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As I understand it, some devices that heat up to a high temperature, like soldering gun, use a step-down transformer, in order to increase the current flowing through the heating element. An alternative option would be to use no transformer, and a heating element with a higher resistance. For the right value of resistor, this would result in the same power dissipation, and therefore the same heat, in the heating element. Why is this not done? What benefit does the step-down transformer have?

Note: This question is not specifically about soldering guns, just any device where a step-down transformer is used with a heating element.

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  • \$\begingroup\$ At some point the cost of the element will exceed the cost of the transformer. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 27 '17 at 4:36
  • \$\begingroup\$ Why is that, @IgnacioVazquez-Abrams? Couldn't you make a higher-resistance heating element using less material, by making the wire thinner? \$\endgroup\$ – ItsTimmy Dec 27 '17 at 4:38
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    \$\begingroup\$ Making the wire thinner reduces reliability, and making it thinner doesn't necessarily make it cheaper. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 27 '17 at 4:42
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A soldering iron has to meet several specifications, beyond getting hot enough to melt solder

  • it must not electrocute you, so must be isolated from the mains
  • it must be physically strong
  • it must get hot quickly enough to be useful

Obviously some of these requirements are more important than others. You cannot tolerate an iron that kills people, but waiting for it to heat up is a compromise some people will accept if it's cheap enough.

There are two main ways to meet these requirements.

The first uses a heating element made of long thin floppy wire connected directly to the mains. For isolation and strength, this is wound onto an insulating former, wrapped in an electrically insulating sleeve, and then further sleeved in a strong, earthed metal tube which heats the soldering tip. This pattern takes a long time to bring the tip up to soldering temperature, as there are so many components to heat up through electrical insulation and additional interfaces.

The second uses a heating element made of a short thick wire, which is strong enough to be used as the soldering tip. This cannot be connected directly to mains as it's far too low a resistance, and it needs to be isolated from mains. Fortunately a transformer solves both problems with a single component. The cost of the transformer can be traded off against the advantage of rapid tip heating.

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One reason is fairly simple - using a transformer provides line isolation and limits the voltage on the iron. Both of these are good things in the event of a fault.

However, I have several cheap irons I don't use much at all that would make you happy, as they are straight wall-plug at 120VAC with no transformer. so it's not like you can't get those. I'd happily sell you mine, though I would not recommend them over a good iron - but it would keep me from dragging the (nearly) useless things around.

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In soldering guns, the current is going directly through the thick wire you are soldering with. It needs a certain cross section to be mechanically stable. That cross section directs the current you have to supply. Making that wire long wouldn't add value but instead make it mechanically unstable again.

That's why soldering guns are using 4..12V at 5..10A.

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    \$\begingroup\$ I think you are low by ~20:1 on current and high by a similar ratio on voltage. \$\endgroup\$ – Spehro Pefhany Dec 27 '17 at 5:30

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