I may not know what I'm doing, but at least I can SEE what I'm doing.
I started with a 1" square piece of double-sided copper-clad PC board. I drilled it in the middle to fit the shaft of the soldering iron, plus a bit so it's a loose fit. I then used a file to make the square into a disk. While I was at it, I tapered the edges of the hole so the copper surfaces wouldn't touch the iron shaft and short. I should have tapered the outside edges as well - more on that in a moment.
Front of completed board.
Back of completed board.
The upper surface of the board is the negative surface, and the bottom is the positive. I soldered the LEDs onto the edges as shown, but one of the leads that bent around the outside to the underside was very close to the edge of the upper copper, and shorted. I moved all the LEDs out a millimeter or so to prevent this, but tapering the edge of the board would probably be a better solution.
Note that this arrangement puts all the LEDs in parallel, which isn't optimal. If one of them goes open circuit, the others get more current, making it more likely that another one will go open circuit, giving the others even MORE current ... until they all blow. But that's not likely to happen without being noticed in plenty of time, and even if it does happen, LEDs are cheap.
The LEDs are powered by the voltage drop across a 24 ohm, high-wattage resistor (for some reason, the resistor isn't labeled, so I don't know the power rating) which is inserted in series with the heating element. A diode bridge rectifies the AC, and the whole thing fits nicely inside the handle of the iron.
The voltage drop across the 24 ohm resistor is 7.5 volts max, or 5.3 volts RMS, minus the diode drops of the bridge. Or it would be if the LEDs didn't clamp it at 4.5 volts or so. They see a total of 135 mA, or about 17 mA each (this is why I used eight LEDs). They're rated for 20 mA, so they ought to be fine.
Putting a resistor in series with the heating element decreases the power of the iron a bit, but not as much as you'd think. The resistance of the heating element is 517 ohms or thereabouts, so the power is cut by only about 4.5% - or at least it WOULD be if some of the current didn't bypass the resistor entirely and pass through the LEDs. In fact, only about a two thirds of the current passes through the resistor - and since power dissipated is proportional to the square of current, that means that just a bit less than half the power! It's more efficient than it appears at first glance.
Mounting the disk
The disk is held in place, and powered, by stiff wires (paper clips) that are themselves held in slots filed in the edge of the two halves of the plastic handle. Once the two halves are screwed back together, the notches hold the wires pretty firmly in place. The solder connections on the inside keep the wires from sliding out, the bends on the outside keep the wires from sliding in, and the disk itself keeps the wires from turning in the notches.
The disk blocks access to the two screws in the front of the iron, so I had to put the wires in place before I attached them to the disk. It is possible to get them in with the disk in place if you use needlenose pliers, but it's a pain. Not all irons have screws there, so it's probably not an issue.
Just waiting to zap someone...
I wanted to put the resistor on the neutral side of the 120 VAC circuit, but the plug on the iron is not polarized, so it's moot. Just remember that this circuit is NOT isolated from the mains, so you can get a nasty shock if you stick your finger where it doesn't belong. There is only a few volts between these wires, so they won't get you by themselves, but there can be a LOT of voltage between those wires and ground, so they can still get you! Not only by electrofrying you, but also by shorting out to a metal chassis, for example, and making the mother of all fireworks right in your face, blowing out expensive components, and setting your project on fire. Make sure to cover up the exposed conductors.
Buck Rogers would be jealous
I used the plastic from a pill bottle to cover the zappy bits. It is still possible to get your finger in there, but you've got to really try now. In retrospect, I would have made it a bit longer so it stuck out past the LEDs for a bit more protection. Also, I think I'd add a flashing LED on the back of the disk, behind the translucent plastic shield, just to make Buck Rogers jealous.
One unexpected feature of this device is that I can now lay the soldering iron down on the table and the tip doesn't touch the surface. Good thing - it no longer fits in the stand I have for it.
Naturally, I very soon came up with another idea. While digging around in my shop, I found a neat little combination laser pointer and LED flashlight with a nifty goose-neck. The batteries had corroded and pretty much destroyed the thing, but the goose-neck was salvageable.
Goose-neck LED laser pointer thing
I simply removed the original LED disk, drilled a properly sized hole, and stuck the goose-neck in. It took all of about a minute. The LED gets a pretty hefty dose of current, so I may go back and add a 10 ohm resistor.
Goose-neck LED laser pointer thing
You can see how it illuminates the tip a lot better than the disk did, even though the disk had eight LEDs. They were not only situated much further back, but the also cast a tiny shadow right in front of the tip. This arrangement lights the tip - you can see the shadow of the tip above.
It's bright, too.
© 2013 W. E. Johns