Power charging bracelet - Page 1
Forum section: electronics / projects

author: TheElectronicsBoy
author_rank: Newbie
timestamp: July 28, 2015, 11:44:04 pm
content: Saw this Dyson power bracelet and thought this is a great idea:http://www.yankodesign.com/2009/06/18/the-power-of-my-wrist/.I wanted to know if someone can draw a schematic of how the output of 5 volts and 1 amp is obtained to charge.

author: ajb
author_rank: Super Contributor
timestamp: July 29, 2015, 03:39:49 am
content: If you google for "Thermoelectric energy harvesting" you'll find some app notes and even some evaluation kits that will get you started. The gist is that you use the Seebeck effect to convert a temperature differential into an electrical current which you use to charge a battery. Once the battery is sufficiently charged you can use this to charge a cell phone. The trouble with this application is that cell phones require a LOT of power to charge (5-10W typically), and the amount of power available to a Seebeck device is going to be very small, because the temperature differential between your body on one side of the bracelet and the ambient air on the other is going to be pretty small unless you live in the arctic. That means it will take many hours to accumulate enough energy to charge your phone for only a few minutes (which the designers admit, impressively, in the text in your link). In order to make it work at all, you'll need some sort of boost converter to boost the voltage from the TEG to the ~4V required to charge a battery, and in order to maximize efficiency you'll need to control the operating point of the TEG. There are ICs geared specifically towards this application, and their manufacturers will have app notes with more details. On the other side of the battery, the circuitry is going to be exactly the same as that in one of those portable USB battery banks--a boost converter to provide the 5V required by the charging device, and either some resistors or maybe a fancy IC to identify the part to a charging device.

author: JimRemington
author_rank: Regular Contributor
timestamp: July 29, 2015, 03:51:45 am
content: Forget the wrist strap scam. To get 5 V at 1 amp out of a Peltier module, you typically need a temperature difference of at least 150 Celsius across the module. To tolerate those high temperatures, you need specially constructed modules intended as thermoelectric generators.Marlow is one source and a single $30 module is capable of providing that amount of power, provided that you can maintain a 180 C temperature difference. Seehttp://www.marlow.com/power-generators/standard-generators.htmlYou can buy TEG camping stoves that use boiling water to cool one side of the module.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 29, 2015, 04:58:49 am
content: Quote from: JimRemington on July 29, 2015, 03:51:45 amIt's actually not a scam. I don't think you understand how the device works.The device harvests the small amount of energy produced to very slowly charge an internal lithium battery inside the bracelet. That takes many hours.It is the battery which provides the capacity and current necessary to power a USB device for a few minutes.The usefulness of the device is limited, since it takes many hours to obtain a few minutes of charging time, but I suppose someone could find a use for it, perhaps as an emergency power source to get one call out when there is no other way to power up the phone. A small solar panel would probably be more effective, but not as easy to carry around as something you can put on your wrist.Nowhere do they claim that it can power a USB device from your wrist heat indefinitely. It actually says right on the page "some hours are sufficient to power its cellphone for a dozen minutes of communication".

author: JimRemington
author_rank: Regular Contributor
timestamp: July 29, 2015, 05:22:48 pm
content: QuoteThanks, but I do understand how it works. The Linear Tech site has plenty of useful information on the power to expect from Peltier devices subjected to such low temperature gradients as might be encountered in wearable devices. I recommend to spend some time on this and related pages:http://www.linear.com/product/LTC3108At a delta T of 5 degrees C, you can expect roughly 0.6 mW power output at 3.3V from a typical TEG/LTC3108 converter. Seehttps://cds.linear.com/docs/en/lt-journal/LTJournal-V20N3-01-df-LTC3108_09-David_Salerno.pdfSo, the wrist band is indeed a ridiculous scam. You can work out from the above about how many hours of wearing the wrist band, at deltaT = 5 C, is required to charge the lithium battery sufficient to allow, say one minute of iPhone operation.

author: electr_peter
author_rank: Unknown
timestamp: July 29, 2015, 08:18:06 pm
content: Is it really Dyson company (company that makes vacuum cleaners and fans)? What a shameAnother scam about energy harvesting. Few points to keep in mind:Peltier element creates electricity from the flow of energy, not just from temperature difference. Thus you need to ensure that hot side stays hot, cold side stays cool and both sides have low thermal resistance. For human body in free air this does not work at all.Efficiency, energy potential, temp difference, etc. are so pathetic that is simply not worth any effort to even try this method. Energy harvesting is for those applications that can live on measly harvested power. Cellphones are too power hungry for this.With efficiency and usefulness so pathetic, who would be stupid enough to wear this thing on their arm for 3 months to charge the battery in case of emergency? Simply buy extra battery pack if you are so paranoid about discharge.

author: JimRemington
author_rank: Regular Contributor
timestamp: July 29, 2015, 08:36:15 pm
content: Speaking of pathetic, I love this statement from the wrist band designers:Quote

author: tszaboo
author_rank: Super Contributor
timestamp: July 30, 2015, 08:54:19 am
content: Quote from: TheElectronicsBoy on July 28, 2015, 11:44:04 pmEasy. You connect a voltage and current measurement meter to it, and photoshop and 3D model the required voltage and current to it, like the rest of this product was made.