I’ve been focusing on power supplies and related equipment lately because I don’t have a lot of bench gear. On one hand, I would like to build equipment from scratch, but on the other, it would be crazy not to look at some of the building block modules available on eBay and other outlets. I could not beat these prices, even if I built just about entirely from scrap. Using modules rather than homebrewing down to the metal seems like cheating, but it not only saves time and money, but often yields better performance and miniaturization.
So, this project consists of building a bench top power supply from three modules: a switching power supply that puts out a constant voltage, a DC/DC converter, and a combined voltmeter/ammeter. The combination is based on a youtube video by GreatScott that made use of a cheap DC/DC converter based on the LTC3780 that is being sold by various vendors at a bargain price on eBay. Some of the same vendors are also selling a red and blue volt/ammeter. There are a few variations of the meter, I selected one rated for 100V and 10A and which did not require an external shunt resistor for current measurement.
I ordered one converter board and one display from two different vendors. The body of the two meters was identical, but the wires were slightly different in gauge and color. The two DC/DC converter boards were apparently identical except for placement of the heatsinks and the shunt resistors, as noted below.
The switcher power supply that I ordered was a commodity item, again available from many vendors on eBay. I went with 12V/5A. For this application, it doesn’t really matter whether the voltage is terribly accurate, but it is desirable that it not be noisy.
The DC/DC converter board is reviewed in detail on the beyondlogic wiki. That site provides some information from the chinese datasheet and reverse engineers some of the control features. The board is said to tolerate input voltage 5-32V, outputs 0.3-30V, and in continuous operation can handle current of 7A and total power of 70W.
There are three small potentiometers on the DC/DC converter board; with the input voltage on the left, they are at the bottom of the board, from left to right: low voltage cutoff, current limit, and voltage output. I adjusted the low voltage cutoff to the extreme low end of its range to get it out of the way.
The voltage output adjustment pot is about 500k ohm. Configured as a rheostat. On the high side, it is connected to output voltage, and the lower end is the upper half of a voltage divider. A 14k resistor serves as the lower half. The divider voltage is sampled by the Vo(sense) line of the LTC3780, which will adjust output to keep the Vo(sense) at 0.8V.
One important implication of that design is that adding a larger potentiometer could destroy the chip. The formula for output voltage is 0.8V * (1 + potentiometer/14k). With a pot much larger than 500k, the output voltage would be driven out of spec trying to compensate for the voltage at Vo(sense).
I had a 500k pot on hand, but found that it was difficult to make fine adjustments. One solution would be to replace it with a multi-turn pot, but the only ones I had on hand were 10k and 100k. I went with the 100k one, but to keep the voltage divider in the right range replaced the 14k SMD (0805) resistor with a 2k8 one. By calculation, this should yield a maximum output voltage of 29.3V, which turned out to be the case.
The one other feature that I wanted was current limiting. The middle 200k potentiometer sets the current limit such that if a load draws more, output voltage drops. This circuit is built around a shunt resistor and an LM358 configured as a non-inverting amplifier. Voltage above the shunt resistor is the positive input, and the potentiometer and other resistors in the feedback loop set gain. When the shunt voltage times the gain exceeds 0.8V plus a diode drop, that voltage is fed to Vo(sense), and the LTC3780 starts dropping its output voltage.
To make practical use of the supply, I wanted to be able to set a known current limit without having to hook the supply to an electronic load each time. Two approaches came to mind: 1) replacing the potentiometer with a microprocessor-controlled one, working out calibration and implementing the control in firmware; and 2) selecting fixed resistances for a few useful current limits. The first approach would require a microprocessor, display, rotary encoder, and a digital potentiometer. That seemed like overkill. On the other hand, I had a multiposition switch in my junque box. It was originally double ganged and wired to switch four inputs between three positions, but it did not take a lot to take it apart, rewire, and polish up.
I made a spreadsheet to calculate the resistances needed to obtain various current limits assuming that I would switch in a resistance between the 1k resistor on the board and the 1M6 resistor at the LM358’s output. The junction between my added resistor and the 1M6 resistor went to the LM358 negative input.
I tried a few values, monitored the current on an electronic load and realized that the currents obtained were systematically off. I had measured the voltage drop across the diode between the op amp output and the Vo(sense), and the 0.8V value came from the datasheet, so the other possibility was the shunt resistor. On my board, the shunt resistor is labeled R008 (8 mOhm) versus the description by beyondlogic of 7 mOhm on their board. Working backwards, I calculated the total observed resistance as 9.3 mOhm, mostly the shunt resistor, but apparently also some contribution from solder, traces, wiring, etc.
Working with a revised spreadsheet that takes the real world resistance into consideration, I worked out a table of values. To be spot on, I used trimmer pots to adjust the higher values. I did not have a trimmer for the lower values, so I used fixed resistances. I could later add trimmers, but the low-end currents are not that critical for my applications.
The meter used has two leads to power its board; these connected to the 12V input of the DC/DC converter. The heavy red wire from the meter went to the positive output jack. The negative output from the DC/DC converter went to the heavy black wire on the meter, and the blue wire from the meter to the negative output terminal. The meter does care about polarity for the current measurement. I found the meter well-calibrated from the factory, with no need to adjust. The meter’s integrated ~50 mOhm shunt is a negligible load.
The supply was built in plastic case, with the AC input and switching power supply in the bottom half, and the DC/DC board, resistance board, and front panel on the bottom half. The only connection between the two halves is the 12V output from the switching power supply; the connection is made through powerpole connectors. In a pinch, the switching power supply can be bypassed, and any other DC source within a reasonable voltage range could be quickly plugged in via the powerpole connection.
Now the make the labels for the front panel…