🚀 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.

|> 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 ---------------------------------------------------------+-----

Best to avoid the first solution. First, diode drops are
only approximately 0.6 volts (they vary in *practice* from about 0.5
to 0.7 volts at room temperature). The drop varies with temperature.
The voltage drop is also sensitive to bulk resistance of the
junction among other things, and is thus hard to predict. The
"regulation" (impedance of the supply) will be much worse than
a battery. So even if the source voltage does not swing, this circuit
is a nix.

The second circuit also has shortcomings, although less so as the
diode voltage uncertainties only show up from the transistor's base
to ground (fewer diodes). The only purpose for the diodes on the
transistor's collector would be to dissipate a majority of the power
(something a properly heatsinked 3055 is MUCH better at doing than
small power diodes). There is no reasonable voltage compliance
available at the collector, as shown, either. You could remove some
or ALL of the collector diodes, choose R to give a current through
R of 1/20 the load current, and go with it. But then, you've
exceeded the cost and simplicity of a simple LM317 regulator
as shown by another poster.

You'd be much better off with a regulator like the 317 as
suggested. It is simpler, inexpensive (costs less than a 3055
transistor), and provides regulation as a side benefit.

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