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# LC circuit power transfer

##### Full Member level 4
This question is not regards to any specific circuit or design but more of a general inquiry.

Say you have a small capacitance , from somewhere of 500 pF upwards to maybe 500 nF maybe p to 1 uF at best.
Now say I wish to transfer AC current through it , for example , at 50 Hz.

Now for a series LC circuit the circuit becomes purely resistive at resonance when the L and C reactances are equal.
So in theory at resonance I could transfer most current, but the problem is that for such a small capacitance the resonant inductance is very large which means a large coil made of rather thin wire in order to achieve that resonance.
So even if the circuit becomes purely resistive , due to the large coil the DC resistance of that cold would be substantial and limit maximum current.

So what I'm asking is , how does one achieve efficient power transfer if one only has small capacitance? Parallel LC? There too the coil still have to be massive due to the need for maximizing inductance as far as I'm aware.

One option is to increase frequency, in the hundreds of kHz range even a small capacitance can transfer alot of power and it's reactance is small, but what if I wish to do it at low frequency ?
The only option that comes to mind is to increase voltage because as far as I know for the same capacitance , increasing voltage increases the total energy either stored in the capacitor as in DC or transferred through the capacitor as in AC.

Thanks.

Don't forget about the voltage across your low value capacitor when you are trying to put largish currents through it at low frequencies

Vc = Ic . Xc
--- Updated ---

whether resonant or not

Hi,

In theory you could use a huge coil... with low reactance.

But in the real world there are limits.

Thus: small capacitance, low frequency and high current .... simply does not work well.

Klaus

Don't forget about the voltage across your low value capacitor when you are trying to put largish currents through it at low frequencies

Vc = Ic . Xc
--- Updated ---

whether resonant or not
I'm not sure I got what your saying? Are you saying what I already said about increasing voltage across the cap to maximize power transfer at low frequency ?
--- Updated ---

Hi,

In theory you could use a huge coil... with low reactance.

But in the real world there are limits.

Thus: small capacitance, low frequency and high current .... simply does not work well.

Klaus
Huge coil with low reactance? How does that work? huge coils usually have ever larger reactances unless your talking about some physical dimensions in which case a huge coil can be made as an air coil but that would have low inductance and therefore not resonate at low frequency with the small value capacitor

Huge coil with low reactance? How does that work?
* "in theory" and
* "but in the real world"
???

Klaus

Yes, sorry, theres no way to do it....only way is like you say, at high frequency...like in an LLC converter.
Think how big the resonant cap would be in an LLC at 50Hz......enormous.

I'm not sure I got what your saying? Are you saying what I already said about increasing voltage across the cap to maximize power transfer at low frequency ?
the voltage would be lots of kV for any serious level of power transfer, e.g. let's take 1nF at 5A ac at 50 Hz, Vc = Xc . Ic = 15, 915, 963.71 volts ac
--- Updated ---

at 500nF it is 31.831 kV ac ( 45 kV peak )

This question is not regards to any specific circuit or design but more of a general inquiry.

Say you have a small capacitance , from somewhere of 500 pF upwards to maybe 500 nF maybe p to 1 uF at best.
Now say I wish to transfer AC current through it , for example , at 50 Hz.

Now for a series LC circuit the circuit becomes purely resistive at resonance when the L and C reactances are equal.
So in theory at resonance I could transfer most current, but the problem is that for such a small capacitance the resonant inductance is very large which means a large coil made of rather thin wire in order to achieve that resonance.
So even if the circuit becomes purely resistive , due to the large coil the DC resistance of that cold would be substantial and limit maximum current.

So what I'm asking is , how does one achieve efficient power transfer if one only has small capacitance? Parallel LC? There too the coil still have to be massive due to the need for maximizing inductance as far as I'm aware.

One option is to increase frequency, in the hundreds of kHz range even a small capacitance can transfer alot of power and it's reactance is small, but what if I wish to do it at low frequency ?
The only option that comes to mind is to increase voltage because as far as I know for the same capacitance , increasing voltage increases the total energy either stored in the capacitor as in DC or transferred through the capacitor as in AC.

Thanks.
Hi,

To be honest, your question is not very clear and we have only been trying to look for a way to squeeze out an answer for you, even when the question is not clear.

In your fourth paragraph, you mentioned "efficient power transfer" and in your fifth paragraph you mentioned "alot of power". As these are not the same thing, it is not clear what your question wants as an answer. So please clarify this.