Lab supply

Linear 0-30V, 2A lab power supply

Disclaimer

This supply has been built and tested and does work. However, I am a hobbyist doing this to learn. You are welcome to follow along, but keep the experimental status in mind and always exercise caution when dealing with electricity!

Abstract

This project aims to design a high-performance, high-stability microcontroller-based lab power supply. I am mostly doing this to learn, but also because I'm tired of cheap switchmode power supplies delivering power riddled with RF noise.

Principle of operation

High voltage section

The high voltage section is very simple and consists of a regular AC inlet with a safety capacitor across it. There is a 3A fuse in series with the hot lead and a two pole mains switch. The transformer is a 24V/4A unit with a 12V center tap. Once rectified, the voltage reaches about 36V at my location. Mains ground is connected to the chassis as well as the ground output terminal.

Analog secion

32-36V AC enters the circuit at the AC1 and AC2 terminals and gets rectified and smoothed into approximately 36-40V DC. This voltage is then pre-regulated down to 32V through Q3 which is gets its voltage reference the backwards bias of D3. This voltage is used to power the opamps. The CNT input is the transformer center tap, which should be around 15V and is used to create the +12V and +5V voltages.

The U1D opamp acts as a voltage regulation amplifier and expects a control voltage between 0-4.096V on its postive input. The feedback loop is tuned to 7.32x amplification, resulting in an output voltage ranging from 0-30V. The Darlington pair at Q3 acts as a emitter follower current amplifier.

Current regulation is based on a high-side current sensing resistor, R27. The differential voltage across the current sense resistor is amplified to a 0-2.5V signal using an INA225 current sense amplifier. This voltage is then sent to the positivie input of U1A. The control circuitry feeds a 0-4.096V signal to the V_ISET input, which is scaled down to 0-2.5V through a resistor network. This voltage is then fed to the negative input of U1A. When the voltage from the current sense amplifier goes above the control voltage on the negative input, the opamp will source current to Q2, which brings the base on Q3 low until the current is limited to the set value. Q3 is needed since we want to keep R24 relatively low to avoid voltage drop at high output currents, while making sure U1A isn't sourcing or sinking too much current. Q1 acts as a simple switch for turning on the current limit indicator LED.

Digital section

The digital section is based on a Adafruit ItsyBitsy 5V, which is essentially an Atmel ATMega 32u4 with some support circuitry. This was chosen to simplify the design and keep everything as through hole. The digital section is responsible for feeding the V_VSET and V_ISET voltages to the the analog section, as well as measuring and presenting actual voltages and currents.

A key component is the LMZ4040DBZ voltage reference, which provides a stable 4.096V reference used by the rest of the circuit.

The control voltages are generated by an MPC4922 12-bit DAC and fed directly to the analog section. The user sets the desired voltage and current using two rotary encoders which can be pushed to switch between a "fine" and "coarse" mode.

The output voltage of the supply is measured by an MCP3202 ADC through a voltage divider network to bring the voltage down to 0-4.096V. Further overvoltage protection is offered by the D12 clamping diode. Current is measured in a similar way, but since that signal is in the 0-1V range, it is first amplified 4.096x by U1B. This signal is also clamped to the 4.096V reference through D10.

Controls

Current and voltage

Current and voltage are controlled using two rotary encoders. Pressing the knob switches between coarse and fine mode. In coarse mode, a blinking cursor will indicate the least significant digit subject to change by turning the knob.

Enable/disable

The enable switch enables or disables the output. Notice that this is a "soft" disable that's simply shorting the posisitive terminal on the voltage regulation opamp to ground. Thus, the output is NOT electrically isolated from the rest of the circuit.

Lock mode

When the lock switch is activated, voltage and current output are locked to their current values. The user can still change the settings, but they are not acted upon. When the switch is flipped, the voltage and current output is changed to the current setting. This is useful when a user is carefully dialing in new settings without changing the output while turning the knobs. When locked mode is activated, the "->" symbol between set and actual values on the display is changed to "LCK".

Grounding

This power supply is designed to be floating, i.e. it is isolated from ground. If the user needs either output (plus or minus) to be grounded, they may simply short it to ground using a ground bar or patch wire. Since the supply uses high side current sense, accidental shorts to ground should not cause damage.

Physical design and enclosure

PCB design

This is my very first PCB design and there are many beginners mistakes present in the design. However, it does work. If I ever redesign this, I would make the +36V path shorter, for example. There are also some questionable component placements. As with any open source project, you are encouraged to improve on it!

Enclosure

I opted for a cheap sheet metal/plastic box that is widely available on sites like Ali Express and Amazon. It works well, but the metal is fairly thin and tends to flex when unplugging the leads.

Cooling

The TIP142 output transistor requires good cooling. I opted for a simple $9 AMD CPU cooler and bolted the transistor and the 7805 regulator to it. I'm currently using isolating pads for the TIP142, but I'm not entirely happy with the heat transfer. I might remove that, although I'm not sure I like to have unregulated 36V exposed inside the case. The fan is PWM controlled using a simple temperature curve algorithm.

Rotary encoders and switches

The rotary encoders and switches (with exception of the mains switch) are all cheap Amazon products and work well so far.