The linear power supply,
shown in the schematic, provides 0-30 volts, at 1 amp, maximum, using a
discrete transistor regulator with op-amp feedback to control the output voltage.
The supply was constructed in 1975 and 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, and Q10 (TO-3 case)
operate as a capacitor multiplier. The 36 volt zener across C2 limits the
maximum supply voltage to the op-amps supply pins. D5, C4, C5, R15, and R16
provide a small 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 4 components and use a CMOS rail-to-rail op-amp.
Current limit is set by R3, D1, R4, R6, Q12, R10, and R13 providing a bias to
U2 that partially turns off transistors Q9 and 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), and R11
is an internal trim-pot for calibration. The meter is a 1 milliamp meter with
an internal resistance of 40 ohms. Switch S1 determines whether the meter reads
0-30 volts, or 0-1 amp.
0-30 Volt Laboratory Power Supply Circuit diagram
A more modern circuit
might use a single IC regulator, such as the MC78XX, or MC79XX series,
immediately after the half wave rectifier, to replace approximately 30 components,
or at least a high precision zener diode to replace D10 as the voltage
reference. The LM4040 is one such voltage reference and has excellent stability
over temperature. IC regulators such as the MC78XX series may eventually become
obsolete as newer IC regulators are designed, however, discrete transistors,
op-amps, and zeners are more generic, have a longer
production lifespan, and allow the designer to demonstrate that he understands
the principles of linear regulated power supply operation.Link
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