OLD soldering iron element resistance

[NM7G]

I found what appears to be a 1940s-1950s iron for sale, with no markings on it whatsoever. The heat element portion is 1 inch in diameter, maybe a little larger. The replaceable tips are at least 5/8 inch dia. where they push into the element. I've seen and used a fair number of big irons, and think this one could be 275W to 375W. The line cord is bad, with no plug. My multimeter showed about 1.3 Ohms across the two frayed where a plug should be. I expected higher. How much cold resistance should it have? I don't know the ratio of hot-to-cold DC resistance for an (assumed to be) nichrome element. Anyone care to offer an educated guess?. I chose not to assemble an AC plug to it and watch the fireworks, at least while someone else owns it.

[KC9UDX]

I chose not to assemble an AC plug to it and watch the fireworks, at least while someone else owns it.

Plug it in in series with a 300W light bulb. The worst that would happen is the light bulb comes on full brightness and stays that way. If so, the iron is shorted. If the bulb comes on bright but goes dim, the iron is OK. If the bulb doesn't light up, use a smaller wattage one.

The resistance will go up when it warms up.

[NM7G]

Good thinking! My brain was out to lunch, because I've done that before. Thanks.
Gary

[G4COE]

Re-: KC9UDX
3 x 100W lamps or even 2 x 150W lamps might be easier to find, just run all the lamps in parallel to up their wattage to 300W as suggested .

1.3 Ohms sounds a wee low to me, but this could well be the 'cold resistance' which would rise some what when heated and passing current.

How about measuring the resistance if the lead to the tip just in case you have a short from live to ground... if isn't a three wire cord, one being ground or earth you don't wanna be using it on transistors n' chips.


Checking the leakage current wouldn't go amiss using a megger, I'd say this would be the first thing to do, your own personal safety also come to mind... you gotta hold the solder while you solder, right?

[NM7G]

Re-: KC9UDX
3 x 100W lamps or even 2 x 150W lamps might be easier to find, just run all the lamps in parallel to up their wattage to 300W as suggested .

1.3 Ohms sounds a wee low to me, but this could well be the 'cold resistance' which would rise some what when heated and passing current.

How about measuring the resistance if the lead to the tip just in case you have a short from live to ground... if isn't a three wire cord, one being ground or earth you don't wanna be using it on transistors n' chips.


Checking the leakage current wouldn't go amiss using a megger, I'd say this would be the first thing to do, your own personal safety also come to mind... you gotta hold the solder while you solder, right?

Also some good advice, and thank you.

I made it a habit to always isolate my irons. Twenty years ago I wired two filament transformers secondary-to-secondary, and power my iron from one primary. 73

Gary

[G4COE]

I made it a habit to always isolate my irons. Twenty years ago I wired two filament transformers secondary-to-secondary, and power my iron from one primary. 73

Gary

You take me back years, when the TV sets and the odd radios were ac/dc types, many early solid state and hybrid TV sets weren't isolated too..... using isolating transformer stopped the occasional 'bang n' sparks'.

I personally wouldn't trust any old soldering iron, especially two corded ones on silicon slabs, ok on tube gear though.

To take the leakage current further, I ought to have added the leakage current could also be dependent on the temperature, meaning what might appear OK when cold could rise to dangerous levels when heated depending on the insulation etc.

Dave

[WB2WIK]

I made it a habit to always isolate my irons.

Me, too. I keep them in a bag that also holds some balls and tees in little pockets.

[G4COE]

Me, too. I keep them in a bag that also holds some balls and tees in little pockets.

Tee off..... I'll have coffee instead !

Dave