Wednesday, May 30, 2018

A LED Replacement Light For Your Soldering Iron

A Communicator Reprise: January 2012

One of handiest tools for the Ham Shack workbench is a Weller soldering gun.   It’s a 100 and 140 watt gun in a good old Bakelite case. This year it’s celebrating its 40th year on the work bench.  The thing  has been dropped so many times, it’s a miracle the gun still works, but thanks to several tubes of 5-minute epoxy and some crazy glue, it’s still in fine working condition.

But recently the little incandescent pilot lamp/tip illuminator burnt out and I just couldn’t find the right replacement bulb… a 2.0 volt bulb with a focusing lens at the end, something you used to be able to find in a common flashlight… a common flashlight 40 years ago. So while hunting in my parts bin, I came across a white light LED and decided to update the old gun with a modern light source.

So I removed the bulb from it’s screw base, found the right current-limiting resistor for 20ma at 2 volts and proceeded to assemble my new LED spot light.
There are a few things to ponder… the gun supplies 2v AC, your LED is a diode, so it’s going to work on one half the cycle, which translates into less light output…
Also the junction breakdown voltage is rather low on a LED, so if one were to use  higher AC voltages, it would most likely “expire” rather quickly.  But it’s only 2 volts, so I wasn’t worried.  You could put a 1N4007 in series on the other lead to help the LED deal with reverse voltages, but only if you were working with higher than 6-9 VAC.

So to hold things in place, I potted the resistor in epoxy putty, which set in 3 minutes and then soldered the LED to it.  I only had a ½ watt resistor, larger than I needed but it fit nicely in the screw base.  So if I wanted a 20mA current draw, that would be R = E / I = 2/.02 = 100 ohms and I just happened to have a 100 ohm resistor in my parts cabinet.  P (in watts) = E x I = 2 x .02 = .04 watts or 40 mW of heat dissipation so a ½ watt resistor wasn’t necessary, but it was the only size I have in stock and size wasn’t an issue.

A few minutes later I had a modern light source in an antique tool… but would it work?  But of course… for ½ the cycles per second -- so the light from it wasn’t as bright as I was hoping, but good enough to shed a bit of illumination on what was being soldered and certainly adequate for a pilot light to verify the gun was on.   OK for younger eyes, but this old buzzard needs a few more candle power!  Why didn’t I put a tiny bridge rectifier on the power leads to feed the LED with better DC?  Cause it was only a 2 volt tap off the coil inside the gun.. and for every diode you insert, you lose 0.7 of a volt.   Why did I need this in the first place you ask?  Well the tips of my fingers and tongue hadn’t recovered from my earlier attempts to see if the soldering tip was getting hot!  

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