# Transfer power effectively from one coil to another through air

I want to build my own wooden alarm clock.

I want the surface to be smooth without any connectors anywhere. My idea is to place one coil beneath the alarm clock, and another pickup coil inside. I suppose that I need to maximize the area.

• What kind of power can I hope to get through?
• What is the optimal frequency? (is 50 Hz ok)
• Will an unconnected iron core help?
• What is best - many or few windings?

Please also tell me, if you know of other reasons why this would not work.

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100% Duty Cycle: When Steven said "100% duty cycle" he meant "always-on AC waveform" –  Russell McMahon Sep 28 '12 at 19:57
@Russell McMahon: I woud like to learn what an "always-on AC waveform" is. For me it makes no sense - always on is DC an not AC at all. –  Hans-Peter E. Kristiansen Sep 28 '12 at 23:07
@Hans-Peter - "Always on" is not the same as "always maximum". When I switch the light on the voltage doesn't become DC, it remains a 50 Hz sine. When you switch a transmitter on it outputs an RF signal, not DC. When you leave it on it will still output an RF signal, so if you leave it always on it will always transmit an RF signal. –  stevenvh Sep 29 '12 at 8:40

This is readily achievable and there is much practical material on=web. Search for inductive power transfer.

Some good examples here but an annoying format. You can probably find the originals outside this system once you see something of interest here

An extremely worthwhile instructable. While he reports problems with operation he has images of inside equipment and circuit diagrams. An excellent resource

Many examples here

An instructable - rough but workable

Possibly same as above - somewhat different material

A hack-a-day example

What kind of power can I hope to get through?

Watts is easy.
10's of watts is not very hard.
Kilowatts is doable

What is the optimal frequency? (is 50 Hz ok)

50 Hz is terrible.
Higher frequency = smaller coils. 50 Hz ineeds vast coils or large cores.
Best to use an assigned ISM (Industrial Scientific Medical) frequency and/or one used by one of the many systems that do this. 'Off the cuff' 125 kHz and 13.5? MHz are two such./ Searching on IPT as above will tell you more.

Not if done well. The transfer is near-field magnetic - not RF.

Will an unconnected iron core help?

Read the many articles. Coils need to be resonant. Use of an iron core is not usual.

What is best - many or few windings?

Read the many articles - follow some examples cited above to start.

Please also tell me, if you know of other reasons why this would not work.

Entirely doable. Start by copying others' examples.

Very good demo video.

Related website here

100% Duty Cycle: When Steven said "100% duty cycle" he meant "always-on AC waveform"

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Not sure, but I don't think ISM allows you to send with 100 % duty cycle (read: continuously). –  stevenvh Sep 28 '12 at 14:27
@stevenvh: You can not induce current without a varying magnet field. You can not create a varying magnet field without a varying current. 100% duty cycle equals DC. –  Hans-Peter E. Kristiansen Sep 28 '12 at 18:24
@Hans-Peter - You can define duty cycle in many ways. IIRC in ISM they talk about duty cycle when they mean the ratio of transmission: if you send for 100 ms, and then are silent for 900 ms, then transmit again for 100 ms and so on, they call it a 10 % duty cycle. –  stevenvh Sep 28 '12 at 18:42
@stevenvh: I have no idea what IIRC and ISM is, but you can not send DC between coils. –  Hans-Peter E. Kristiansen Sep 28 '12 at 18:43
@Hans-Peter - I'm telling you: sending, transmitting. That means radio signals. ISM is Industrial-Scientific-Medical, and it's an acronym for applications which have been assigned a number of RF bands. When I say "send for 100 ms" that means "send the RF signal for 100 ms". (BTW, IIRC means "If I Recall Correctly). (And BTW means "By The Way"). –  stevenvh Sep 28 '12 at 18:47