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Dieter Aschmann

User Interface for Unilab2 Power Supply

Status: Proposal
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March 28, 2019 , Latest update: June 28, 2019
u-i-display-unilab2.jpg
This project is an add-on for the Unilab2 power supply, published in and at Elektor Labs.

The Unilab2 has a potentiometer to set the output voltage and the maximum current. A carbon film potentiometer tends to wear out prematurely and they are a bit "old school". Also, for reasons of cost, the Unilab2 does not have a display. Setting the current limit is a bit like flying blind, as there is no feedback. The actual value of the current is available as a proportional voltage (on K3) but with quite some tolerance and the voltage must be corrected to obtain the real valiue of the output current.

My approachnter

A 4 x 20 LCD shows nominal voltage, actual power, and the current limit. One rotary encoder is used to set the output voltage and another to set the maximum current in 0.1 V / A steps. Three preset pushbuttons with LED indicator have been added as well. With these buttons one can select one of three fixed output voltages: 3.3 V, 5.0 V, and 12 V.

The actual value of the current is measured with an ACS712 [not suitable for new designs, use AC723 instead] current sensor and captured at the PC1 analog input. The setpoint for the current limitation can be chosen in this mode.

The two LEDs available at the Unilab2 board can be moved to the display board as well.

Prototype

The project was created in Target V19 and the prototypes are finished. Of course, the board dimensions (160 x 110 mm, W x H) are tailored for my enclosure. A 100 x 60 mm cut-out in the top half provides space for the display. The the copper side of the board is mounted to the front panel with the display plugged "in reverse" on a header on the component side. The rotary encoder is on the right together with the three jacks Plus, Minus, and Ground. The preset voltage pushbuttons are nicely alligned.

When in Preset-Voltage Mode, pressing the built-in pushbutton of the rotary encoder will switch back to Variable-Voltage Mode.

The firmware is written in Bascom. The program and all its functions have been tested extensively on a Breadboard and works fine. I've measured the setpoint voltage at the wipers of the MCP42xxx with two multimeters and they showed exactly the desired values. The Unilab2 expects 0...5 V at the wipers of P3 and P4.

Conclusion

If a higher adjustment resolution is desired, a digital potentiometer IC with 512 steps can be used. The MCPxxx has 256 steps.
 
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