I am trying to make a room heater. It comprises of a few coils of nichrome wire connected in parallel with each other and in series with a regulator to control the voltage. I am also having a step-down transformer connected in parallel with this setup powering a small dc-motor to spin a rotor and push the air out. I was wondering how I could connect the nichrome wire coils to my regular copper wires which I am using to connect them to rest of the circuit without the copper melting at the junction of the two wires where the hot nichrome will be of a temperature higher than the melting point of copper. How do I join them without this happening? I haven't tried this but I think this is what is going to happen if I do. Any ideas anyone?
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4\$\begingroup\$ A nice heavy crimp should do the trick \$\endgroup\$– JIm DeardenJan 15, 2018 at 15:40
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\$\begingroup\$ I've been told phosphoric acid works well for soldering NiCr but haven't tried it myself. You might also try mild HCl. Works well for soldering SS. \$\endgroup\$– Robert EndlJan 15, 2018 at 20:12
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\$\begingroup\$ cheap soldering irons simply twist the wires together... \$\endgroup\$– dandavisJan 16, 2018 at 2:48
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4 Answers
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Attach both wires to opposite ends of larger strip of metal using screws or welding. The attachment strip can be stainless steel or something that will not corrode or otherwise be damaged by the higher temperature at the nichrome attachment point. The attachment strip needs to be large enough so that it does not get too hot from the current and so that temperature at the copper attachment point is acceptable for the copper wire. The copper wire may need a high temperature insulation near the attachment point.
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That kind of depends how hot your plan on driving the ni-chrome.
If you are well under 1,085C, the melting point of copper, a good crimp will do with appropriate insulation on the copper.
If you plan in going closer to the ni-chrome max, 1,400C, you will need an appropriately sized intermediate block of material like stainless steel. The block has to be sized so the TD from the ni-chrome side to the copper side is enough to bring the temperature well below 1,000C.
Here is a general temperature colour chart.
If your wire is at the yellow/white end, you need some intermediate material, if it's down at the dark red end, a direct crimp may be sufficient.
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1\$\begingroup\$ @jonk you know I though material mattered too, but on researching it, apparently material does not affect it much. Since iridescence is an atomic "surface" effect, it's more about atomic shells/boundary layers than mass of the material. If you want to get a real tight number you do need lots of experience, but for a quick guide it's fine. If the OP's wire is at the yellow/white end, his fastening needs an intermediate material, at the faint red end.. not so much ;) \$\endgroup\$– Trevor_GJan 15, 2018 at 17:47
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1\$\begingroup\$ No. It has nothing whatever to do with the thermal conductivity. This is pure physics and it's something that was vexing businesses back in the early 1800's until Kirchoff came up with an answer that advanced research methods. (Integrating sphere.) Later, Planck was forced to accept the idea of atoms as "little, perfect oscillators" in order to develop a way to derive his equation. (He didn't believe in atoms at the time.) But this is fundamental physics -- as in QM. I'd have to write out stuff, I guess, and can't do it here as a comment. \$\endgroup\$– jonkJan 15, 2018 at 20:27
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1\$\begingroup\$ The surface, shape, material itself etc., all impacted observations and this was a horrible problem for people smelting metals, for example. It's still a horrible problem for pyrometry (where I specialize.) I can tell you that FABs spend (since they paid me) lots of money to solve this problem for them. \$\endgroup\$– jonkJan 15, 2018 at 20:29
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1\$\begingroup\$ Hehe. When I was talking about those cubes, I meant that the cubes are contained inside an oven where there is no edge effects for radiation, so that the cubes are completely and entirely throughout at the exact same temperature. The corners are at the same temp as the core. (I went to some trouble to make sure that was also true using multi-zone control systems.) These are the kinds of experiments that were so vexing for iron workers and the like, long back, and helped fund scientists because they needed a solution/answer. \$\endgroup\$– jonkJan 15, 2018 at 20:33
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2\$\begingroup\$ I don't ever go to parties. So luckily I don't have those problems. :) No birthday parties; never been to a bar -- not once in my life. And I do not go to barbecues, either. Social gatherings are anathema. I do things where the relationships are structured at the outset -- teaching as a teacher, working on projects as a consultant, starting a business or two from time to time. Stuff like that. But the roles are clear and understood. A party is pretty scary to me. I wind up in a corner somewhere waiting for the time to expire. \$\endgroup\$– jonkJan 15, 2018 at 20:41
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A simple method that seems reliable running a nichrome coil up to ~900°C is to use copper wire as a heat sink and attach to the nichrome with cable sleeves - those thin wall tinned copper tubes with a plastic "funnel" and belled entry at one end, normally used to make decent quality crimped terminations with finely stranded wire. Choose a sleeve that slides over solid copper wire, which in this case is ~1mm OD. Rip the plastic off the sleeves, slip the plain ends over the copper about ½ way, solder in place and also fill the belled end of the sleeves with solder. An advantage is that ordinary soft (electrical) solder can be used and no special flux required.
In my case ~80mm of the copper wire is exposed to ambient air. Nichrome coil ~32 awg and 3mm OD is attached by straightening a section ~10mm long at each end, reheating the soldered sleeves and dipping the nichrome into molten solder.
Strictly, it is not the same as a soldered joint because the nichrome will not be wetted but it is electrically sound (conducts well) and mechanically robust, since the comparatively heavy gauge copper sucks the heat away from the connection far faster than the nichrome can put it back in. Of course if the joint is somehow allowed to get hot the solder will melt and quickly fall out; then the show is over. One can also crimp the sleeves to the copper with nichrome coil in between (ie concentrically) but it is not as elegant and this also introduces a lot more heat to the copper wire and sleeves.
Plain old resin core solder works well, just keep it cool. 
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Sleeves with plastic funnels still attached
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1\$\begingroup\$ Those sleeves can be found by searching for "ferrules." You can also buy them with no insulation to save you the step of ripping off the plastic part. Since you are new, let me say Welcome. Probably you noticed that this is a rather older question and already has an answer that has been accepted and upvoted. You will probably gain reputation faster if you answer newer questions which don't yet have accepted answers. Anyway, welcome to EESE. \$\endgroup\$ Mar 8, 2020 at 1:50
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2\$\begingroup\$ Thx for all that and yr advice is sound. Was aware this one was way out of date but it was something I'd been wanting to find a quick and easy solution for, for far too long. Once it was proven, it had to be told. \$\endgroup\$– JSB-PPMar 8, 2020 at 23:07
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1\$\begingroup\$ This site shows up in a lot of search results, so that makes sense. Thanks for contributing. \$\endgroup\$ Mar 9, 2020 at 0:24
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Simple solution is round nichrome wire around itself (I mean twist) couple times for one or two inches part of attaching point. Single wire has more resistance and gives more heat but at the end it will be much thicker wire so has much less resistance and will be cooler than the single wire section. Nichrome is also resistant to heat transfer so heat can't go through to the end. Then you can attach the copper wire to tip of the thicker nichrome wire part. (Sorry for my English)
