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Inductive charging / power

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Hi all,

I'm after a circut to either make a bulb/led glow inside a small plastic car when it's placed on a stand or have a battery inside the car (to power the led/bulb) that will be recharged when the car is placed on a stand, i guess it's something similar to one of those rechargeable toothbrushes,

I would like to power the charger/source from some sort of plugpack

Any ideas would be greatly apperaciated......


I saw something similar to this at radio shack the other day. It was a little remote control tank, and when placed on the stand (properly inserted) the battery inside of the tank would charge.

I'm a little confused on what you're trying to accomplish. Are you wanting the LED to light up only when the car is on its stand? Or are you wanting a LED to glow when the car is off of the stand, powered by a battery which is recharged when you place it on a stand?

If you're wanting a LED to glow when the car is on the stand, you wouldn't really need a battery if you're using a wall wart (Plugpack). Conversely, you wouldn't really need a plugpack if you have a battery on board, unless you want to be able to charge the battery.

More info please... :D
hi, sorry for the incoherant babble,
a led that glows when it's on the stand and doesn't when it's away from the stand (no battery) is the first choice, but if i have to use a battery (perferably rechargeable and it gets recharged while sitting on the stand)with the led to make it happen then we can work with this aswell.

hope this helps........

cheers :)


any more info needed??
An LED takes very little power, an oscillator placed under the stand could transmit enuf energy to the car to light the LED. The receiver would just be a coil and diode ( or maybe the LED could be the diode). An oscillator frequency of 100kHz would make it small and simple.
thanks for the info, can you give me any hints as to the circuit design - what values for components, what would it be best to run the circuit on 5 - 12v?? or higher voltage plug pack???

cheers :)

It sounds just like my "sonic" toothbrush. I put it in the "charger" and if in the correct orientation it charges even without any conductors making contact. My guess is that there is a coil powered by AC in the charger and a coil in the toothbrush that are in close proximity when it position - to behave as a transformer without a core. The magnetic lines of force induce a voltage on the non-powered coil.

If I wanted to accomplish this I'd start here: 3 vac supply and a coil (no core or slug) that has an inductance of about 80 mH. Why 80 mH - no other reason than 80 mH looks like 30 ohms at 60 Hz and 30 ohms across 3 vac gets 100 ma. Match that with an equivalent coil separated by some plastic and see what you get. If the coupling is good you ought to see something close to 3 vac on the non-powered coil.

Coils - I've seen them (no core) wound for crossover applications and other uses.

This might not be entirely right but it's the best I can do in 5 minutes.
I threw this design together to illustrate the idea, I think it works.


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hi all,
Thanks heaps for the info, to answer the question about the led, yes we are looking to power it by induction


In the above circuit what type of caps are the 0.1uf & 0.33uf :?:
i assume the 1uf is an electrolytic.....


hi all,

is there any way to get the coils smaller, i'm assuming that a 2in form is

2in in diameter???

thaks for your fast answer.....

for the powered side about an 1in would be good, for the led side as small as possible 1/4in????????? does the thickness of the wire have a bearing on the amount of turns needed and / or the amount of current produced??

No, the size of the wire is only important when the resistance gets significant. The 1/4" diameter for the receiving coil will give two problems:1. More turns will be needed, so the transformer ratio will reduce the current. The smaller diameter should be closer to the primary to get good coupling. The coupling is related to the ratio of the coil diameter to the spacing between coils.

I notice that I neglected to provide bias for the transistor, that error is corrected in this schematic. I simulated the circuit, there is about 10 mA current in the transmit coil, so the receive coil current will be about 3mA with perfect coupling. I think the LED will be rather dim. A 1" receive coil would work much better.


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Thanks for the circuit, works well, would you be able to tell me how many turns is needed for a 1/2 inch form......

also to increase the distance between the coils what can be done.....


thanks heaps again for your help!!
For 1/2 inch diameter coils, transmit 11.8 turns #30, receive 10.7 turns #30. One way to be able to increase the spacing between coils, is to run the oscillator on higher voltage (more power).
I've noticed that the 100 ohm resistor connected to the emitter of the tranny is getting quite warm - is this to be expected?

i'm running the circuit on a 12V DC 300mA plug pack......
the resistor is a 0.5w metal film type.....

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