This Dual supply circuit is suitable for installation of power analog (audio or measurement), it provides a bipolar voltage with voltage source between 20 and 50 V (in audio power amp, for example). Regulated output voltage is 15V, but can be adjusted from + /-12V or + /-18V by simply changing two components. The maximum output current must not exceed 100 mA each branch.
If you ask, where the transformer and diode rectifier?
This circuit is intended to be transplanted into an existing circuit (eg audio amp), which already provides symmetric voltage between + /-20V and + /-50V.
Reduction and voltage regulation
The settings output voltage is provided by two zener diodes D1 to D2 for positive and negative branches. The polarization of the two diodes are provided respectively by R1 and R2. “Power” of the series was entrusted to two transistors BD139 and BD140, which should also be provided with a heatsink to ensure proper cooling. It may seem strange when should put some of the heatsink for low power output (max 100 mA). But the reality is that both transistors will be able to dissipate high power in this case max current .
The output voltages are not equal to 15V!
If you measure + / -14.5 V output instead of + /-15V, do not worry, it’s normal. Voltages + /-15V are determined by the zener diodes, which are likely to have a voltage different from one model to another (the dispersion characteristics are quite large). Moreover, the two transistors are connected in voltage follower, meaning found on the emitter, the voltage applied to the base, unless the fall of base-emitter voltage is in the range of 0, 6V. In practice, you should find out, a voltage can be between 14 V and 15.5 V. Outside this range, you can consider that there is a problem and you should check the wiring and the condition of the components.
Modification of the output voltage
Just replace the zeners D1 and D2. The value of 12V is limited, if you go under, the voltage drop across the transistors will continue to rise and they will heat up more. The value of 18V or 24V, however, is more acceptable, but be careful if the supply circuit contains op amp, the maximum voltage generally accepted for these is + /-18V! You may need to decrease slightly the value of resistors R1 and R2 if the zeners are 24V and the input voltage is “only” 30V.