In an earlier post, I described an arduino-based clock that I made for my amateur radio station. I had to interrupt work on it for a business trip, but got back a few days ago. I’m now far enough past jet lag to operate heavy machinery, so I finished mounting the board on an aluminum faceplate and installed it in the station.
I’ve been meaning to add a clock with multiple time zone display to my radio operating position for a while. The time in UTC is displayed on my computer when I’m logging, but it’s small and it disappears when the screen sleeps. Also, I use the same room to take work-related conference calls, and it would be helpful to have a clock that displays the local time in Madagascar and the time on the US East Coast, where most of my overseas calls are scheduled. The fast, reasonable approach would be to wire up a microprocessor, a real-time clock chip, and an I2C-driven LCD display that could show all three zones at once. However, the only LCDs I have on hand have small fonts and aren’t that bright. On the other hand, I have a bunch of 4-digit x 7-segment LED displays that burn as bright as Sauron atop Barad-dûr. I went with those.
Continuing on the theme of power supplies and related, I thought I would try my hand at making a bench top variable power supply based on a universal laptop adapter. My rationale is that these adapters are made to run on 11-16V, so I could run it off whatever power is available (the 12V bus on my workbench, a battery, from mains by using another power adapter upstream, or even off the car); considering where I live and the reliability of electrical power, this seemed like a good idea. Also, these brick power supplies are, well, built like a brick, and are designed to tolerate abuse.
I originally came across a posting about converting a Kensington model 33197, which seems idea for this purpose because it has two wires for power/ground, and one each for sensing voltage and current that are set by resistors to a fifth wire that carries 5V. Since then, I’ve seen other articles along similar lines.
My plan was to do something similar, but I wanted to add the twist of being able to set a current limit above which power would be entirely cut to the load until reset. I anticipated using a microprocessor and thought the project wouldn’t be to complicated… but that turned out not to be the case. The power adapter is finicky about turning on into loads, doesn’t like being reset, and I haven’t managed to get more than 15W output from it. That said, I now have it working, but its capabilities fall short of what I had anticipated.
It’s been a chicken and egg race for the last couple months – I was working on power supply projects, but at the same time needed an electronic load so I could test the supplies at known current draws. I have had a working electronic load for a few months, but only got around to making it pretty and boxing it up in the last week. The design that I came up with is not very sophisticated, but it uses common parts and it seems pretty solid. The picture at right shows my recently built variable power supply working into the electronic load.
No, not an article on QRP fox hunts (which I miss, being a bit out of range), but a construction project to make a pair of devices for tracing down wiring. The fox sends a tone down a wire and the hound sniffs around until it finds the right wire by detecting the tone. I found some commercial versions online at a reasonable price, but there isn’t much to these devices, so I figured I should be able to put them together from parts lying around. In what has to be one of the few examples of truth in advertising still to be found on the web, I came across neat circuits for these devices on neatcircuits.com.
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.
Remember that ATX power supply that I fried last year? I replaced it with another mail-ordered supply and then it sat on the shelf long enough for me to get around to fixing it: a new regulator, some MOVs, a cap and the fuse, and it was ready for action — except I had no computer to install it in. I kind of doubt that I’ll build another tower form-factor computer, so I turned it into a high current bench power supply.
It has to be more than ten years since I’ve done this, and I believe the last time I didn’t even have an ATX supply. At that time, supplies needed a hefty load to get going and remain stable. I recall using a car headlight. Literally: an old-style (maybe halogen?) automotive headlight. I reviewed a couple videos on the subject to see what people were doing with more recent supplies and found one that I liked and more or less followed.
Catching up on the bench back catalog: here’s a useful little module – a soft latch that isn’t finicky, works when you first plug it in, draws effectively no power when not changing states, and operates over a relatively wide range of input voltages. What’s not to like?
The basic idea is that mechanical switches, even simple toggles, are much more expensive than push buttons. It would be nice to be able to push a cheap push button and have it turn devices on and off. Some bad ways to do this would be to have the circuit twiddling its fingers, burning power, waiting for button presses – for example, having a microprocessor down stream of a voltage regulator idling until it detects a change in state on the pin connected to the push button. Sure, the microprocessor could be throttled down in some energy-saving mode and might only consume microamps, but the upstream voltage regulator would be consuming power, certainly true for linear regulators, and to a lesser extent for switchers.
Since the SOTA genie escaped the bottle, the blog has leaned towards operation rather than bench work, but rest assured that projects have been percolating in the background. Here’s an update on a few of them: a custom LED display, a soft latch circuit, and a high-current converted ATX power supply. Let’s start with the LED Display.
For an upcoming project (actually, one that was moth-balled last June, stuck in a butter-cookie tin, and just recently again saw the light of day, followed by several days of pouring over notes and head scratching to remember where I had left off), I needed an LED display that would show voltage, amps, and power. Sure, you could figure the power out from the amps and volts, but I wanted it to change in real time to give me feedback while I make adjustments.
Well, I hope it is the final chapter. I’ve written before about fixing and then learning that I hadn’t quite fixed my Weller WESD51 digital soldering iron. After much manipulation, I think I ended up cooking the PIC chip on the PC board, so I just ordered another board. Long story short: replacing the old board with a new one worked and I now have a functional solder station. I have some details below about where I ordered the replacement board and how I installed it in case anyone runs into a similar situation (or, given that cyclone season is again upon us with massive voltage fluctuations on the mains, in case my unit gets toasted again and I need to follow my own instructions).