Re: OT: Electronics advice needed to make a combined emergency charger / USB supply lead for my PDA

[ian.bird@xxxxxxxxxxxxxxxxxxxxxxx]

Whilst I don't know the answers to your questions I have some
observations.

1) Double check the continuous current your USB port can supply. In days
gone by it was only 300ma.
2) The LED needs connecting direct from the positive rail to the negative
rail with an appropriate resistor (all upstream of the diode). 1k will
give a nice low current draw.
3) Battery charging circuits are usually more complex than you have to
stop over charging etc.

It all seems a little simple for a relatively complex requirement but
sadly my knowledge is not in this area. A quick google threw this up as
the first hit which should help
http://www.discovercircuits.com/B/batt-chrg.htm
or
http://www.vicr.com/documents/application_notes/an1_battery-charger.pdf
for some basic info.

There will also no doubt be some capacitors needed somewhere along the
line. More head scratching needed I think.

Hopefully there will be some more in depth answers along the way.

HTH

Ian






"Mark A Wright" <mawright@xxxxxxx>
14/07/2004 13:08
Please respond to ukha_d


To:     <ukha_d@xxxxxxx>
cc:
Subject:        [ukha_d] OT: Electronics advice needed to make a combined
emergency
charger / USB supply lead for my PDA


I'm trying to make a combined emergency charger / USB charger for my PDA.



I've got all the hardware apart from the electronic components. What I
really need is some help in designing the circuit.



With my limited knowledge of electronics I've come up with this...



-[R1]-[LED]-[D1]-+------------[D2]--+

[R2]