This is the variable DC power supply circuit. This power supply has regulated output, can be adjusted from 3 to 25 volts and the current output is limited to 2 amps as shown, however it may possibly be improved up to 3 amps or more by applying a smaller current sense resistor (0.3 ohm). Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. The 1458 may be substituted in the circuit, but it is suggested the supply voltage to pin 8 be limited to 30 VDC, which can be achieved by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The 2N3055 and 2N3053 transistors need to be attached on proper heatsinks and the current sense resistor must be rated at 3 watts minimum.
The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively. The power transformer ought to be capable of the preferred current while keeping an input voltage at least 4 volts higher than the expected output, but not surpassing the maximum supply voltage of the op-amp under minimal load conditions. The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will deliver regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is acquired utilizing the center tap of the transformer with the switch in the 18 volt position. All components/parts should be available at Radio Shack with the exception of the 1558 operational amplifier.
Variable DC power supply 3-24V / 3A circuit source page: www.bowdenshobbycircuits.info