Multi-Output Controller Board with ATmega16: Heating, AC and Galvanic Outputs

Overview

A multi-output control board that I designed in 2022: an ATmega16 microcontroller drives three kinds of outputs, a heating output with 10 steps, an adjustable AC output with 10 steps, and several low-voltage bipolar outputs (of the galvanic kind). An LCD is connected over a serial link. I drew both the schematic and the PCB in Altium Designer myself.

For the website I focus on the board itself and its structure, and avoid the details of the application.

What the board does

• Central control: an ATmega16 with an 8 MHz crystal. It has 10 command lines for each stepped output, an enable line for each output, two PWM outputs for the galvanic part, and a serial link to the display.

• A heating output with ten steps. Ten optocouplers each put a different resistor into the firing path of a triac and give ten power levels. A relay cuts the output off completely, and the controller drives the relay through a transistor.

• An adjustable AC output with ten steps. Ten optocouplers put resistors from 5.6 to 100 kilo-ohms into a phase-control circuit (a diac and a triac) and work like a digital potentiometer. A separate optocoupler enables or disables the output.

• Galvanic outputs. Two PWM signals from the microcontroller become analogue voltages through RC filters, operational amplifiers amplify them, and a transistor stage and a relay build and select the output. The bipolar supply is needed for this part.

• Isolation. The command side (low voltage) is separated from the mains side by optocouplers, and the outputs are cut off by relays.

The diagram below shows the structure of the board.

Power supply

A transformer and a bridge rectifier (KBP210) bring the input to about 26 V DC. From it, an LM2576 step-down converter makes about 18 V, LM7812 and LM7805 regulators give twelve and five volts for the relays and the logic, and an MC34063 converter makes the negative 18 V for the galvanic part. Several inductors and large capacitors for filtering are on the board too.

PCB design

The board has two layers, with through-hole parts (DIP), about 148 components, two relays, several terminals and four mounting holes. The power tracks (the mains side and the transformer input) are wider than the signal tracks, and the high-voltage part is kept away from the logic. The schematic has five sheets: the central control, heating, high frequency, power supply and galvanic.

Specifications

• Microcontroller: ATmega16, 8 MHz crystal

• Three kinds of outputs: two 10-step outputs and one bipolar analogue output with PWM

• MOC3020 and MOC3021 optocouplers, BT136 triac, DB3 diac, relays

• Supply: LM2576, LM7812, LM7805, MC34063 (negative voltage)

• Link: UART to the display

• Two-layer board, designed in Altium Designer

Result

A complete control board with isolation and several outputs, from the schematic to the PCB, which brings an ATmega16 together with power outputs and an analogue part on one board.