Switch Mode Altoids iPOD Charger using 3 'AA' batteries

The goal of this project was to build an efficient Altoids tin iPod (firewire) charger that runs on 3 (rechargeable) 'AA' batteries. This project started out as a collaborative effort with Sky on PCB design and construction, and I on circuit and firmware. As it is, this design will not work. It is presented here in the spirit of "the concept of a derivative project"
( http://www.instructables.com/ex/i/C2303A881DE510299AD7001143E7E506/ )

"????-- a project that uses another project as a stepping stone for further refinement, improvement, or application to a totally different problem. The community of DIYers that we're all a part of can really do some amazing things working together as a community. Innovation rarely happens in a vacuum. The obvious next step is to let the community help refine and evolve ideas that aren't yet ready to be finished projects."

We submit this now so that other iPod enthusiasts could pickup where we left off.

There are (at least) two reasons this charger _does not_ work:

1. The transistor doesn't let enough current flow to fully charge the inductor. The other option is a FET, but a FET needs a minimum of 5 volts to switch fully on. This is discussed in the SMPS section.

2. The inductor is simply not big enough. The charger doesn't produce nearly enough current for the iPod. We didn't have an accurate way to measure the iPod charging current (save cutting apart the origional charging cable) until our parts arrived from Mouser. The inductors recommended are nowhere near large enough for this project. A suitable substitution might be the coil Nick de Smith uses on his MAX1771 SMPS. Its a 2 or 3 amp coil from digikey:

( http://www.desmith.net/NMdS/Electronics/NixiePSU.html#bom )

This device can provide a small amount of power to a USB or firewire device, but not enough to charge an (3G) iPod. It WILL power, but not charge, a totally dead 3G iPod.

Step 1 Switch Mode Altoids iPOD Charger using 3 'AA' batteries

The goal of this project was to build an efficient Altoids tin iPod (firewire) charger that runs on 3 (rechargeable) 'AA' batteries. Firewire delivers 30 volts unregulated. An iPod can use 8-30 volts DC. To get this from 3 AA batteries we need a voltage booster. In this instructable a switch mode power supply based on a microcontroller is used.

Standard disclaimers apply. High voltage....deadly...etc. Think about how much your iPod is worth to you before connecting it to this little stun gun in a tin can.

For all the math and dirty details of SMPS, read the nixie tube boost converter instructable:

http://www.instructables.com/ex/i/B59D3AD4E2CE10288F99001143E7E506/?ALLSTEPS

Read on to see how the nixie tube SMPS design was adapted to be an iPod charger....

---

A ton of previous work inspired this project. One of the first DIY chargers used a combination of 9 volt and AA batteries to charge an iPod through the firewire port (works for all iPods, mandatory for 3G iPods):

http://www.chrisdiclerico.com/2004/10/24/ipod-altoids-battery-pack-v2

This design has the problem of uneven discharge among the batteries. An updated version used only 9 volt batteries:

http://www.chrisdiclerico.com/2005/01/18/altoids-ipod-battery-pack-v3

The design below appeared on Make and Hackaday while this instructable was written. It is a simple design for a 5 volt USB charger (this type will not charge earlier iPods, such as the 3G). It uses a 9 volt battery with a 7805 5 volt regulator. A stable 5 volts is provided, but the extra 4 volts from the battery is burned off as heat in the regulator.

http://www.instructables.com/ex/i/9A2B899A157310299AD7001143E7E506/?ALLSTEPS

All of these designs have one item in common: 9 volt batteries. I think 9 volters are wimpy and expensive. While researching for this instructable I noted that an 'Energizer' NiMH 9 volt is only rated 150 mAh. 'Duracell' doesn't make rechargeable 9 volters.

A 'Duracell' or 'Energizer' NiMH 'AA' has a healthy 2300 mAh of power, or more (up to 2700 mAh ratings on newer rechargeables). In a pinch, disposable alkaline AA batteries are available everywhere at a reasonable price. Using 3 'AA' batteries nets us 2700mAh at ~ 4 volts, compared to 150mAh at 9 or 18 (2x9 volts) volts. With this much power we can live with switching losses and extra energy eaten up by the SMPS microcontroller.

Comments

[Blackice504]

when i try to load the Diagram your sch file does not work i am using PCB express

[spock155]

Just a question, how much money did you spend aproximatly getting all the parts and such?

[westonN]

Use the 555 timer and a power MOSFET. for a school project I had to build a boost converter using that configuration, look up the article on wikipedia. this design is becoming to complicated

[spock155]

Agreed.

[westonN]

what voaltage is this desinged for

[snarfer]

Another thing to keep in mind is the input capacitance. In my experimentation I found that even a logic level FET can require an external driver, such as the TC442x series. The Fairchild FDS9926A mentioned above has an input capacitance of 650 pF. You might find that it heats up quite a bit unless you use a driver circuit. Another alternative that could be interesting is a depletion mode FET, such as available from Analog Devices, IXYS, Supertex, and Clare. Depletion mode FETs are already on at 0 gate volts. Look it up.

[ssizzle52]

wow why dont you make it a little harder to make

[thematt3]

Koolieo

[Einsteins Circuitry]

Very well done!

+1

[burningsuntech]

Nice job, ian. Certainly one for my collection of circuits.

[theprofessor]

another thing that comes to mind, a 555 can be used to controll a buck/boost converter, there was an application note on that somewhere, would most likely be easier for somone to understand without knowing jack about EE. I'll look at it and if i find a break in my class load see if i can concoct something and post it.

[carlton stedman]

Sure, to provide a constant duty cycle, but that won't work when the input voltage or output current draw changes. Doing it the analog route you'll (typically) control the duty cycle with a PWM signal. This is not trivial.

[James (pseudo-geek)]

"1. The transistor doesn't let enough current flow to fully charge the inductor. The other option is a FET, but a FET needs a minimum of 5 volts to switch fully on. This is discussed in the SMPS section." darlington pair maybe?

[Parth]

this is sweet

[hjjhjhgjjh]

Sweet. Now you can switch between ipods and stuff. Nice project good job.

[theprofessor]

even easier is to use a 4 comparitor scheme to generate your triangle wave and then to rectify it, there also are switch mode chips (pricy, but if your just building one what a buck fifty) using a pic here is excessive and slow, my experience with class D is slew is your enemy.

[hjjhjhgjjh]

I need help tranfering a design to eagle cad so I can make a pc board or pcb.

[bsyris]

Guys, the easiest thing for you to do now is to use an NPN Bipolar transistor instead of your NMOS FET. The pinout should even be the same (BCE vs GDS). Radio Shack carries the TIP120, which should suit your app. It should just plug right in and you can run out and try it! They only require a couple volts to turn on and the 120 is a Darlington so your PIC should be fine driving it.

http://www.radioshack.com/product/index.jsp?productId=2062617

Of course, Somnione is right: next time just use a dedicated DC/DC converter.

[Somnione]

Why use the pic when there are specialized ic's for that purpose? (up/down dc-dc) 34063 (just search on google "34063 dc dc pdf" for a datasheet). They are found in most car cellphone chargers, but are used as step-down from 12 to whatever a cell uses. I remember there was even a dos application(i believe it was from motorola) for calculating the values of resostors, coils, etc in the circuit for such a device.

[ian]

Any uC can drive a SMPS, specialized parts can be hard to get in the right package or a year after the article is written. Using a uC also demonstrates all the principles of the SMPS, rather than the instructions in the manufacturer's datasheet.

[toaste]

What's the series resistance of your inductor? Too high a resistance here can dramatically reduce the performance of your boost converter. One expiriment you should try is using two inductors of double the target value, connected in parallel. I know they're big components, but halving the resistance ought to help a lot.

[toaste]

The above is just one example of why I appreciate spell check more than most... experiment...

[Kipp]

Dan is right. There are FETs that are design for SMPS, like the Fairchild FDS9926A, which actually turns on at about 1v. At 2.5v it claims an on resistance of 43mOhm. The continuous drain current is a couple of amps. It's a dual N-Fet packaged in an 8pin SOIC, so gang them together and get half the resistance. The part is $0.71 each at Digi-Key. If you need a DIP adapter, they are about $6 at Digi-Key (part #A724-ND) but may be available elsewhere for less.

[ian]

Thanks guys. I found three fairchild FETs at mouser that have a Vgs of 2.5 or 3 volts. I will order them and give it a try.

[dan]

you can get FET's that turn on fully as low as about 2.5V.