🚀 go-pugleaf

In article <sehari.721280522@class1.iastate.edu> sehari@iastate.edu (Babak Sehari) writes:
>In <1992Nov7.141550.538@ludd.luth.se> d89-ahg@ludd.luth.se (Anders Hedberg) writes:
>
>>A friend wants to listen to his CD player powered from the car.
>
>>The CD-player wanna have 3Volts at max 600 mA, and the car has a battery
>>of 12 volts.
>
>>I thought of a LM317 or something similar, but I've never designed something
>>alike, and I need to know how to calculate the resistor values.
>
>>/Anders Hedberg
>
>There are many ways to do this the cheapest way is to put:
>
>  12-3
>--------- = 13 diodes in series with the input voltage of your CD player,
>    0.7
>
>Like this:
>
>+12V -|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|- +3V
>GRD  --------------------------------------------------
>
>The problem might be generated if the input voltage swings.  To avoid
>this problem try putting a 2N3055 transistor as follows:
>
>+12V |>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-|>|-    ---+3V
>                                        | \_/ Emitter
>                                        R  |
>                                        |--|
>                                        |
>                                        ||>|-|>|-|>|-|>|-|>|-|
>                                                             |
>GRD ---------------------------------------------------------+-----
>                                    
>
>R=1000 ohm
>Trans=2n3055
>Diodes= 1N4001 or 1N4005
>
>                         With highest regards,
>                              Babak   Sehari.
>-- 

My advice is to IGNORE this advice!!  For two reasons... both of which
may result in destruction of the poor CD player.

1) DIODES are terrific temperature sensors. Meaning the voltage drops
may vary considerably over the ranges likely to be seen inside the car.
The CD player may be destroyed if voltage too high or even too low is 
applied. Stacking them up like that only compounds the problem.  A 
0.1 volt change in the diode will result in a 1.3 Volt change in a
string of 13 DIODES.

2) The power requiremens of the CD player have to be known before
the current handling capacity of the diodes can be selected. The voltage 
drop across diodes is dependant on the current across it.  Also if
there is a failure.. does it fail OPEN or CLOSED circuit?  Just imagine
the smell when you apply 12 V across the CD ROM  :)

I would go for the linear regulator with some good filtering and
make sure you fuse it properly.

Ken.