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Ohm’s Law Is a Bitch

Jimi Henton, the local breeder from whom we got Aero, Jack, and Dash, brought me her dog grooming dryer some time back to see if I could figure out what was wrong with it. Carol has the exact same dryer, a Chris Christensen Kool Dry. It’s basically an SCR-controlled variable-speed fan in a box, putting out 114 CFM through a hose.

Jimi said it wasn’t blowing as much air as it used to, even after she cleaned the filter and made sure nothing else was gummed up with dog hair. It still blew, and the pot still varied the fan speed, but it wasn’t as loud and clearly didn’t have its out-of-the-box oomph. Worse, she’d had a new motor installed last year. The first one had gone for eleven years before dying; this seemed kind of premature.

I wanted to compare the two dryers to get a sense for how much air was being lost in Jimi’s. I have no way to measure airflow here, but sitting on the laundry room floor I noticed Jack’s little soccer ball, much reduced from its original size, but still round enough for my purposes. With only a little skill I managed to get the ball levitating over the nozzle, as any kid who’s bright enough to put a vacuum cleaner in reverse has done. On Carol’s dryer, the ball wobbled between 18″ and 24″ above the nozzle. On Jimi’s, it was maybe 4″.

So there was work to do, somewhere. Upon opening the dryer up, at least one problem was obvious: The 1,025 watt AC motor was wired to the speed control with #24 telephone wire, and too much of it. (You know, the stuff with the two-color, bands-on-solid insulation.) Close inspection showed two cold solder joints, coincidentally (heh) where the #24 wire hit the speed control pot. The plastic insulation on the phone wire was blackened with heat. The dryer slowly was cooking itself from the resistance of all that skinny wire. No need for a fork; it was done.

Jimi had ordered the motor from the manufacturer and then had somebody local put it into the dryer. She called him an amateur. No. I’m an amateur, with a callsign to prove it. I do electronics because I love it. Whoever installed this motor was…an idiot.

All fixed now, using some #14 stranded wire and soldering skills I learned when I was eleven. Both dryers now loft the soccer ball two feet hgh. Ohm’s Law is a bitch, dude. Please go back to sharpening scissors.


  1. Bob Fegert says:

    I’m also an amateur…and you are right, that guy is an idiot!

    #24 wire…lol

  2. Tom R. says:

    Jeff, Maybe he was trying to make a resistance heater in there. Looks like he succeeded. I learned to solder when I was about 10 and got a heath AR-3 receiver for Christmas and I STILL say that solder was my FIRST programming language.

  3. Lee Hart says:

    One of my electronic engineering magazines has a regular column “Designed by Monkeys”. They find endless examples of products that appear to have been *designed* by the same sort of idiot that fixed your friend’s dryer. Only in this case, it results in millions of defective products, not just one.

  4. Jack Smith says:

    Probably not so much the IR drop through the wire as the heat causing bad solder joints.

    1025 watt motor at 120V draws 8.5A and no. 24 AWG wire has a resistance around 0.025 ohm/ft, so the IR drop for one conductor is thus 0.2V per foot. If the no. 24 wire is 2 ft long (seems unlikely to be longer than that based on your description) and we consider that it has two conductors, the total voltage drop across the wire loop would be around 0.8V. That’s negligible in terms of the supplied 120V AC. (There’s also a voltage drop across the triac even if set at maximum conduction angle.)

    The no. 24 wire would dissipate a fair bit of power, of course, 6.5 watts in round numbers. It’s going to get quite hot and I doubt the insulation is rated for the associated temperature rise, but it may have been cooled by airflow through the enclosure.

    The speed control device is probably not an SCR but rather its close relative, a triac. In ye olde days, the triac was triggered by a separate device such as a diac but I imagine the one you looked at integrated the trigger and power control into one device with just the speed pot being external.

    This leads me to a different failure mechanism … my conjecture is that the triac was not properly triggered due to the bad solder joint, which was, in turn, caused by wire overheating or perhaps it was just a lousy soldering job to begin with. The triac probably ran in half-cycle mode which would reduce the motor’s power significantly. Fixing the solder joint cured the triac’s defective trigger signal and restored it to normal operation.

    1. Ahh! I hadn’t thought of that. Those solder joints (three of them, actually) were the worst I think I’ve seen in my 50 years as an electronics tinkerer, and like most of us, I’ve touched up and re-flowed a lot of joints on a lot of other people’s Heathkits. (Once with the power on. Caught 400 volts through a roll of solder. Yowch.)

      I built a triac/diac soldering iron heat controller back in the late 70s that I still have, though with a modern soldering station I use it only for “portable” work where hauling the station around would be a nuisance. You’re right about that; the speed control had only one active device.

      As a Xerox repairman I had to occasionally fix other Xerox repairmen’s repairs. I wonder sometimes if any of them ever had to fix any of mine.

      1. Jack Smith says:

        I have the same soldering iron controller, diac + triac, that I built in the early 1970’s, Last time I tried it, still worked. (Housed in a metal electrical outlet box, with a dual outlet on the triac output side.)

        1. If you got the circuit out of the ARRL Handbook, I’m guessing it is indeed the same one. Put mine in a minibox, I think because I used to hoard miniboxes, and still do. Miniboxes, to me, are what electronics projects are shaped like.

  5. William Meyer says:

    Many years ago I had a friend in Silicon Valley who worked for a once great CPU manufacturer, on the CPU test instruments. His work was to make the instruments capable of testing higher-speed CPUs (in those days, that meant 6+ MHz). We talked about the modifications he made:
    – more iron (a bigger PS transformer)
    – more copper (heavier wiring from PS to loads)
    – more capacitance (PS filtering)

    There were other, more esoteric changes he made, but without those fundamental improvements, the rest were ineffective. And these were commercially produced instruments! Over the years, I have similarly exotic improvements in many products.

    Not only is Ohm’s law a bitch, but power supply specification, as easy as it is, can be a bigger bitch, when thoughtlessly done.

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