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.
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).
My dog Ginger has a problem: she really likes food. That, coupled with her uncanny ability to convince family members that she has not been fed adds up to an overweight dog. She is not particularly sensitive about the subject and besides, she doesn’t read this blog (does anyone?) so I don’t think she’ll mind if I write about our attempt to find a technologically sound solution to her overeating, which is really a matter of our overfeeding her.
Ginger is supposed to get two scoops of dog food, one in the morning, one in the evening. Mornings, however, are chaotic. I get up and shower, and my wife adds food to the bowl. Then, while my wife is in the shower, Ginger whines and I figure she hasn’t eaten, so bowl number two. Often we’re out the door before the kids have emerged from their rooms, and at least one of them will take pity on Ginger, who clearly has not been fed at that point, and she gets her third bowl. Pretty much any time someone walks past her bowl, it seems that she hasn’t been fed. You would think we’d be on to her now, but there are times where in our rush to get out, we have made the mistake of each assuming the other one fed the dog. Ginger reminds us constantly of these rare occasions.
You know how you never see negative results published? Well, here are some. Recently, I thought that I had used experimentation and logic to find a clever fix for my transiently functional solder station. For the entire day after I soldered in two capacitors, it worked beautifully. On, off. On, off. On, off. And every time I turned it on, the LED display lit up, the soldering iron got hot, and everything was peachy. Until this evening. I turned it on, finally hopeful that I had found an enduring fix and… no. Nothing. No LEDs. No heat. It mocks me.
Once again, with experience borne of many unproductive disassemblies, I tore the solder station apart and poked around. This time, poking the CLKOUT pin didn’t help. I clipped off the cap on the CLKIN. No help. I clipped off the other one, no help. I’m still getting power to the MPU, the oscillator is still oscillating. What gives? I didn’t keep at it long enough to see if it would randomly turn on again at some point.
You’ve won this round, Weller, but I’ll be in Europe next week — where they sell 230V Hakkos, and I’m just about ready to buy one.