If you have a coil and a capacitor in series and put an AC signal across the series pair, the voltage across each of them will rise a lot at resonance, especially if the coil has low resistance.
So, you can choose if you take the output across the capacitor or the coil. It will be a much magnified version of the input.

You probably could use bipolar electrolytics, but the circuit you used them in might not have a very good resonance.
This is because large capacitors (which these capacitors are ) need a very large current through them to develop big voltages.
So, unless you can deliver such currents, you are better off calculating your resonance so that the components have a reactance of at least 200 ohms.

For example, a capacitor of 0.796 uF and a coil of 31.8 mH would resonate at about 1000 Hz and could give quite a good resonance.
You could resonate at 1000 Hz with 15.9 uF and 1.59 mH but the resonance would be very poor if you only had a signal generator to put the AC across the circuit.
In the first case, the components have a reactance of 200 ohms. In the second, 10 ohms.

The 15.9 uF capacitor could be a bipolar electrolytic but it would also have to have low ESR which is Equivalent Series Resistance.

If you are thinking of trying this, the output resistance of the signal generator appears in series with the tuned circuit, so you would normally put a small resistor across the signal generator. Maybe 10 ohms or so.

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Last edited: Sep 27, 2009
3. Sep 27, 2009

Averagesupernova

A series resonant circuit will have a LOW impedance at resonance causing the voltage to sag.

4. Sep 27, 2009

Idea04

For a low current application would a ceramic capacitor work better.

5. Sep 27, 2009

Idea04

Tell me if I'm wrong with this , But I thought that if you had a supply current of 1 amp then a 1 Farad cap would work. And if you had a supply current of 3.3 micro amps a 3.3 micro Farad cap would work.

6. Sep 27, 2009

vk6kro

The voltage across each of the components will rise dramatically at resonance.
This rise in output is very useful if you have a low impedance source driving a high impedance load as you can get a substantial voltage gain.

Tell me if I'm wrong with this , But I thought that if you had a supply current of 1 amp then a 1 Farad cap would work. And if you had a supply current of 3.3 micro amps a 3.3 micro Farad cap would work.

No, that isn't true.
It has more to do with the reactance of the components. Any capacitor and coil will resonate at some frequency, but the actual voltage step-up depends on the reactances of the components. The voltage is equal to the reactance times the current and reactance depends on frequency. Even at 1 Hz a 1 Farad capacitor only has a reactance of 0.16 ohms so 1 amp through it would produce a voltage of 0.16 volts

For a low current application would a ceramic capacitor work better.
Better than an electrolytic? Yes, much better.
Ceramic capacitors work well in resonant circuits. They change capacitance with temperature but low temperature coefficient ones are available.