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Capacitive wireless power transfer question

Started by Unknown July 5, 2016
Could some electronics guru please shed some light on this ? Consider the following set up. 

I have an oscillator running at 25 MHz, driving a 
10 pF parallel plate capacitor. The capacitor is 
made of two single layer copper clad plates of 
appropriate dimensions, separated by 1.2 mm of FR4. The capacitive reactance at this frequency is about
32 Ohms. 

What test can I perform to check that power is 
transferred across the capacitor plates ? One
plate of the capacitor is grounded, I have a 
garden variety oscilloscope and a Fluke DMM.

Thanks in advance for your answer.
On 2016-07-05 08:08, dakupoto@gmail.com wrote:
> Could some electronics guru please shed some light on this ? Consider > the following set up. > > I have an oscillator running at 25 MHz, driving a 10 pF parallel > plate capacitor. The capacitor is made of two single layer copper > clad plates of appropriate dimensions, separated by 1.2 mm of FR4. > The capacitive reactance at this frequency is about 32 Ohms.
1/(2 pi 25M 10p) = 640 Ohms. How did you get 32?
> > What test can I perform to check that power is transferred across the > capacitor plates ? One plate of the capacitor is grounded, I have a > garden variety oscilloscope and a Fluke DMM.
No power is transferred. You'd have to have something in your circuit to absorb power, and your description mentions none. Jeroen Belleman
On Tuesday, July 5, 2016 at 8:08:54 AM UTC+2, daku...@gmail.com wrote:
> Could some electronics guru please shed some light on this ? Consider the following set up. > > I have an oscillator running at 25 MHz, driving a > 10 pF parallel plate capacitor. The capacitor is > made of two single layer copper clad plates of > appropriate dimensions, separated by 1.2 mm of FR4. The capacitive reactance at this frequency is about > 32 Ohms. > > What test can I perform to check that power is > transferred across the capacitor plates ? One > plate of the capacitor is grounded, I have a > garden variety oscilloscope and a Fluke DMM. > > Thanks in advance for your answer.
There's no power being transferred "across" the capacitor. You are just charging it up and discharging it. Ideal capacitance is non-dissipative so there's no energy going anywhere. FR4 isn't a particularly perfect dielectric so there may be some power dissipation in the dielectric, but that power isn't going anywhere. You could detect the dissipation in the FR4 by measuring the temperature of the copper on the grounded side of the capacitor, which might be perceptibly warmer when the oscillator was running. An interchangable thermistor might do it. -- Bill Sloman, Sydney
On 07/05/2016 08:08 AM, dakupoto@gmail.com wrote:
> Could some electronics guru please shed some light on this ? Consider the following set up. > > I have an oscillator running at 25 MHz, driving a > 10 pF parallel plate capacitor. The capacitor is > made of two single layer copper clad plates of > appropriate dimensions, separated by 1.2 mm of FR4. The capacitive reactance at this frequency is about > 32 Ohms. > > What test can I perform to check that power is > transferred across the capacitor plates ? One > plate of the capacitor is grounded, I have a > garden variety oscilloscope and a Fluke DMM. > > Thanks in advance for your answer. >
charge pump? ----+---+ C2 ^ |R |C2 ---||---+ | | ___ ^ | | ___ | | | ___ ___ ___ (input signal from the left) measure voltage on c2 for various R(a poti) if you look up charge pumps you'll see the efficiency will drop with higher load currents, because charging the capcitor from a Voltage source will burn half the energy in the source resistance. The higher the load current, the more voltage difference on c1 to compensate each cycle.
On Tue, 05 Jul 2016 00:10:04 -0700, bill.sloman wrote:

> On Tuesday, July 5, 2016 at 8:08:54 AM UTC+2, daku...@gmail.com wrote: >> Could some electronics guru please shed some light on this ? Consider >> the following set up. >> >> I have an oscillator running at 25 MHz, driving a 10 pF parallel plate >> capacitor. The capacitor is made of two single layer copper clad plates >> of appropriate dimensions, separated by 1.2 mm of FR4. The capacitive >> reactance at this frequency is about 32 Ohms. >> >> What test can I perform to check that power is transferred across the >> capacitor plates ? One plate of the capacitor is grounded, I have a >> garden variety oscilloscope and a Fluke DMM. >> >> Thanks in advance for your answer. > > There's no power being transferred "across" the capacitor. > > You are just charging it up and discharging it. Ideal capacitance is > non-dissipative so there's no energy going anywhere. FR4 isn't a > particularly perfect dielectric so there may be some power dissipation > in the dielectric, but that power isn't going anywhere. > > You could detect the dissipation in the FR4 by measuring the > temperature of the copper on the grounded side of the capacitor, which > might be perceptibly warmer when the oscillator was running. An > interchangable thermistor might do it.
This is a perfect explanation for why radio transmitters never work. Because nearly all of them have a DC blocking cap somewhere in the power amplifier chain, and it fails to transfer any power. Thanks! Although, it does make me wonder why people persist in building their transmitters that way... -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On Mon, 04 Jul 2016 23:08:46 -0700, dakupoto wrote:

> Could some electronics guru please shed some light on this ? Consider > the following set up. > > I have an oscillator running at 25 MHz, driving a 10 pF parallel plate > capacitor. The capacitor is made of two single layer copper clad plates > of appropriate dimensions, separated by 1.2 mm of FR4. The capacitive > reactance at this frequency is about 32 Ohms. > > What test can I perform to check that power is transferred across the > capacitor plates ? One plate of the capacitor is grounded, I have a > garden variety oscilloscope and a Fluke DMM. > > Thanks in advance for your answer.
Wait -- with one plate grounded, how can power be transferred? It can only be burned up in various losses. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On Tue, 05 Jul 2016 11:24:50 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Tue, 05 Jul 2016 00:10:04 -0700, bill.sloman wrote: > >> On Tuesday, July 5, 2016 at 8:08:54 AM UTC+2, daku...@gmail.com wrote: >>> Could some electronics guru please shed some light on this ? Consider >>> the following set up. >>> >>> I have an oscillator running at 25 MHz, driving a 10 pF parallel plate >>> capacitor. The capacitor is made of two single layer copper clad plates >>> of appropriate dimensions, separated by 1.2 mm of FR4. The capacitive >>> reactance at this frequency is about 32 Ohms. >>> >>> What test can I perform to check that power is transferred across the >>> capacitor plates ? One plate of the capacitor is grounded, I have a >>> garden variety oscilloscope and a Fluke DMM. >>> >>> Thanks in advance for your answer. >> >> There's no power being transferred "across" the capacitor. >> >> You are just charging it up and discharging it. Ideal capacitance is >> non-dissipative so there's no energy going anywhere. FR4 isn't a >> particularly perfect dielectric so there may be some power dissipation >> in the dielectric, but that power isn't going anywhere. >> >> You could detect the dissipation in the FR4 by measuring the >> temperature of the copper on the grounded side of the capacitor, which >> might be perceptibly warmer when the oscillator was running. An >> interchangable thermistor might do it. > >This is a perfect explanation for why radio transmitters never work. >Because nearly all of them have a DC blocking cap somewhere in the power >amplifier chain, and it fails to transfer any power. > >Thanks! > >Although, it does make me wonder why people persist in building their >transmitters that way...
Snicker >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | JU$TICE HA$ BEEN $SERVED, MY A$$
On Tue, 05 Jul 2016 11:24:50 -0500, Tim Wescott wrote:

> On Tue, 05 Jul 2016 00:10:04 -0700, bill.sloman wrote: > >> On Tuesday, July 5, 2016 at 8:08:54 AM UTC+2, daku...@gmail.com wrote: >>> Could some electronics guru please shed some light on this ? Consider >>> the following set up. >>> >>> I have an oscillator running at 25 MHz, driving a 10 pF parallel plate >>> capacitor. The capacitor is made of two single layer copper clad >>> plates of appropriate dimensions, separated by 1.2 mm of FR4. The >>> capacitive reactance at this frequency is about 32 Ohms. >>> >>> What test can I perform to check that power is transferred across the >>> capacitor plates ? One plate of the capacitor is grounded, I have a >>> garden variety oscilloscope and a Fluke DMM. >>> >>> Thanks in advance for your answer. >> >> There's no power being transferred "across" the capacitor. >> >> You are just charging it up and discharging it. Ideal capacitance is >> non-dissipative so there's no energy going anywhere. FR4 isn't a >> particularly perfect dielectric so there may be some power dissipation >> in the dielectric, but that power isn't going anywhere. >> >> You could detect the dissipation in the FR4 by measuring the >> temperature of the copper on the grounded side of the capacitor, which >> might be perceptibly warmer when the oscillator was running. An >> interchangable thermistor might do it. > > This is a perfect explanation for why radio transmitters never work. > Because nearly all of them have a DC blocking cap somewhere in the power > amplifier chain, and it fails to transfer any power. > > Thanks! > > Although, it does make me wonder why people persist in building their > transmitters that way...
Whups -- sorry, didn't notice the OP's comment about one end grounded. But -- power transfer across capacitances is a real thing. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On 7/5/2016 9:41 AM, Tim Wescott wrote:

> > But -- power transfer across capacitances is a real thing. >
Doesn't the capacitor contribute the "imaginary part?" ;-)
On Tuesday, July 5, 2016 at 7:40:55 PM UTC+5:30, Johann Klammer wrote:
> On 07/05/2016 08:08 AM, dakupoto@gmail.com wrote: > > Could some electronics guru please shed some light on this ? Consider the following set up. > > > > I have an oscillator running at 25 MHz, driving a > > 10 pF parallel plate capacitor. The capacitor is > > made of two single layer copper clad plates of > > appropriate dimensions, separated by 1.2 mm of FR4. The capacitive reactance at this frequency is about > > 32 Ohms. > > > > What test can I perform to check that power is > > transferred across the capacitor plates ? One > > plate of the capacitor is grounded, I have a > > garden variety oscilloscope and a Fluke DMM. > > > > Thanks in advance for your answer. > > > charge pump? > > ----+---+ > C2 ^ |R |C2 > ---||---+ | | ___ > ^ | | ___ > | | | > ___ ___ ___ > > (input signal from the left) > measure voltage on c2 for various R(a poti) > if you look up charge pumps you'll see the > efficiency will drop with higher load currents, > because charging the capcitor from a Voltage source will > burn half the energy in the source resistance. > > The higher the load current, the more voltage difference > on c1 to compensate each cycle.
Thanks. Maybe I am not reading the diagram correctly, but you are suggesting a series connection of C2-R-C1, with C1 grounded ? Am I correct ?