[ltp] self-made external battery for laptops project

[Bert Haskins]

Probably a thousand people will tell you this but...
That first diode is backwards.
-- Bert

Richard Neill wrote:

> Is a fuse going to help? Just that there are many instances of power
> supplies "containing suicidal transistors that give up their lives to
> save the fuse". Seriously, fuses are their to prevent fires. But they
> take ~seconds to blow. Whereas a high voltage ignition-spike can only
> last 100 microseconds and kill things in that time!
>
> I would recommend you add the following to your protection circuit:
>
> INPUT +                                                  OUTPUT +
>
> 0--------[--X--]------+------------+-----------+-----------0
>                        |            |           |
>            fuse        |            |           |
>                        |      zener |           |
>                        |            |   /       |
>                      __|__       /-----/     _______ +
>                      \   /      /  / \                 capacitor
>               diode   \ /         /   \      _______
>                      -----        -----         |
>                        |            |           |
>                        |            |           |
>                        |            |           |
>                        |            |           |
>                        |            |           |
> 0---------------------+------------+-----------+-----------0
> INPUT GND                                               OUTPUT GND
>
> The diode is there to prevent against reverse polarity. Rather than have
> it in series with the circuit (which would waste 0.7V), put it in
> reverse parallel. Then, if the power supply is reversed, this diode will
> short-circuit, and blow the fuse. Note - you need a pretty hefty diode
> to short out a car battery! At least 20x the current rating of the fuse.
> You might also want a series diode as well for a better safety margin,
> if you can afford the 0.7V drop.
>
> The zener is rated at a little over the voltage you expect to supply to
> your laptop. It will help with surges and over-voltages. Again, in
> normal use, it should not conduct, and the aim is to protect the laptop
> by crowbar-ing the supply for long enough that the fuse will blow.
>
> The capacitor will also halp absorb spikes, as well as smoothing the
> supply. Don't make it too large, or it will overload something else at
> start-up. But don't make it too small either. You need a sufficiently
> high voltage rating (perhaps 33V or more), and it needs to be good
> quality (eg Audio grade). You might do well to make it from an
> electrolytic and a smaller non-electrolytic in parallel.
> [I recently built a switch mode supply, used a cheap electrolytic, and
> the high frequency pulses were not well absorbed by the cap. Result:
> capacitor went high-impedance, and bye-bye to the switcher IC. In the
> process, my circuit got fried too!].
>
> Sorry if I'm boring anyone now :-)
>
> Good luck
>
> Richard
>
> peter nol wrote:
> >>Laptop and data worth more then that. My $0.02
> >
> > Naturally, one has to build a PROTECTING cuircuit before passing anything to
> > the laptop.
> > A proper FUSE is absolutely NECESSARY as i have stated. /so anyone reading
> > this in archive make sure you read THE WHOLE thread and look at the
> > schematics at the linked article/
> > We wouldn't want to hurt our baby, would we?
> > I did a search on google on thinkpad mailing list-
> > I wonder if cross-posting my question to other lists would be rude /is there
> > a lot of people subscribing to severall lists?/
> >
> >
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>
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