Looking into building an induction charger

[lilimike]

I've been browsing for a while but I can't find enough specific information to get me started.

I want to build an induction charger that will charge two AA batteries in series.
I understand there is a coil in the charger and another coil in the portable device picking up the current by induction but I would like to know how to calculate the coils. I have found Inductor calculators that will give me an inductance value based on diameter, gauge and length but unfortunately knowing inductance value doesn't really help me unless someone tells me what value I need to accomplish my goal.

Also I presume such a device can only trickle charge and does not use feed back to lower the charge as the batteries are close to full charge and I suppose batteries would never get close to full charge in this type of configuration.

This being said I need to be able to leave the portable device (batteries) on the charger for a long period of time and if possible keep them on the charger full time when the portable device is not in use.

Any help, link, example would be appreciated.

Mike

 

[Reloadron]

This link should get you started with a schematic and the info for the two coils.

Unfortunately when my last electric toothbrush was dying my wife replaced it and tossed the old unit out. I am now waiting for the new unit to die so I can dissect it.

Ron

 

[crutschow]

The battery charge regulation circuitry would need to be with the coil in the device being charged.

 

[lilimike]

Thanks Ron,

I had seen this link, I don't know if I have some kind of deficiency but when I see those circuits using ASCII characters it triggers a small current in the index of my right hand which makes it click the mouse button away but I am trying to resist and and stay on this page.

What is this MOV device at the AC input?

Mike

 

[lilimike]

Carl,

Are you referring to the diode? should I add anything else?

Mike

 

[crutschow]

Carl,

Are you referring to the diode? should I add anything else?

Mike

Well, you need something to regulate the charging current to the battery if there is not already circuitry for that. It can be a simple as a resistor, if you are charging an NiMH battery at a low rate (such as a 15 hour charge time). For faster charge times and Li ion batteries, you will need an integrated circuit type regulator designed to charge the specific type of battery you have, that detects the charge state of the battery and reduces the charge current when charged.

 

[lilimike]

Based on what I have seen, NiMH would be my best choice, it will be within a device that will discharge for about 4 hours a day and I want to be able to keep it on the charger for days or weeks. Now the link Ron provided indicates only a diode in series with the coil. Do you suggest adding a resistor?

What would be its value?

Mike

 

[Reloadron]

Thanks Ron,

I had seen this link, I don't know if I have some kind of deficiency but when I see those circuits using ASCII characters it triggers a small current in the index of my right hand which makes it click the mouse button away but I am trying to resist and and stay on this page.

What is this MOV device at the AC input?

Mike

Yeah, they make my head hurt. The MOV is a metal oxide varistor. It is there to protect against transient voltages from the mains. The circuit would work without it, however I would use about a 200 volt MOV for 120 VAC mains power. Now although not mentioned I am assuming 120 VAC mains so if you try to build this be careful! The battery is a 2.4 volt NiCad as they mention.

Overall it is a simple circuit a likely much like the electric toothbrush I want to dissect.

Ron

 

[lilimike]

120V AC is going straight in there? wow! I guess I will add a fuse, is 1A ok?
Should D1 on my schematic be 1N4148?

This is better for the eyes:


Edit: Forgot to say I made a few mods, Bridge(for clarity), Switch, Fuse, LED + Resistor


Mike

 

[Reloadron]

I would venture .5 amp fuse on the mains would be fine. As to D1, yeah, I would try a 1N4148 for starters. Considering the coils are AWG 30 I wouldn't worry about current limiting at first but measure it and go from there.

Ron

 

[ericgibbs]

120V AC is going straight in there? wow! I guess I will add a fuse, is 1A ok?
Should D1 on my schematic be 1N4148?

This is better for the eyes:
View attachment 44590

Edit: Forgot to say I made a few mods, Bridge(for clarity), Switch, Fuse, LED + Resistor


Mike

hi Mike,
Do NOT build that circuit you have posted, if you plug that into the mains you will get a bang and a dark brown smell..!

 

[lilimike]

Thank you Eric for the warning,

But is it something I did wrong or the circuit all together?
Is there anything I can do to improve it?

Mike

 

[ericgibbs]

Thank you Eric for the warning,

But is it something I did wrong or the circuit all together?
Is there anything I can do to improve it?

Mike

Mike,
Consider that 1K and LED across the rectified mains, they have 170Vpk across them, so that about 160mA thru the LED and 17Watts dissipation in the resistor.!!
The two 1K' s in the +V and 0V rail dissipate about 5W each........

If the component values are correct then the 60kHz appears as just a short burst near the positive peak.??

 

[lilimike]

Ok, I see what you mean.

Does this make more sens?


I am in the process of learning LTSpice so eventually I will test my circuits before I post them.

Mike

 

[lilimike]

A few questions on the circuit from previous post:

1 - If I want to recharge 10 devices or more, is it ok to connect 10 coils (L1) in parallel? otherwise do I have to replicate 10 times all components after the bridge?

2 - About the coil, 200 turns of #30 on 1½" I guess 1½" is the external diameter of a tube where the coil starts to be rolled around. Does it make a difference how many wires will be side by side or row by row as the next row will have a diameter slightly larger than 1½" and so on for each additional row?

3 - This will probably have the same answer as #2, will 199 turns or 201 turns make an important difference (based on what I am using it for)?

(My first experience with coils)

Mike

 

[ericgibbs]

A few questions on the circuit from previous post:

1 - If I want to recharge 10 devices or more, is it ok to connect 10 coils (L1) in parallel? otherwise do I have to replicate 10 times all components after the bridge?

The inductor is part of the 60kHz osc circuit, so you cannot parallel more coils.

2 - About the coil, 200 turns of #30 on 1½" I guess 1½" is the external diameter of a tube where the coil starts to be rolled around. Does it make a difference how many wires will be side by side or row by row as the next row will have a diameter slightly larger than 1½" and so on for each additional row?
Its the outside diameter of the blank former.
Make the length of the winding as short as reasonable, dont forget the coupling required to the remote pick up coil.

3 - This will probably have the same answer as #2, will 199 turns or 201 turns make an important difference (based on what I am using it for)?
No problems.

(My first experience with coils)

Mike

hi,
I would initially build the circuit and debug it.
Calculate the wattage of the resistors used, also remember there is no mains isolation so the circuit is a shock hazard and could kill.

 

[lilimike]

Thanks Eric,

Its the outside diameter of the blank former.

I have spend the day at a bunch of surplus stores and I could not find anything.
Also tried to Google for blank coil former with no luck either. Does this plastic thing go under another name?

Anyone have any links to where I can buy this? I don't need one, I need 100s... well unless the first one doesn't work then I will only need one.

Make the length of the winding as short as reasonable

Are you referring to the width of the former? or the length of the wire? or maybe its the same? the wider the former is the shorter the wire will be?

dont forget the coupling required to the remote pick up coil.

Are you refering to the other circuit with 160 turns on a 13/16" diameter?

I would initially build the circuit and debug it.

I am struggling with LTSpice, I have my circuit already in there but still learning. Since I am dealing with the main I want to test as much as I can on LTSpice.

Would a 1:1 transformer make it safer? I figured not but maybe so?

Mike

 

[Reloadron]

OK, if I were going to build this project the odds of you finding a perfect coil form are like one in millions so I would resort to making one. I would start with a piece of nylon dowel rod and then place it in a lathe and cut to the size I wanted.

Now if that is not practical I would go to a drug store and find a cheap, as in real cheap electric toothbrush and perform a lobotomy on it. I think the better Oral B models only cost about $35 USD and there has to be some cheaper versions out there. Hack it apart and note the design, The coil forms and spacing.

Since this is air core it really matters not how you fabricate a coil form. Just work with the basic design. Experiment a little for performance and understanding how it works. That would be my suggestion anyway.

Ron

 

[lilimike]

Thanks Ron,

i have a feeling I will be experimenting a bunch with this project, the original design calls for the charger part having a coil diameter of 1½" and the charging side 13/16"

It appears to me, it would have been more logical to make both coils 1 inch and change either the size or number of turns but i guess there is a reason for these specific sizes and that's what is making things difficult for me.

Mike

 

[jamesstanley]

did you ever get round to building this?

hi just a quick question did you ever build this, i am looking at doing something similar,like the powermat that is out in the shops. just wanted to know how you got on