How to implement an adjustable capacitance?

Discussion in 'The Projects Forum' started by nickagian, Nov 24, 2011.

1. nickagian Thread Starter Active Member

Mar 12, 2010
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Hi to all!

I would like to implement a circuit that will electronically control the resonance frequency of an LC-network by providing an additional, electronically adjustable capacitance. The nominal resonance frequency of the network is 1.5kHz and I would like to decrease this frequency even down to 50Hz. So I guess a Varactor is not very suitable.

Does anyone have any idea?

Actually I have thought of perhaps using the topology of a capacitance mutliplier or the negative impedance network (both implemented with BJTs or MOSFETs), but (1) I am not sure if I can indeed control the equivalent capacitance by controlling the biasing of the transistors and (2) I have not seen anywhere in the net any detailed analysis on the implementation of such circuits.

Thus, any help on this direction would be really helpful!

Nikos

2. thatoneguy AAC Fanatic!

Feb 19, 2009
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718
You should look at a VCO for the range you are trying to reach. There aren't any variable caps that would have any function at 50Hz.

If making a filter, using an active filter with a variable resistor will allow you to change low frequencies.

3. nickagian Thread Starter Active Member

Mar 12, 2010
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So you think that my ideas are not suitable, too?

I'm not making a filter (actually it's just an LC-structure which will be used as is to achieve resonance inside a magnetic field), but even then how can a variable resistor be controlled electronically? The digital pots I don't think they are suitable.

4. thatoneguy AAC Fanatic!

Feb 19, 2009
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718
I don't know what your ideas are. I do know that for a capacitor to have any useful reactance at 50Hz it needs to be in the mircroFarad range, and all the adjustable caps are in the picoFarad range, a million times too small.

5. Hi-Z Member

Jul 31, 2011
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Well, you could make use of the Miller effect here, whereby the apparent capacitance at the input of an amplifier is the feedback capacitance multiplied by the voltage gain of the amplifier +1. You'll find plenty of info on the internet, and you could implement the (variable) gain block using op-amps, if desired.

6. MrChips Moderator

Oct 2, 2009
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You could look into a switched capacitor filter.

7. thatoneguy AAC Fanatic!

Feb 19, 2009
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718
He said he's not making an oscillator.

I'm guessing he's trying to get a tank circuit to operate at a low frequency, then "boost" the inductor output with a magnetic field, and get more power or something similar.

Last edited by a moderator: Nov 25, 2011

Dec 26, 2010
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The OP may be following a false trail that goes along the lines of getting a system to resonate, then once a current has built up in the circuit, reducing the capacitance so that a larger voltage is achieved, (at a higher frequency).

If such a system were built, for instance using a mechanical variable capacitor, the system might indeed contain more energy immediately after the capacitance was reduced. There is of course a fallacy here if this is regarded as an energy source, namely that mechanical work would have to done on the capacitor to reduce its value. The net result would no more be a source of free energy than is a Wimshurst Machine.

9. Wendy Moderator

Mar 24, 2008
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2,540
Bet this is part of a HHO scheme. I'll be tracking the threads.