[ltp] self-made external battery for laptops project

[Richard Neill]

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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