Tuesday, February 27, 2018

AC Line Powered LED

From what I've seen of AC line powered LED circuits, they tend to be pretty elaborate, and they always include a rectifier diode in reverse parallel with the LED, to limit the reverse voltage that the LED will be subjected to. Then I came across this receptacle tripler in my home with an AC line powered LED in it, and thought I'd look into how the LED was incorporated. What I found was the simplest circuit imaginable.

There's no manufacturer's name on it, but embossed on the back of it is "MODEL: CT3-1V". Also embossed is the CSA (Canadian Standards Association) logo. Two M3 threading screws fasten it together, so it was easy to open for an examination of its innards. Here's a view of the interior.

And here's the schematic that I worked out for it.

Ignore the thermal fuse and the MOV for now, and note that the LED with a current limiting resistor is directly across the AC line. There's no reverse diode across the LED. Everything I've read about powering LEDs from the AC line says that you're not supposed to do it that way, but there it is and it works and it has the CSA's blessing.

So, the LED's reverse breakdown voltage must be up to the task; that's the only explanation. And looking into the matter a bit, that's what I found. If you google "led reverse voltage", you'll find some threads that  reveal that led reverse breakdown voltage is commonly well in excess of the 5V figure that most LED data sheets give.

So there you have it -- powering an LED directly from the AC line is actually a cinch. All that's needed is an appropriate value of current limiting resistor.

Notes On The Schematic
  • The MOV (Metal Oxide Varistor) provides transient ('surge') suppression. Power bars that claim 'surge suppression' all have an MOV in them. That's what their surge suppression consists of -- a single MOV.
  • The thermal fuse is there to protect against catastrophic MOV failure. A huge overvoltage surge could conceivably cause an MOV to flame out, hence the nearby thermal fuse.
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Friday, January 26, 2018

Tool Review -- Weller SP25N Soldering Iron

I've been resurrecting my interest in electronics lately, and that's led me to shop around for a soldering iron suitable for kit assembly work and the like. I was looking for something affordable -- industrial quality soldering gear is pretty pricey.

At the Home Depot, I found a Weller item that looked promising. It's Weller's SP25NCN. (I think the 'CN' suffix indicates that it's the Canadian market version. The basic model number is SP25N.) Here's a view of the iron plugged in with its 'headlights' on.

(I'll reserve judgement on the efficacy of the LED lighting.)

The tool is well thought out and comfortable to use. The 25 watt heater is entirely adequate for most electronics work. (The screw-in tip, P/N MT1, has a 10-24 thread.)

Something that surprised and disappointed me, though, was the life span of the conical tip. After only a few hours of use, tip erosion was severe. It doesn't photograph all that well, but here's a view of the eroded tip.

It's as though the tip has been evaporating. That really surprised me, because I have long experience with Weller's industrial soldering tools, and I'm accustomed to Weller's soldering tips being robust, long-lasting items. I certainly wasn't expecting short tip life from a Weller product.

Anyway, I wrote to Weller about it, and they've promised to send me a replacement tip. I'll see how that one holds up. Maybe the tip that came with my SP25N was just a fluke defective one.

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An Observation On Tip Configuration

I find that the conical tip shape is less than ideal for fine work. It seems to me that molten solder wants to migrate up away from the fine, conical point of the tip, which kind of defeats the purpose. A better configuration in my experience is a chisel shape, like on this ancient Ungar miniature 'tiplet', photographed next to the SP25N's tip.

The chisel-shaped tiplet tends to accumulate molten solder right where it's needed.

The Ungar firm is no longer with us. I think it was acquired by Weller, and its excellent low-end line of soldering irons and tips was discontinued. That's a shame, because Ungar's low-end product line was good, affordable gear. I still have a very few of their 1/8" screw-in tiplets, and that led me to ponder how I might put them to good use. I came up with a way to do it.

Modifying The Weller SP25N For Undersize Tips

I drilled and tapped the barrel of the SP25N to accept a 6-32 setscrew, and that gave me a soldering iron that takes the old Ungar 1/8" tips, like so.

I've certainly voided the soldering iron's warranty, and I doubt that Weller would approve, but the arrangement works nicely. One could even use a piece of 12 or 14 AWG solid copper wire as a tip.

So now I can have the use of my old Ungar tiplets, and whatever other undersize tips I might come across.

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Update -- THURSDAY, FEBRUARY 8, 2018

UPS brought me two MT1 tips today, graciously provided by Weller as warranty replacements. It will be interesting to see how they hold up.

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Tuesday, January 23, 2018

Arduino Impressions

I had mentioned to my son a while back that I might look into the Arduino, as a way to relaunch my decrepit electronics technologist's career. My son responded to that by getting me an Arduino starter kit for Christmas. Here's a view of that opened up on the workbench.

It's quite an array of stuff. (That little arrangement on the breadboard is an RGB LED wired up for a demonstration.)

The quality of the hardware is superb; everything is beautifully made. One little feature is almost too much of a good thing -- the resistors are all one percent tolerance, instead of five percent. The colour codes on one percent resistors are a bit difficult to read. Five percent tolerance resistors would be entirely adequate for the kit's purposes, and the colour coding on those is much easier to make out. One wonders what the kit's creators were thinking on that point.

Anyway, my son installed the software for the kit on my laptop, and left it with me to have at the array of exercises. As an old electronics hardware guy, the hardware aspects were all no-brainers, and I had no trouble getting the programs ('sketches', as Arduino likes to call them) to run.

All well and good, but there's a fly in the ointment. As one with minimal background in computer code and programming, I find the Arduino software impenetrable. I've had some exposure to microprocessor instruction sets back in the day, and that sort of thing made reasonable sense to me. I was able to actually work with it and manipulate it. The Arduino stuff is thick as a brick, and leaves me baffled. I'd like to be able to do original, creative work with the Arduino, but I fear that it would be my life's work to fathom its software. The documentation is not terribly helpful -- nowhere can I find the sort of clear, concise presentation of an instruction set that I was accustomed to seeing way back when.

The literature that I have seen seems to imply that the software is fairly intuitive and readily learned. I'm calling 'hogwash' on that. The software looks to me like an arcane, opaque mass of syntactical booby traps -- the province of those who are adept at the C and C++ languages that it's supposedly based on.

I'm still hopeful that I'll find some interesting applications for the Arduino that will motivate me to keep at it, but they'll likely have to be things for which the software code already exists. I've found a metal detector project that looks promising, but I don't expect that I'll be writing original code for the Arduino anytime soon.

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Thursday, November 2, 2017

Tealights As Jack-O'-Lantern Lights

It turns out that tealights are not a great choice for lighting a jack-o'-lantern. They don't last nearly as long as they ought to. Here's a view of one that I pulled out of a jack-o'-lantern after only about two hours. (The lights are rated for four hours.[1])

The wick got consumed completely long before the fuel ran out. Being out in the cool autumn air, and sitting on cold pumpkin flesh inside the pumpkin, the tealight's body couldn't warm up enough that the fuel could melt fast enough to keep feeding the wick with liquified fuel.

I still like the idea of using a tealight to light a jack-o'-lantern, though. Tealights are cheap; I got this package of 32 for $2.00 at the 'dollar' store.

So, I'll do a little experiment. I'll make a wooden holder for a tealight, so the light's metal cup will be insulated from the cool air and the cold pumpkin's flesh. I have a 2 1/2" diameter softwood off-cut from a lathe turning that should make a good holder. I bored a 1 1/2" diameter pocket in that to receive a tealight, like so.

I'll install that in my jack-o'-lantern and set it out by the back door to the workshop, where no one will see it and think that I'm trying to make a multi-night festivity out of Halloween, like the twelve days of Christmas or something. Here's a view of my experiment underway.

- - -

Well, so much for that experiment.

The tealight with the wooden insulator lasted over two hours, but met the same fate as an uninsulated tealight -- it consumed its wick long before it could exhaust its supply of fuel.

So there we are. The tiny flame of a tealight can't overcome the chilling effect of a low ambient temperature on its fuel supply, and liquifiy sufficient fuel to keep itself going.

Next year, I'll try using a dollar store votive candle as a jack-o'-lantern light, and see how that does.

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[1] I checked out the 'four hours' rating under ideal conditions indoors, and four hours turned out to be a conservative figure. With a tealight perched on a scrap of softwood inside the workshop, the light burned for well over four hours. By about half way through that amount of time, the fuel inside the light's metal cup had liquified completely -- there was no possibility of the wick running out of fuel and consuming itself before the fuel supply was actually exhausted.

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Tuesday, September 26, 2017

An Installation Tool For Staked Garden Fixtures -- Or -- An Earth Poker

I recently built a tomato trellis for my son's garden. Here's a view of it.

I got the plan for the trellis from a book, Classic Garden Structures by Jan & Michael Gertly.
Note the 12" long rebar stakes attached to the bottoms of the legs. Those weren't in the book's plan. I added them to improve the trellis' anchoring. Here are closer views of one of the stakes.

It dawned on me that installation of the trellis might be a bit problematic -- shoving the trellis' four stakes into the ground simultaneously didn't strike me as an approach that was likely to work. So, I made a poker tool to help prepare holes to accept the stakes. Here's a view of that right next to a stake.

The handle is a 4 1/2" length of 1 3/8" diameter hardwood dowel. The shank is a 15" length of 1/2" diameter steel rod, salvaged from an ancient Qume daisy wheel printer. Here are some closer views of the tool.

The rivet through the handle is a 3" common nail. I seated the shank into the handle with five-minute epoxy, since the shank turned out to be a little bit loose in the hole that I'd bored for it. I turned a 60° pointy end on the shank in the metal lathe.

So there we are. I expect that the trellis will still be an awkward thing to install, even with the aid of this tool. We'll see how it goes.

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Friday, September 15, 2017

A Soil Sifter

In two earlier posts, I've dealt with a couple of gardeners' tools -- a seed row compactor and a seeding tool. I so enjoyed those two projects that I was moved to go casting about for another gardening tool to build, and make a threesome of it. I found a plan for a soil sifter that looks like it might be useful. Here are scans of the article from the book "Build It Better Yourself", by the editors of "Organic Farming and Gardening", published by Rodale Press of Emmaus, Pennsylvania.

Following are a few notes on the version of the thing that I constructed.

a) I made the length and width bigger by 1/4", so that the snaggy edges of the hardware cloth screen would be set back a bit from the frame's perimeter.

b) Frame corners are joined with plugged pocket screws and Gorilla glue.

c) No finish is needed, but I gave the frame an application of tung oil.

Here's a dimensioned sketch of the end pieces' shaping and handle cut-outs.

d) The book calls for 3/4" staples for fastening the hardware cloth screen to the frame. 3/4" staples strike me as an oversize, awkward fastener for the task. I used No. 6 screws with flat washers. Here's a view of that.

And here's the completed sifter out among some comrades.

Bill of Materials
  • Qty 2: 3/4" x 3 1/2" x 18 1/4" SPF for frame sides.
  • Qty 2: 3/4" x 5 1/2" x 10 3/4" SPF for frame ends.
  • Qty 1: 12" x 18" rectangle of 1/4" mesh hardware cloth.
  • Qty 8: Kreg No. SML-C125, No. 8 x 1 1/4" screw.
  • Qty 8: Kreg No. P-PNT, paint grade pocket-hole plug.
  • Qty A/R: Gorilla glue.
  • Qty A/R: Tung oil.
  • Qty 20: No. 6 x 1/2" pan head screw.
  • Qty 20: No. 6 SAE flat washer.

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Monday, September 11, 2017

An Electric Lawn Mower Motor Failure

I actually know better than to pick up discards like this from the roadside, but I've got nothing else pressing to do, and I thought I might learn something about electric mowers.

It's an elderly Sears Canada model C935-35516-1. The odds of it being repairable are slim to nil. But I have no experience with or knowledge of electric lawn mowers, so I may as well open this one up and at least discover what its problem is. At worst, I'll have one more hunk of scrap metal for the next run to the scrapyard.

The blade is not in too bad a condition, but when I try to turn it it won't turn but a bit, and there are crunching noises -- not a good sign.

A 3/4" A/F hex nut holds the blade on. Removing that with an impact wrench reveals the blade's 1/2"-20 spindle stub.

There's a plastic centrifugal fan blade that's seized in place. That fan blade has to come off in order for the motor to be removed.

- - -

To make a long story short, after much puzzling out and dismantling, I got the motor out of the deck and opened it up. Here's what happened to the motor.

One of the permanent magnets came loose from the frame, met the spinning rotor and shattered, jamming the rotor. Here's another view after I'd removed some bits of the shattered magnet.

That's what's known as catastrophic failure. That motor is unrecoverable.

So, that's a roadside find that didn't pan out. All I'm left with is scrap value, which is precious little since the mower's deck is all plastic.

Anyway, I did learn a bit about electric lawnmowers -- they use permanent magnet DC motors. That's how they're able to have adequate power with a motor of minimal size and weight. A solid state bridge rectifier is what makes it possible for the motor to operate from AC household current. Here's a view of the rectifier mounted on top of the motor.

There's quite a good article on electric lawn mower wiring and servicing here.

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