Previously we decided I needed a better lab power supply for working synth modules; picked out a switch mode power supply to do most of the heavy lifting,, a case, and desirable output terminals. This time we’re going to look at the electrical design that will generate the three output voltages ±12 V and 5 V, and how current will be measured and displayed.
Let’s start with the easy stuff. I want the display to do two things let me know the system is on and display the output currents for each output voltage. To accomplish goal one I’ll pick an LCD with a back light which is nearly all of them.
Goal two is slightly more challenging. We need to pick the number of digits displayed and the display format. That is will all channels be displayed at the same time or will they be displayed one at a time? If they’re displayed one at a time how will we transition between which measurement is displayed. The number of digits displayed should be based on the current consumption for euro rack modules. From what I’ve seen this ranges from ~10 mA to ~200 mA. So we’ll want three characters for the measurement, two characters for units and an additional character to separate the measurements. For the overall format I wanted to see all of the measurements at the same time (Two other display options cycling between measurements using a switch or a timer seem fiddly). So with three channels and six characters per channel I need a display that’s at least 18 characters long; fortunately I have a 2×24 character display.
The final thing required for the display is a micro controller to take the output of the current sensor and write it to the display. I didn’t want to break any new ground here and just grabbed the nearest Arduino nano.
Circuit wise the display and controller look like this. the display has a typical 16 pin interface, and I’m using the four bit data transfer protocol.
Output voltage generation
For regulation I picked out a couple of classic regulators the LM317 and LM337. These are adjustable regulators so I picked out some 5 k multi turn pots to trim the output to the desired voltages. Finally, I followed the reference design in the datasheet for sizing and laying out the capacitors and diodes. For the 5 V output initially I duplicated the circuit configuration for the 12 V rail to minimize the part count on the BOM, and got all the way to ordering boards before realizing this would be a bad idea. Why is it terrible? glad you asked. On initial assembly the prior to tuning the output with the trim pot the regulator output could be anywhere between 0-14 V. The display and controller are powered from the 5 V rail and do not react well to voltages over about 6 V. Thus in the original configuration shown below a good portion of the project could be smoked just by turning it on for the first time. So I changed the 317 for the 5 V rail to a fixed output 5 V regulator for the first revision.
Next time in this series we’ll take a look at current measurement.