AJB2K3,
Ante's suggestion is a very good one and should work fine. If that part is not available to you, here is a converter circuit that is close. This is only a start and you will have to make some changes. In fact it is a basic building block for a simple step down regulator that can have many purposes if you make the base voltage variable with a pot.
Change the zener diode to allow the voltage you want. R1 determines the base current. V/R=I. The transistor should be one which will allow the current you require. I cannot find a data sheet on the ECG184, but a post on another site gave the description: The ECG184 Power Transistor is a bipolar silicon NPN 4 Amp, 40 Watt power transistor. Another website stated that the ECG185 is rated at twice the power rating. Personally, I would recommend finding transistors that are very common to your geographical area.
Hope this is somewhat helpful.
MP
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