YmeBP - a simple way of getting approximately 3 volts from that toy transformer would be to put a 3.0 volt Zener diode across the output of the transfomer, with a resistor inline with this diode.. You will have a very close voltage to that of what you are looking for across the Zener diode. The resistor values will have to be determined with this formula:
Zener diodes are widely used to regulate the voltage across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value.
In the circuit shown, resistor R provides the voltage drop between UIN and UOUT. The value of
R must satisfy two conditions:
- R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6[1] device, a 5.6 V 0.5 W zener diode, has a recommended reverse current of 5 mA. If insufficient current flows through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher then nominal and could rise as high as UIN). When calculating R, allowance must be made for any current flowing through the external load, not shown in this diagram, connected across UOUT.
- R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX, then IDVB < PMAX.
A zener diode used in this way is known as a
shunt voltage regulator (
shunt, in this context, meaning connected in parallel, and
voltage regulator being a class of circuit that produces a stable voltage across any load).
These devices are also encountered, typically in series with a base/emitter junction, in transistor stages where selective choice of a device centred around the avalanche\zener point can be used to introduce compensating temperature co-efficient balancing of the transistor
PN junction. An example of this kind of use would be a d.c.
error amplifier used in a
stabilized power supply circuit feedback loop system.
I know a lil far fetched in understanding this explanation, but in electrical terms its what you are looking for..