Linear Bench Power Supply




DETAILED DESCRIPTION

This project is a simple linear bench power supply with adjustable voltage and current. It is meant to be a learning experience, putting ideas of circuit design, feedback, stability into practice building a complete practical power supply. I have intentionally avoided using a 3 terminal regulator or purpose built chip and instead implemented the feedback for voltage and current using a couple of opamps. The idea is for this to be a practical final product while giving pactice implementing some basic circuits.

Linear Bench Power Supply




Specs:

  • 0-30V
  • 0-2A
  • separate fixed 5V output
  • Display of setpoint and measured current and voltage

Beyond this, everything is kept as simple as possible with no additional features to compliacate the design. The display is a 20×4 character LCD controlled by an Arduino which reads the currents and voltages. The voltage and current are set by 2 pots on the front panel. There is no data logging or serial comms, although this could be added later if desired with an update to the arduino firmware.

Overview of operation

The design is fairly simple and uses a single npn darlington transistor as the main pass transistor in a common collector configuration. This provides current gain but no voltage gain, which is provided by a small signal npn transistor in a  common emitter configuration. The current and voltage feedback loops are controlled by two opamps which compare the measured output values with setpoints generated by two potentiometers on the front panel which are driven by a 5V reference. The output voltage is sampled via a 6:1 votlage divider (converts the 0-30V output to a 0-5V signal) and the current is sampled by measuring the voltage drop over a 100mOhm resistor in series with the output. Everything is mapped down to the 0-5V range so that they can be directly measured with an arduino using the 5V rail as the reference voltage. There is additional circuitry for the mains rectification, the transformer tap select and a 7805 5V regulator which provides the reference voltage, powers some of the chips and is routed to the front panel to provide a fixed 5V output.

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Feedback loop

Voltage Control

The main feedback loop and output stage is shown in the figure above. Opamp U1A has it’s non-inverting input connected to the wiper of a pot on the front panel, driven by a 5V reference signal which provides the setpoint between 0V and 5V. The inverting input comes from the 6:1 voltage divider R5/R6 which divides the 0-30V output down to a 0-5V signal. The output voltage is amplified by the common emitter amplifier created by R1, R3 and Q2, with the pnp transisor Q1 required as the common emitter amplifier is inverting. The current gain is then provided by the common collector amplifier formed by Q3. Q3 is the main pass transistor and so must have high gain and be capable of dissipating a lot of power (see below) – I use a TIP120 which is an npn darlington transistor rated to 5A with a DC current gain (hfe) of ~1000.

Read more: Linear Bench Power Supply




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