[mdlug] Old 386 needed - Power Supply

[Jay Nugent]

Greetings,

On Tue, 21 Apr 2009, Aaron Kulkis wrote:

> gib at juno.com wrote:
> >  
> > I was looking for some power supplies for some old laptops.  I noticed that JCC Computers on Inkster just South of Ford RD (Dearborn Heights) on the West side of the street had several.  
> > Do you know the Voltage and amps?  Can you tell if the connector looks like a modern standard?
> > 
> > 
> 
> If all else fails, AC -> DC power converters are extremely
> easy to make -- and you can probably make one which is
> significantly more robust, both electrically, and mechanically
> than what you buy at a computer store, for approximately
> the same cost from parts available at any Radio Shack.
> 
> 
> 
> The circuit is this
> 
>                   +-------+
>     o--)||(-------| ~   + |------+-----------+-------o  +
>        )||(       |AC   DC|   |  |           |
>        )||(       |       |   +-----+        |
>  AC    )||(       |       |      ^  |    ----+----     DC  
>  in    )||(       |       |     / \      ----+----     out
>        )||(       |       |    /___\         |
>        )||(       |AC   DC|      |           |
>     o--)||(-------| ~   - |------+-----------+-------o  -
>     Transformer   +-------+    Voltage   Capacitor 
>                   Rectifier   Regulator   (Large)
>                    ($2.50)  (Zener Diode)


   Close, but no joy.  You *will* need a current limiting resistor in 
series between the transformer and the zener diode.  When the zener 
reaches cutover it cannot handle the full output of the transformer, so a 
current limiting resistor is needed.  You can *almost* think of this as a 
voltage divider (using two resistors) where the voltage drop across one 
resistor (in this case the zener) is always constant.

   But over, Aaron is correct.  You *CAN* build a perfectly good power 
supply for FAR less than the cost of an off-the-shelf laptop supply.

   Note:  Many laptops could accept a very wide range of input voltages 
simply because they had their own internal regulators.  I have run many 
old 386/486 laptops that said they needed 16-20 volts on a 12 volt supply.  


   If the voltage is too low --- the laptop simple won't work or will fail
when a floppy drive spins up, or something requires a little more uumph
causing the voltage to sag and the motherboard resets/reboots.

   If the voltage is too high --- well, that's a different matter as the 
"magic smoke" may leak out and it will never run again :(

   
   Enjoy!
      --- Jay Nugent  WB8TKL

Train how you will Operate, and you will Operate how you were Trained.
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