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

[Jim Noble <yahoo-groups@xxxxxxxxxxxxxxxxxx>]

Mark A Wright wrote:

> With my limited knowledge of electronics I've come up with this...
>
>
> -[R1]-[LED]-[D1]-+------------[D2]--+
>
>                [R2]
> From USB
> +5V                +5v
>
>                4x 1.2v
>               Rechargable         PDA
>                  AAA
>
>                    -5v
>
> Common
> -----------------+------------------+
>
>
> Assumptions
> I'd like the unit to:
> Power the PDA when plugged in to USB port.
> Recharge the (emergency) batteries when the PDA is unplugged
> Act as emergency charger / external power supply when USB is not
present /
> plugged in.
> Useful but not essential:
> Illuminate LED to indicate power is coming from USB / recharging
> So in my diagram I've used
> R1 To limit the current from the USB port to under 500ma.
> R2 Is this needed? Thinking was to limit the current from batteries
> when USB
> was not present.
> D1 is to prevent reverse flow back to the PC.
> D2 stops the PDA charging the rechargeable (emergency) batteries.
> My Questions?
> Will this work or have I got it very wrong?


Wow, where to begin...

From the USB power, you're dropping voltage across R1 (current x
resistance to get the voltage drop), the LED (LED forward voltage -
usually around 1.2v?), D1 (forward voltage drop probably 0.7V), and R2
(IxR=V). Even with R1 = R2 = 0, that's not going to leave enough to
charge 4 1.2V rechargable batteries connected in series (and I think
most chargers charge batteries in parallel).

Where does the -5v come from/why?

You're dropping another (?) 0.7v across D2 to the PDA (what voltage does
the PDA expect to see?)


I don't think you can achieve what you want there with only passive
components. Many year ago maplin used to sell a small IC that could be
used to build a battery backup circuit, they don't seem to have anything
similar now though. You could try RS and the like.

Something like this maybe:

http://www.rohm.com/products/databook/reg/pdf/bd4201fv.pdf
http://www.maxim-ic.com/appnotes.cfm/appnote_number/190

That would get you part way there.

> What values do I need for the resistors?
> What part numbers will the diodes be?
> Do I need a current limiting resistor on the LED?
> What happens if everything is connected (USB Supply / Batteries and
PDA)?

If you're lucky, the LED will glow feebly :-)

There might be a passive solution if you could add a couple of switches...

Jim