0 30 Volt Power Supply

Here is the circuit of a linear power supply, which provides 0-30 volts, at one amp, maximum, using a discrete transistor regulator with op amp feedback to control the output voltage. The supply was constructed in 1975-6 & has a constant current mode that is used to recharge batteries.

With reference to the schematic, lamp, LP2, is a power on indicator. The other lamp (lower) lights when the unit reaches its preset current limit. R5, C2, & Q10 (TO-3 case) operate as a capacitor multiplier. The 36 volt zen-er across C2 limits the maximum supply voltage to the op amps supply pins. D5, C4, C5, R15, & R16 provide a tiny amount of negative supply for the op amps so that the op amps can operate down to zero volts at the output pins (pins 6). A more modern design might eliminate these four parts & use a CMOS rail-to-rail op-amp. Current limit is set by R3, D1, R4, R6, Q12, R10, & R13 providing a bias to U2 that partially turns off transistors Q9 & Q11 when the current limit is reached. R4 is a front panel potentiometer that sets the current limit, R22 is a front panel potentiometer that sets the output voltage (0-30 volts), & R11 is an internal trim-pot for calibration. The meter is a one milliamp meter with an internal resistance of 40 ohms. Switch S1 determines whether the meter reads 0-30 volts, or 0-1 amp.

A more new circuit might use a single IC regulator, such as the MC78XX, or MC79XX series, immediately after the half wave rectifier, to replace about 30 parts, or at least a high precision zen-er diode to replace D10 as the voltage reference. The LM4040 is such voltage reference & has excellent stability over temperature. IC regulators such as the MC78XX series may finally become obsolete as newer IC regulators are designed, however, discrete transistors, op-amps, & zeners are more generic, have an extended production lifespan, & permit the designer to demonstrate that they understands the principles of linear regulated power supply operation.

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