1. General info
This is a laboratory power supply with a stabilized output, continuously adjustable from 0 to 20VDC and from 2mA to 4A. The current limiter makes it useful in experimenting with electronics because you can limit the maximum current before applying the voltage. This power supply has also build-in protections from parts failure and output overload.
The power supply can function in two regimes:
- Voltage source: The voltage is fixed and set by the red potentiometers.
- Current source: The current has reached the maximum value set by the blue potentiometers. The blue LED is on. The voltage is regulated automatically so that the current stays at the fixed value.
This is a modified circuit initially found at www.electronic-lab.com. This circuit can also be found as a commercial DIY kit, or module, and is very popular. Simulations in on this were performed in EveryCircuit and schematic drawings are exported from EasyEDA. Both of these programs are web based and free.
We split the above circuit to smaller sub-circuits and analyze the workings of each sub-circuit. The circuit can be split into four parts:
- Rectified and stabilized power supply with a negative voltage rail - powers the circuit and the output of the circuit.
- Voltage reference according to which output is controlled.
- Output amplifier - amplifies the referent voltage.
- A current limiter - turns on/off the constant current source and holds the output current constant.
2.1. Rectifier with a negative rail
The power supply powers from step-down mains transformer with two secondaries rated at 18VAC and 2A. If you connect this in parallel you get 18VAC and 4A. This is the effective value, and the amplitude is a square-root-of-two times higher: 1.414 · 18V ≈ 25V. This means that we can charge the capacitors up to the amplitude voltage, but effectively we can get around 20V on the output.
Standard Graetz rectifier rated for at least 5A and capacitor C1 form a standard full-bridge rectifier for getting DC output. In our circuit we use op-amps 741 that output voltage levels from zero to some value. To get a zero level output from an op-amp like 741 its negative input must be bellow the zero level.
Working of the negative rail circuit in stable regime is as following: C1 is charged to 25V. In reference to the chosen ground voltage the secondary coil point T_2 oscillates with amplitude of 25V. When in positive half-period current flows from T_2 through C2 and D5. The resistance in this branch is small so C2 is charged during the positive half-period. During the negative half-period current flows from ground through D7 and C3 (charges it), then through D6 and discharges C2. Capacitor C3 is charged to about 9V which is defined by the current flowing through the Zener diode: 5.6V + 12.5mA · 270Ω = 9V. The C2 voltage oscillates around 16.5V as this capacitor charges and discharges. Visit this link to see simulation of the sub-circuit.