Why does this solder connection only connect for a short while?

Hi,  I recently (as of last night) built a hot wire foam cutter and it has a small problem. Every time I plug it in, the wall wart produces a high pitched whine and the wire does not heat up. I have to unsolder the wire and re-solder it every time to make it work. Is there a way to resolve this problem? The nicrome wire is attached to a loop of copper wire and the end of that copper wire is soldered to my power source.

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seandogue5 years ago
I just happened across this and although quite old, I suppose it's worthy of saying something..

Nichrome is incredibly hard to solder, in fact, due to its metallic makeup, it really can't be soldered successfully using electronics solder. The best you can hope for is a really good cold-solder junction.

As was mentioned, the best connection will be via a crimp connection, either literally by a crimp connector or via a screw and nut.

For quick connections, alligator clips are very handy, especially since Nichrome has a tendency to fail on application of too much current.
Modarius (author)  seandogue5 years ago
I eventually gave up on using nichrome and switched over to guitar wire which is a little more durable but thanks for the reply :)
np. If you ever do decide to use Nichrome again, or for that matter nichromel, just remember what I said. I spent 10 years providing direct electrical engineering support for combustion research at NASA GRC in which 'NIchrome' type wire was used extensively for initiation of the combustion processes under study and know first hand that it just will not solder worth diddly squat. It's cheap as dirt anyway and tends to fail often, so it really makes more sense to use a non-solder operation anyhow.

best wishes.
Modarius (author) 6 years ago
That did the trick! I soldered a switch between the power supply and the nicrome wire and now it works. There must be a surge protector built into the wal-wort and that was tripped every time I plugged it in without the switch. Thank you all for your wonderful advice that helped my solve this puzzling problem.
Re-design6 years ago
You're not getting the wire hot enough and you are making a cold joint.

Let your iron get hot.
Heat up the wire until the solder will melt on the WIRE not the iron.
I you see that the solder is soaking into the wires then it's a good connection.
Now that I read your question again, you may be getting the joint hot enough to remelt the solder and ruin the connection. If you nicrome wire is too short then there is not enough resistance and all the wires will heat up.
Surely the insulation on the green wire would at least be getting soft, if not melting or burning if subjected to that kind of temp. It should only be rated for around 75 Degrees (celsius), much less than the temp needed to remelt the solder. (about 190?)
If it starts out as a bad connection in the first place then it may get much hotter than the wire next to the connection.
framistan6 years ago
There are a couple things wrong with your connection. BEFORE you soldered it, you should have made a good mechanical connection by twisting the 2 wires around each other. Solder then applied would complete the connection. The way you have it.... you are using the solder as BOTH the mechanical connection (for strength) and also the ELECTRICAL connection. Comments by Re-design also i agree with. Solder should not be considered a kind of WELDING. it is NOT a kind of weld. Solder connections will break if any stress is on the connection OR if any vibration occurs or movement of any kind. That is why larger electronic components are also GLUED to pc boards... to eliminate component "wiggling" and working the solder connections loose. If your wall-wart is producing a noise I suspect you are OVERLOADING the wallwart. If so, it will soon burn out or overheat. be carefull.
To this I would add that, in the flight simulators that I worked on, it was forbidden to use such solder connections for wiring looms because of the vibration issue. On these machines, crimped connections were the order of the day. In a crimped connection the wires are inserted into a small tube of metal called a ferrule (I think) and then this is squashed down onto the wires with a special tool that squoze the ferrule down neatly trapping the wires. But your description hints at another possible problem. I think that there might be something in the "Initial turn on" issue. Especially the bit about the buzzing on first turn on. Is there any way that you could introduce some resistance to the circuit when you first turn it on and then reduce this resistance once the power supply has got going.
Modarius (author)  karnuvap6 years ago
Mm, you have a good point here, maybe it works only because I break the connection and then re-solder it while the transformer is plugged in. A simple switch would solve this. I shall give it a try
smitec086 years ago
are any of the wires solid core. if so there may be a clear insulated coating on it which would block an electrical connection. I had that problem recently casued a lot of troble before i worked it out. try geting your multimeter and testing the resistance betweeen points both before and after soldering. hope that helps
Modarius (author) 6 years ago
OK, I tried twisting the wires together with a very high amount of heat applied to solder it together and it still refuses to stay connected. I should clarify, the joint doesn't physically break, it just refuses to make a good solid connection. Any more ideas? And thank you for your good answers :D