A 12V
Car Charger For ASUS Eee Notebook
The ASUS Eee is a fantastic ultra-portable notebook with almost everything
required for geeks (and nothing that isn’t). Plus it features fantastic build
quality and is very well priced. If you live in New Zealand you can get them
from DSE; at the time of writing they are the exclusive supplier. I worked out
it’s the same cost as importing one once you include all the duties and tax,
plus you get the advantage of a proper NZ-style mains charger. Anyway, being so
small I thought it would be nice to be able to carry this around in the car.
Unfortunately I couldn’t find a car charger available anywhere at the time so I
decided to tackle the problem myself. As a bonus this provides an opportunity
for an external high-capacity battery.
Commercial Equivalent:
I thought at this stage it would be worth noting that a commercial car charger
is now available for less than it cost me to build this from Expansys and is
available in most countries (select your location on their site). It outputs
9.5v from 10-18v in at up to 2.5A. I’d actually recommend it over the design
here is it seems to perform better at lower voltages (that one works down to
10V). However I have kept this page up as a reference for those who enjoy
tinkering.
Design:
The charger included with the Eee is rated at 9.5v, 2.315A. There isn’t a fixed
voltage regulator available for this exact voltage, so the circuit needed to be
designed around an adjustable regulator. I decided to design the charger around
the LM2576 “Simple Switcher” IC from National Semiconductor. There are tons of
ICs like this available, many of which are a bit more efficient, however I
selected this one because it is readily available and relatively cheap. It also
has a lower drop-out voltage (~2V) than many other chips I looked at which is
important when powering the device from a car or 12v SLA battery.
This circuit could have used a standard three pin regulator IC such as the
LM317, however most types require an external transistor when handling so much
current and not to mention the fact that they are very inefficient; they draw
the same amount of current from the input as the load and the difference in
power is dissipated as heat. The main problem with using the LM2576 is the fact
it needs quite a large inductor due to its somewhat low switching frequency.
The inductor I used is made by Pulse Engineering, part number PE92108KNL. I’d
prefer a smaller one, however I couldn’t find one capable of supplying the
required current that I could purchase in single units. Besides the PE92108KNL
is apparently designed specifically to work with the LM257x series.
The circuit also includes a low voltage cut-out based on a 9.1v Zener diode
and BC337 transistor that will shut down the regulator if the input voltage is
below 11.5V. This prevents unstable operation of the regulator at lower input
voltages, and also helps prevent accidental flattening of the supply battery.
Substituting this transistor for similar type may affect the cut-out voltage;
the Vbe of the transistor should be 1.2v.All of the components used should be
pretty readily available in most areas. I got everything from Farnell. Jaycar
also sells everything except the inductor. Make sure you specify high
temperature, low ESR capacitors as these help result in more stable operation
and better efficiency of the charger.
Unfortunately the end result is a charger that is slightly bulkier than I
would really like. I attempted to fit this inside an old mobile phone charger
case so the whole thing could hang out of the cigarette lighter, however I ran
into trouble making the circuit stable enough and dissipating all the heat. Due
to the high current involved compared to a mobile phone charger the components
are much bulkier so it’s pretty tricky to get all to fit! If I do get it
finished I’ll add an update.
Parts List:
- 2× 10k resistor (R1 & R4)
- 2× 22k resistor (R2 & R3)
- 1× 1.5k resistor (R5)
- 1× 120μF 25v electrolytic
capacitor (C1)
- 1× 2200μF 16v electrolytic
capacitor (C2)
- 1× 1N5822 Schottky diode (or equivalent)
- 1× 9.1v 0.5W Zener diode
- 1x BC337 NPN transistor
- 1x LM2576T-ADJ IC
- 1× 100uH, 3A inductor (e.g. Pulse PE92108KNL)
- 25°C/W or better minature heatsink (e.g.
Thermalloy 6073)
- Cigarette lighter plug with 3A fuse and 2.1mm DC
plug (e.g. DSE P1692)
- 2.1mm DC chassis mount socket
- 1.7mm x 4.75mm (ID x OD) DC plug and cable
- Small plastic enclosure
Building It:
Make yourself a PCB using the template below (600dpi). I simply laser print (or
photocopy) the design onto OHP transparency sheet and then transfer the toner
onto a blank PCB using a standard clothes iron. Any missing spots can be
touched up with a permanent marker before etching. This is quick, usually
results in pretty tidy boards and hardly costs a thing. There is a tutorial on
a variation of this method at http://max8888.orcon.net.nz/pcbs.htm.
Install the components on the PCB and triple check the layout before
soldering. It is much easier to start with the low profile components such as
resistors and diodes, then install the larger components after-wards. Don’t
forget the wire link; this is shows as a red line on the layout guide above.
Remember to smear a small amount of heatsink compound on the regulator tab
before mounting the heatsink.
For a case I used a small plastic enclosure from DSE, part H2840, as it was
all the local store had in stock that was remotely suitable. The PCB is
designed to fit into this particular case, however any small box should be
suitable. If you have a dead laptop charger lying about it might be worth
ripping the guts out of that and salvaging the case. If your enclosure is
different you may need to modify the design to suit, so I have provided the
schematic and PCB design files for download. They were created using Eagle. The
Eee uses a standard 1.7mm DC power connector with a positive tip.
Testing:
Connect the circuit to a 12v supply. If you use a car or lead acid battery
ensure you have a 3A fuse fitted in line with the circuit before connecting it,
just in case. Use your multimeter to check that the circuit outputs about 9.45v
with no load. Connect a 12V, 21W lamp (e.g. old brake lamp from a car) or
similar load across the output and check that the voltage doesn’t vary much.
You should now be able to connect your Eee. The circuit design should be good
for up to 2.5A, so there is plenty of margin for the Eee to fully function and
charge its own battery off this supply.
SLA Battery Carry-bag:
Jaycar have a really cool carry bag with a shoulder strap designed to perfectly
fit a 12v 7AH sealed lead acid battery. The bag features a fused cigarette
lighter socket and is the perfect compliment to this charger. It works well
with the Eee and provides hours of extra use. The shoulder strap means it’s not
too bothersome to carry about and the charger circuit itself zips up neatly
inside the bag. The under-voltage cut-off means the battery will never run
completely flat, and the Eee will simply cut over to its internal battery once
the SLA runs out. I got my SLA battery from Rexel as they are much cheaper
(approx NZ$18 including GST last time I bought one) and they don’t sit as long
on the shelf as many other suppliers.
Disclaimer:
This circuit is intended for people who have had experience in constructing
electronic projects before. The circuit design and build process are provided
simply as a reference for other people to use and I take no responsibility for
how they are used. If you proceed with building and/or using this design you do
so entirely at your own risk. You are free to use the content on this page as
you wish, however I do ask that you include a link or reference back to this
page if you distribute or publish any of the content to others.
Source:
Marlborough Wi-Fi
