[mdlug] Old 386 needed - Power Supply

[Raymond McLaughlin]

Jay Nugent wrote:
> Greetings,
> 
> On Tue, 21 Apr 2009, Aaron Kulkis wrote:
> 
>> 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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Thank you Aaron and Jay,

I have been, and continue to mull a more detailed reply, but will here
quickly point out that I have, on hand, several serviceable AT and ATX
powers supplies. Each of these provide both a +12 (yellow) and a -12
(blue) output. Shouldn't this look like 24v to any DC device connected
to it? I might want to drop this to 22v for the laptop. The circuits you
describe are something I "used to know" but need some brushing up on.

Just quickly, shouldn't the 24 volts described above provide a
satisfactory "first approximation" of the 22 that the laptop requires?

Also any comment on my supposition that "pin positive" is more common
than the reverse?

Raymond McLaughlin