Inductive charging. How ? - Page 1
Forum section: electronics / projects

author: free_electron
author_rank: Super Contributor
timestamp: August 21, 2012, 01:19:37 pm
content: Anyone got any hard info on how it's done ?Im talking about deices like the powermat.Any appnotes ? Coil design info ?I have a product idea and would like to include inductive charging. Just a plate om the desk. Drop the unit on it and it charges.There has got to be chips and appnotes out there that do this stuff... Just havent found one yet.I want to charge a single cell lipo battery in about 14 hours.

author: mtkaalund
author_rank: Contributor
timestamp: August 21, 2012, 01:42:44 pm
content: Hum, if you research transformer designs without iron cores, you will get some idea on how the transformer should be. But believe me, you probably need to make a different couple of designs and try them out.on the primary side, you need some kind of switching to get the transfer, on the secondary side you need some sort of rectifier circuit, and lipo charger circuit.If I was to going to do this, I would take a look on a SMPS chip, and use this as the primary switching side.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 21, 2012, 01:43:07 pm
content: It doesn't look that easy...

author: AndyC_772
author_rank: Super Contributor
timestamp: August 21, 2012, 01:53:50 pm
content: Have a look on the Texas Instruments web site and search for "Qi".

author: SeanB
author_rank: Super Contributor
timestamp: August 21, 2012, 01:54:45 pm
content: Going to be anything but efficient. Commonly the mat has either a large pancake coil wound under it, with the devices having a similar coil to harvest energy from the very high impedance coupling, or you have a ferrite core that is used to increase permeability, but at the expense of needing close alignment of the 2 coils.I do not see any way to make what is basically a crystal radio into something as efficient as a wire.If you are going to have a charge station and a design that is waterproof or encapsulated with no external metal, then you can have a close coupled coil and matching one in the unit, with a ferrite core and cup that increases coupling between them. Still a massive power loss, but the efficiencies will be better, and might reach 50% of the input power being coupled, depending on how close you can couple them and how big a magnetic circuit you can fit in the station and device.

author: T4P
author_rank: Super Contributor
timestamp: August 21, 2012, 02:12:50 pm
content: It usually introduces what looks a massive airgap between the two coils, seems good on paper but no, the main coil needs to pass way over 2A @12v for 1A on the receiving sideDefinitely not efficienthttp://www.instructables.com/id/Wireless-Ipod-Charger/

author: olsenn
author_rank: Frequent Contributor
timestamp: August 21, 2012, 02:38:37 pm
content: Getting unregulated power transferred is quite easy; it just uses basic induction. The advanced looking circuitry you see in your PowerMat (or similar device) is for communicating with the device placed on the charger. Several functions are performed, namely a handshake to see that there is a device waiting to be charged, a heartbeat to determine that the device hasn't been removed from the charger. The receiving end also has circuitry in it to perform these tasks as well as to convert the AC wave to DC and regulate the voltage (to 5V or whatever is required).

author: deephaven
author_rank: Frequent Contributor
timestamp: August 21, 2012, 02:51:36 pm
content: Freescale have some app notes:http://tinyurl.com/d5c7k3d

author: G7PSK
author_rank: Super Contributor
timestamp: August 21, 2012, 03:35:35 pm
content: %0/60 HZ works quite well as used in tooth brushes all the electronics is in the appliance for battery charging. I ripped one of those Braun's apart once and that was all there was in the base a coil and iron core to go up into the secondary.

author: mamalala
author_rank: Unknown
timestamp: August 21, 2012, 05:54:52 pm
content: A modified Royer Converter, as implemented by Peter James Baxandall, is a nice simple circuit that does this too. No need for fancy chips.Check out this page (it's in German, you may want to use Google-Translate or similar):http://www.mikrocontroller.net/articles/Royer_ConverterGreetings,Chris

author: free_electron
author_rank: Super Contributor
timestamp: August 21, 2012, 06:15:00 pm
content: i don't care about efficiency.from the picture posted by pa0pbz it loks like they use RF . i see one big fat coil made form lits of tiny paralle wires. and a second cingle strand coil. pickup ?i found one paper where the make a resonant circuit , both on transmitter and receiver.I only need to charge a 700mAh cell in 14 hours so if i can transfer 100mA i am good.Air gap is going to be in the order of 1 cm.This is a system that is used underwater and sits in a fully sealed container. once built and tested it is placed in the cousing and the housing is completely filled with an epoxy. it cannot be opened. its a bouy that can be deployed to track temperature , salinity and tides. the thing is extremely low power. every six months the buoy is pulled, data is dumped over the bluetooth link and the buoy is recharged by placing it on the charger mat.i may actually go and buy the powermat. i just found that they sell a receiver block that comes out with a 5 volt plug.if i rip that block apart i have the right guts.it uses a bluetooth link

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 21, 2012, 06:37:46 pm
content: Quote from: free_electron on August 21, 2012, 06:15:00 pmI think the distance between the mat and the receiving end is a lot less than 1 cm and I'm not sure either what the epoxy will do,but it never hurts to experiment a bit.The picture I posted is from a HP Touchpad charger, I believe it runs on 100 KHz. There's some teardowns on the net of both the charger stand and the Touchpad itself, so you could have a look at these.Tuning both the sending and receiving coil should improve the efficiency, or at least that is how it feels.

author: richcj10
author_rank: Unknown
timestamp: August 22, 2012, 02:15:43 pm
content: I have done this for a client that needed 9v at 100mA at 3+ inches.It is simple. It uses a ZFS drive like the Ipod charger instructable.It needs a huge start up current (about 3 amps) but when ruining it pulls about 500mA.I run it at 24v. If you want more details, let me know.

author: FenderBender
author_rank: Super Contributor
timestamp: August 26, 2012, 12:28:45 am
content: This seems so wasteful for common devices like phones. What...You cant plug in a little USB plug? Too hard?

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2012, 12:47:02 am
content: Quote from: free_electron on August 21, 2012, 01:19:37 pmYou are basically making a transformer where the two halves are separable.There are two modes of operation where power is transferred between isolated circuits. One is by electromagnetic waves (radio transmission), which is low power and inefficient. The other is by electromagnetic coupling (which is high power and potentially efficient).For a charging circuit you will want electromagnetic coupling, in which case transformer design theory will provide all you need. You can go air cored (charging mat, needing a high frequency), or magnetic cored (electric toothbrush) where a lower frequency may suffice. For practical design insights you might try disassembling an electric toothbrush (if you can get past the potting compound).

author: hans
author_rank: Super Contributor
timestamp: August 26, 2012, 09:03:49 pm
content: I've watched a graduation presentation of fellow students about Qi wireless charging at college.They said Texas Instruments has several chips for this. It's indeed basically an air based transformer; except it works in the order of 100kHz to 1MHz or something.Communication is done by changing capacitors in the resonance network so the resonance frequency changes, if I recall correctly. Or it was a second order (like charging at 100khz, communication at 1MHz). Qi describes more about that and the protocol. They said there is quite a lot of communication back and forth to set up a power level etc.In short, the advantage of wireless charging is convenience. They also said that wireless charging is probably also only going to be added on expensive devices , because the additional cost today is still significant (like a few dollars, which is insignificant on a mobile phone, because it adds a nice feature for consumers).I believe I saw this Devkit from TI:http://www.ti.com/tool/bqtesla100lp?DCMP=hpa_press_releases&HQS=Other%252bPR%252bbqTESLA100LP-prChips:http://www.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1992&uiTemplateId=NODE_STRY_PGE_TFrom seeing there graduation presentation, they went through all the math of transformer coupling factors, resonance, communication, etc.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 28, 2012, 04:14:47 am
content: Quote from: FenderBender on August 26, 2012, 12:28:45 amMaking it wireless has advantages in robustness and waterproofing if your application needs it. Also, it can allow for (slowly) charging a device that is still in a bag.I have managed to make a wireless power demo out of two Parallax RFID readers (Radio Shack had a clearance on those a few years back). Power one up and hold the the other one near it. The LED on the unpowered reader will light up. And strangely, the power usage of the active reader actually decreases with the unpowered reader near it.

author: hans
author_rank: Super Contributor
timestamp: August 28, 2012, 07:06:42 am
content: I believe it isn't uncommon to see power consumptions drop when the 'load' is removed. There is something with the L/C combination, quality factor of the coil etc. I don't remember exactly how. But anyway, I believe all the 'extra' power is being dissipated in the transformer coils.I guess the output power of RFID readers is very low to make them not burst into smoke or flames.