A Tale of Two Power Supplies

Recovery Ward

Okay, supposing that you, our loyal reader, were qualified to solve the problems we found, or knew a qualified person to do it for you. (Refer back to the disclaimer at the beginning of the article.) What would that look like and how well would the repaired units perform? The dust is still settling on the workbench, but let’s take a look at the older supply first, in all its renovated glory.

Old supply - repaired

What is this, HGTV for ants?

Those are some pretty hefty interior updates. The exterior has changed as little as possible, preserving the vintage look as might sometimes be desired. There’s a new fuse holder, a three-wire power cord, and a screw through one of the transformer mounts in place of the original rivet. But inside, all of the wiring has been revised, re-routed, and heat-shrink-insulated to mostly isolate the high-voltage section. The three-wire power cord has a compression-type ring terminal for the green ground lead, and the chassis and transformer frame are secured to that ground terminal with a machine screw and acorn-style lock nut. The black hot lead is first fused and then switched to both the transformer and indicator light, before returning on the white neutral lead.

Meanwhile, on the low-voltage side, the wiring has been rerouted and secured so that it mostly stays out of the high voltage side while keeping a reasonable distance from that heat-generating resistor and the transistor mounting bracket. A load bank test was run. Just over 3A of current, or about 75-80% of the maximum rating was drawn for about 15 minutes with the cover in place. We then removed the cover and checked for hotspots with an infrared thermometer. The following diagram shows the approximate results:

Old supply temperatures

So hot, you could fry a reg.

Now we see the limits of a linear regulator design. The ambient temperature at our test bench was around 75 F (24 C), but the back of the chassis was easily reaching 140 F (60 C). That’s the threshold of pain for typical human skin, and if this tool were operating in an open garage or workshop on a warm summer day, the final temperature rise could be proportionately higher. Inside, three hotspots were identified: the transformer (125 F/52 C), the transistor bracket (175 F/79 C), and for that crispy fresh-fried feeling, the power resistor was at a scorching 210 F (99 C). The unit is only suitable for temporary service, as that kind of heat is not practical to remove passively when driving a high continuous load.

New supply - repaired

Minor changes, major benefits.

Now, what about the updates to our new unit, with its similar electrical design but external heatsink? The exterior didn’t change a bit, although the power cord was replaced. Since we had to correct wiring errors on the high-voltage side, it only made sense to do a cord replacement and correct the color scheme, too. Once again, we ended up with the US-spec black, white, and green. The grounding lead is configured identically to the other repaired supply, and so is the power routing method: black “hot” lead through the fuse, then switch, then load, and return on the white “neutral” lead. The same load bank test was performed, and another diagram prepared:

New supply - temperatures

It’s a sauna in here, too.

For this unit, with a similar (75 F/24 C) ambient, the rear heatsink and the top cover were running just slightly cooler (135 F/57 C) after 15 minutes. Inside, the transformer was running a bit warmer than its competitor (140 F/60 C), and the power resistor was an equally blistering 205 F (96 C). Similar design, similar results, although the external heatsink did seem to help. Even so, the same caveat applies—in a warmer ambient environment, the temperatures of these critical components would rise by a proportionate amount, and that could lead to deep fried fillet-o-fingers.

Online sources vary on their claims about thermal burn risk (link references Wikipedia). But for individuals with thin or sensitive skin layers, it is generally agreed that even 130 F can lead to burns in less than a minute if the user remains in contact with the heat source, and 140 F can begin to cause damage within a few seconds in some cases.

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Aaron Vienot

Engineer by day, hobbyist by night, occasional contributor, and full-time wise guy.

35 Comments
    • cegras
    • 3 weeks ago

    Great article!

    Reply
    • LiquidSquid
    • 3 weeks ago

    Thanks for the trip down memory lane. I have a few of these 13.8V supplies around (Car battery voltage for running HAM equipment and other car related crap)
    What is cool is a tiny SPS wall-wort for $10 can have more output power than one of these behemoths, and is much safer and far more power efficient. However, the SPS will also be far more noisy as far as electrical noise.

    Reply
      • ludi
      • 3 weeks ago

      Indeed, although the attractiveness of a bench supply is that the power switch and terminals are Right There for attaching whatever. The coworker I referenced in the video segment is squarely in the millennial generation and had somehow turned up one for hobby use, so that’s a pretty good sign that these things are as popular as ever among the DIY crowd.

      Reply
    • dragondaddybear
    • 3 weeks ago

    I love the dual post style. Also, I really love the humor.

    Reply
      • juzz86
      • 3 weeks ago

      Me too! Good job ludi, a great read mate!

      Reply
    • continuum
    • 3 weeks ago

    Slightly off-topic question, what’s with the grey text in parenthesis after the links? Is it some attempt at WCAG 2.0 compliance without proper tags in the CMS? Or some attempt at footnotes without actually doing footnotes? Just curious since I’ve never seen a site do that before…

    Reply
      • Ben Funk
      • 3 weeks ago

      It seems to be a way to differentiate a real link from an automatic one.

      Reply
        • ludi
        • 3 weeks ago

        Bingo. It also gives mobile users an easier way to determine if they want to visit the link, since mouse-hovering isn’t implemented as conveniently on a touchscreen.

        It may or may not be used [ever again / by any other authors / cheese ], but I thought I would try it out as an experiment.

        Reply
    • willmore
    • 3 weeks ago

    Before anyone says it again, at the voltages you’re going to find in low voltage sides of these boxes, the caps aren’t going to give you any shock at all. Now, in a modern switching mode power supply, the front end caps can have more than 2x peak AC voltage on them which is around 340V. That will gladly kill you.

    Reply
      • ludi
      • 3 weeks ago

      Yep. And as it happens, both designs in the article (and all three designs shown in the video feature) are constructed such that they bleed down pretty quick when shut down. Since neither I nor the TechReport could invest the proper resources to discuss all possible hazards illustrated in this review, we opted to cover it with multiple, plainly-worded, “don’t try this at home” disclaimers.

      Reply
    • willmore
    • 3 weeks ago

    Can you get some close up shots of the control board and pass transistor for the older power supply? We can probably pull some date codes off of there. Can you see what value the large resistor on the output is? It’s likely a shunt for overcurrent protection. I’d be the control board has an LM723 on it.

    Nice teardown–now on to the new design…

    Reply
    • DPete27
    • 3 weeks ago

    “See how I just nonchalantly fingered the solder terminals on the bottom of those larger capacitors without verifying that they’re discharged (or stating this step had been done prior)…don’t do that”

    I applaud the video content and i think it’s a strong start in the right direction. If I can offer any constructive criticism (no offense intended) it would be to spice up the delivery of the [admittedly dry] review content from the speaker. The effects were a welcome addition to keep things lively, but [IMO] the idea is to make this [boring] power converter sound like the coolest/funnest thing on the planet.
    Obviously most of us Millennial+ viewers are turned off by the LTT-style of delivery, but remember that Gen-Z is responsible for probably an order of magnitude more views and likes comparatively. I’d say that LTT generally doesn’t have the most in-depth or insightful content, but their STYLE of delivery is what attracts views in droves.
    I don’t have the answer whether to go full psycho on the video content in order to cater to the younger crowd while allowing the written content to continue drawing readership from the older generations, or if there’s a happy medium that can be struck.

    Reply
      • ludi
      • 3 weeks ago

      If that’s what you think people want, I’ll wait for you to make that video 🙂 It’s harder than it looks.

      Reply
        • DPete27
        • 3 weeks ago

        I absolutely agree.

        Reply
    • chuckula
    • 3 weeks ago

    Bonus safety tip since I see a nice big blue capacitor in there: Capacitors often hold substantial charge if you turn off the power supply without discharging them.

    So you need to watch out for a scenario where you run the power supply, fully disconnect it from the wall, and still have to worry about the potential for getting a nasty shock if you open it up to do work without making 100% sure the Capacitors are discharged

    Reply
      • dragondaddybear
      • 3 weeks ago

      When I was younger that was an evil trick I would do. I’d hand a charged cap (using a transformer and a battery to charge it) to someone and shock them. It was a take on the sock box I did in school. Very appropriate/fun/evil way to learn electron flow, basic circuits, and transformers.

      Reply
        • crabjokeman
        • 3 weeks ago

        We used to play “pass the capacitor” in lab, where we’d soft toss around an electrolytic capacitor and try to catch it by the cover to avoid getting shocked.

        Reply
    • chuckula
    • 3 weeks ago

    Sweet article and finally some subject matter on TR that can get you killed if you don’t do it right!

    Reply
      • Krogoth
      • 3 weeks ago

      “Our shilling can’t repel potential of this magnitude!”

      Reply
    • Krogoth
    • 3 weeks ago

    Unlimited Power!!!!!!

    Reply
    • redocbew
    • 3 weeks ago

    OMG WHO IS THIS WTF SO SAD

    Reply
    • usacomp2k3 (AJ)
    • 3 weeks ago

    Enjoying the read so far. Does Aaron have a forum handle that we recognize?

    Reply
    • In the video he clarifies that he’s ludi—as well as in the comments yesterday where he accidentally doxed himself, lol.

      Reply
        • ludi
        • 3 weeks ago

        Aaron? Never heard of him. *flips trenchcoat and scurries off*

        Reply
        • Can one of the mods ban this joker for running multiple accounts? 😉
          Seriously though, thanks for your work on this, it could be life-saving information and will directly impact my future purchasing decisions (in a good way).

          Reply
          • usacomp2k3 (AJ)
          • 3 weeks ago

          Great job sir! I really enjoyed the rest of the read. My dad’s dad was a big electrical hobbyist (worked for Ma Bell back in the day) and while the curiosity is genetic, the skill is not. It’s still somewhat black magic to see someone understand what’s going on.

          If you’re even in the Orlando area, you’d love Skycraft (https://www.skycraftsurplus.com/). They have all sorts of vintage electronics like old Agilent Oscilloscopes.

          Reply
            • ludi
            • 3 weeks ago

            Thanks!

            My dad is a lifetime electronics technician and also a hobbyist. I had the early advantage of learning a lot from him early in life, including proper soldering technique, and then went deep into DIY audio around 15 years ago. There are also a lot of good YouTubers that enjoy delving into electrical equipment, which is a great teaching shortcut that didn’t really exist 10+ years ago.

          • Mr Bill
          • 3 weeks ago

          Well written, useful, interesting, and humorous… [quote]We thought about doing so as a side-feature for this review but didn’t want to risk being stoned to death with Mountain Dew bottles.[/quote]
          You should write reviews more often.

          Reply
            • ludi
            • 3 weeks ago

            Sadly, I don’t tax my computer hardware as much as I used to, and that gives me less relevant material in which to geek out. The video editing and encoding for this review was the hardest I’ve worked my main desktop system in over a year, and my last from-scratch system build was a low-end Skylake HTPC nearly three years ago.

            If TR sticks around for a while and I find something interesting, I’d be open to doing it again.

          • Aether
          • 3 weeks ago

          Great lunchtime read; thanks!

          Reply
    • Captain Ned
    • 3 weeks ago

    The traditional “brand name” for inferior Chinese-sourced components is “Yum-Cha”.

    Reply
      • ludi
      • 3 weeks ago

      Interesting. I’ve been heavily into DIY electronics for a while and hadn’t heart that slang yet.

      Reply
        • ludi
        • 3 weeks ago

        *heard

        Reply
        • GhostofNappa
        • 3 weeks ago

        I’m not sure what this Yumcha is, but it sounds like a Raditz to me.

        Reply

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