This weekend I made good progress toward eliminating a first world problem that’s bothered me for ages. I have an old variable DC power supply rated for 0-40V 0-3A that’s good enough for most things I typically need a power supply for. It’s a old Lambda, near relative to those seen on the cover of this 1974 catalog:
It had a few niggling complaints. So:
The meters are like 2″ analog things and not ridiculously accurate. This weekend I did range-changes to a pair of nice Newport 4.5-digit digital panel meters. With these installed, I have 4 digit precision on current and voltage, and nice big bright LED displays to boot. The voltage was easy enough, just convert to the proper range and install a potentiometer/voltage divider to get me to 40.00V indications. Good deal. It’s off by a few millivolts at low voltage but that’s ok enough for me. The current meter was a problem. First I had to find a 0.1 ohms resistor with enough power capacity. Running the output from the power supply through this gives me a scaled 0.1V/A voltage output. The problem is that the meters I have are for 0-10V input. So I had to do some reverse-engineering and figure out a 19M ohms string of resistors to increase the sensitivity to 0-2V. This was more work than it sounds, and calibration gave me fits for quite a few hours. I now have a 0 to 3.000A indication. It’s off by a few milliamps at high current, but that’s ok for me too.
These meters were not installed from the factory. There is not room inside the power supply for these, and if there were it would require cutting a bit of steel which I didn’t feel like doing, and then they would have still protruded. I could have run the wires out the side or through a ventilation hole (the case is 100% ventilated panels) but that would be inelegant. I could have run the wires from the front panel binding posts but that’s a horrible bodge (!) So I ran the wires to the screw terminal strip on the rear panel.
There is a strip of rear panel screw terminals for various connections including remote voltage sensing, which is where the voltage control section gets its reference. I moved the remote sense wires internally from the rear to the front panel where I take all my volts from anyway. So there were two unused terminals on the back, which I used as a proportional current monitoring output. Nice. At work, I have a digital panel meter for my main bench supply that just stays on, burning 3W of power whether I’m using the power supply or not. At home, I want it done better, so I did more reverse engineering and figured out where the internal power connections are to be found. I added a wire to a “spare” rear panel screw terminal so now there is a rear panel output with a switched mains supply from the power supply – connected after the power switch and fuse in the power supply! The neutral and ground for the meters both connect at the main inputs, and then my panel meters now run off a *switched* main supply. The turn on and off with the power supply they monitor. Very trick.
I had my choice of several meters. I tried and failed to figure out a range change on a 3.5 digit meter of a different model for the current readout but in the end I’m glad to have 4 digits and both meters are the same model so that’s also nice. These have screw terminals also so the mains connection is daisy-chained between the power supply and meters.
The power supply had a broken V- Out front panel binding post since forever, and at one point I bodged the V- on to the ground binding post, so the output was ground-referenced but at least usable. This weekend I employed a bandsaw (thanks JB!) and hacksaw to do the fitting/bodging, and I now have a set of larger/easier to use front panel binding posts which are also separated again so the Vout is no longer permanently grounded. Nice.
The voltage wouldn’t reach all the way to 40. I tweaked that so now I can get just over 40V output. I tried to adjust the calibration of the voltage meter on the supply itself and fried a nice Bourns 10 turn pot in the process of failing.
The current limit adjustment on the power supply was flaky. Difficult to turn at best and it would sometimes cut out so I had to jiggle the handle (like an old toilet) to get the power supply to give voltage out. I replaced that bad stiff old 3/4 turn potentiometer with a 10-turn pot for current limiting. Nice.
The voltage had a coarse and fine coaxial pair of 3/4 turn knobs. The fine adjustment knob had a bent shaft and wasn’t great to operate. I pulled that out and installed a separate 3/4 turn fine voltage adjustment. This required drilling a fresh hole in the faceplate, but that was easy enough. I accidentally drilled a pilot hole in the wrong side of the faceplate but I ended up needing to relocate a cable clamp and this made a handy mounting hole, so no worries. I used the power supply a bit for calibrating the panel meters and didn’t like the ability to only adjust in 0.006v or so increments with the 3/4 turn potentiometer, so I installed a 10-turn fine voltage control pot instead. Very nice.
The final step was to install a mounting bracket for the new panel meters. This was fabricated from brace lifted from a vintage piece of test equipment from an old job that closed down around my ears and gave me a bunch of stuff as I was on my way out the door. That holds the top of the meters’ cases, near the front. The rear of the meters was spaced away from the power supply bottom panel with the use of the aforementioned fried potentiometer and a twin that was previously broken, safety-wired in position. The panel meters now are securely mounted under the power supply AND come alive only when the supply is switched on. Nice.
The bodge-grade mounting solution, seen from the side (hidden well under my bench):
All of this reads as much more simple than it was in person to do. Taking the supply down means crawling under the workbench and fighting the heavy thing off of 5 screws that hold it to the underside of my work bench. Then the cover had terrible little screws (which I replaced) and the faceplate also had horrible little screws (which I replaced also). Now the exterior hardware mostly matches. It is a hassle to get this faceplate off but I had to to it several times this weekend. I had the output/heatsink module out a couple of times. I had the cover off many times and the faceplate off many, many times. And then I installed it and . . . the voltage was backwards. My panel meter sensing wires were accidentally connected in reverse polarity and it gave negative voltage readings on the meter. In the end, I decided to not dismount everything once more and just crawled under the bench waist-deep with my arms poked out in front of me like I used to do, back when I was working on airplanes, to switch the wires ’round.
After a day-and-a-half of work my work area looks like a tornado hit, and the power supply has one extra knob and a fresh set of jacks on the front. It sports a pair of big beautiful panel meters and is back under the bench, a bodged-up-laboratory-grade instrument after all the upgrades!
and: the way this power supply is mounted, I had to figure out how to remove my soldering iron (which is also screwed to the bottom of the bench) and remove the iron, TWICE, during all this. I had mounted it myself but over the last decade or so forgot how I did it. I was impressed with my own handiwork, so that was neat. Having permanently-mounted stuff under the bench is sure convenient in use but for maintenance it kinda stinks!
If you scrolled all the way down, I congratulate you and present a reward: A shot of the bottom of my bench! You see here: a soldering iron, a power supply with new meters and an extra knob, and a multimeter measuring the output of the supply for comparison. All of this, thanks God, cost me $0 plus some time. 😀