how do i drop the output of a power suppy?

Question

i have a maplins regulated supply at 13.8 volt 3 amps i need to drop this to 9 volts or if poss variable anyone know how to do it ,its just to run the lights in dolls houses but there are 6 houses i dont want to use 6 transformers and this is the only one i could find with enough amps there are quite a few lights

Answer 1

The easiest way is probably diode drop, each silicon diode will drop the voltage approx 0.7 volts almost irrespective of current so 5 in series will get you about 9 V
3 amp diodes are available from Maplin 1N5400 at 12p eachl so 60 p for the set. for mounting diodes I connect them across a pair of terminal blocks, zigzag style which saves soldering and leaves them out in the breeze as they can get hot.
Playcraft / Jouef used this diode drop system for a model railway speed controller which works well but has only about 10 speeds available.
I abandoned bulbs in favour of LEDs Light Emitting Diodes for buildings on my model railway some time ago, diodes are smaller lighter, use less current, and voltage, only 3 volts, they run cooler than bulbs, the light is softer less garish and apart from clear LEDs which seem to be 4.5 volt and need 3 batteries, they can be powered by 2 x D, C, AA or AAA batteries or as I do use a 3 volt Gameboy power pack £1 from Poundland.
If you use LEDs on 3 Volts and you do not need resistors.

Answer 2

If this is a regulated supply then you may be able to modify the feedback resistor network to allow it to regulate at 9V instead of 13.8V. This feedback network is commonly made from two fixed resistors in series with a potentiometer, with the potentiometer being in the center of the resistive ladder. Changing the relative values of these resistors while keeping the total resistance value constant will allow for different voltage outputs. However, this will reduce the continuous power output capabilities of the unit somewhat. Email me if you need more details.

WK

Answer 3

You can use a 7809 regulator in each dollhouse. This will allow 13.8V input, and will keep the output at 9 volts. 7809 can only handle 1A, so you will need to make sure you don't exceed the specification. The 78xx series regulators are 3 leg TO-220 devices. There is an input leg, ground and output leg.

http://margo.student.utwente.nl/stefan/chipdir/giicm/7809.txt

If you want to make a variable unit, look into the LM-317:

http://www.fairchildsemi.com/ds/LM/LM317.pdf#search='lm317'

Answer 4

a step down transformer or resister bank