by Mark Thoren
In this blog post, we will look at several approaches to building your power supplies. I’ve had a lifelong fascination with homemade test equipment, due to a steady diet of Popular Electronics articles I read as a kid and owning several second-hand Heathkit instruments. But I’ve slowly accepted that most of the time, time is more valuable than money. So while I’ll build the occasional filter, preamp, attenuator, or other little “helper circuit” to augment my “real” benchtop instruments, I never make my own – except for power supplies.
Both my home and office labs have several homemade power supplies, most often open-frame offline linear supplies from Condor or Power One, installed in repurposed 1990’s era hard drive boxes. I’ll often augment these with an adjustable linear regulator like the 3-amp LT3083, in addition to wiring out all of the supply’s fixed voltages. The HDBPS (Hard Drive Box Power Supply) shown in Figure 1 provides +/-15V at 200 mA, fixed 5 V at 2 A, and 0.5 to 5.5V adjustable at 3 A. It’s simple, whisper-quiet, and 100% reliable.
Figure 1. Hard Drive Box Power Supply (HDBPS)
Digital Displays, and More Power!
The DC2132 75W benchtop supply is based on one of the LT3081 datasheet application circuits. The LT3081 is a 1.5A single resistor regulator with a current limit, current monitor, and temperature monitor. The DC2132 parallels two LT3081 for a total of 3A, and an LT8612 buck pre-regulates the (up to 40 V) input to the LT3081s. A clever circuit sets the LT8612 output to about 1.7V above the LT3081’s output voltage, drastically reducing power dissipation.
Of course, I had to have one of these ingenious boards, but to justify the (minor) expense to the company, I used it to develop an LCD panel meter example for the LTC2492 analog-to-digital converter (ADC). The code runs on an Arduino Uno (or clone) and is available in the Linduino repository on GitHub here.
I didn’t have a hard drive box handy, so I repurposed a deceased HP3469B meter case, as shown in Figure 2.
Figure 2. DC2132 with Panel Meter Display
More Digital, Network Control
This supply works great, but what if I want to control the supply from Python or C++ over a network connection?
That’s where the Circuit Note 508 benchtop Power Supply with Raspberry Pi Control comes in. The CN0508 copies most of the DC2132 circuit verbatim. An AD5683 DAC with LT6015 amplifier controls the output voltage, and an AD7124-8 measures, well, everything: Input voltage, output voltage, output current, preregulated input to the LT3081s, and the individual temperatures of the two LT3081s.
Figure 3. CN0508 Raspberry Pi Controllable Power Supply
The board is controlled by a Raspberry Pi, with software running either locally or on a remote Windows, Linux, or Mac host. There is a control panel in the IIO Oscilloscope utility providing a convenient, semi-polished GUI look.
Figure 4. IIO Oscilloscope CN0508 Control Panel
The CN0508 is the first reference design leveraging ADI Kuiper Linux for Analog Devices products. Kuiper Linux comes with a collection of kernels and boot files for various FPGA and processor platforms, with broad support for mixed signal and other devices (including the AD7124-8 and AD5683 on the power supply board.)
Less Money, Student-Friendly
The idea for a hand-solderable, minimal-cost version of this circuit with maximum accessibility to students, faculty, and hobbyists had also been rattling around in my head for a while. So, let’s design a version that keeps as much functionality as possible and as a bonus provides a bunch of learning experiences. Full instructions, design files, and LTspice simulations for a 45 W, hand-solderable, all-through-hole benchtop supply are available on the ADI wiki here. The finished supply is shown in Figure 5.
Figure 5. Fully Assembled 45 W Benchtop Power Supply
The LT3081 is conveniently available in a 7-pin TO-220 package, just right for hand soldering. The choices for a hand-solderable monolithic buck converter at this power level are limited. So we’ll pick a blast from the past, the venerable LT1374. The current limit potentiometer circuit is kept as-is. Sweeping 15 V over a single potentiometer turn is a bit “touchy”, so the RSET circuit has jumper-selectable ranges of 1 to 6V, 6 to 11V, and 11V to 16 V, avoiding range transitions at common voltages (3.3V, 5V, 10V, 15V).
Figure 6. Benchtop Supply LTspice Simulation
The power input is compatible with the CN0581, USB-C PD controller. This essentially acts as a barrel jack replacement, and lets you use an old laptop charger as the offline supply. Be aware of grounded, 3-prong power supplies that tie the USB-C ground to Earth ground. Get a 2-prong adapter if this is a concern. A barrel jack is included in the layout as well.
So now you’ve got a handy, robust 45 W power supply, and the satisfaction of having built it yourself. Leave it bare, or dress it up (or down, depending on your sense of aesthetics) with an old hard drive box, and have fun building another project to power with it.