# Making a 12-24v DC variable power supply

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1. Feb 28, 2017

### JoeSalerno

1. The problem statement, all variables and given/known data
So, for a school project I have to make a resistance soldering iron. As far as I know, this requires a high current going through high resistance wire (such as stainless steel in my case). I need to make this run from a standard 120v 15A wall outlet. I have figured out so far that it would be a good idea to run a D.C. power supply in the cable (similar to a laptop) to take the voltage and amperage down for safety. I need to be able to also make this safe to touch in case somebody touches the leads. I was thinking that grounding the circuit would do this, but I'm not sure.

2. Relevant equations
So I'm not exactly sure what equations will need to be used, but I'm familiar with the simple ones such as
W=I*V and I=V/R.

3. The attempt at a solution
like I said, I believe I need to include a DC power supply to make this safe, as well as ground the circuit. If anybody knows the specifics of how much current is needed to make the tips of the iron go to ~800C similar to other soldering irons, that would be great.

Last edited by a moderator: Feb 28, 2017
2. Feb 28, 2017

### Staff: Mentor

I would approach this by researching how Weller and other standard soldering iron manufacturers do this. Yes, you should transform the AC Mains down to safe voltages, and use some suitable voltages to heat the tip. Have you seen how Weller irons use the magnetic properties of the tip to regulate the tip's temperature?

3. Feb 28, 2017

### Staff: Mentor

BTW, Nichrome wire is usually used as a heating element -- I'm not sure how that factors into your question.

4. Mar 1, 2017

### JoeSalerno

So, I looked into the magnetic method for controlling the heat, but that seemed to work for traditional heating elements. I need to create a soldering iron that would be used in "resistive soldering" where the solder bridges the gap in the circuit. This would create a lot of heat, allowing the electricity to flow through a high resistance lead such as stainless steel. Once the electricity flows, the solder heats up very quickly snd melts. I need to know how to take power from the wall and transmit it to the iron leads SAFELY. I know I could just plug it into the wall, but I would like to do this in a much safer manner. If anyone knows how much current this type of iron needs and how to transmit it from the wall to the iron, that would be awesome. Sorry if that at all came out sarcastic

5. Mar 1, 2017

### Staff: Mentor

For basic safety reasons you'll want to employ an isolation transformer to prevent shock hazards from exposed mains connections. That transformer can also reduce voltages to safe levels (something you can research).

You'll need to provide more details about the physical setup to estimate power requirements. As it stands we have no idea if you're intending to solder hair thin connections to an integrated circuit die or 000 gauge wires on a battleship.

Can you provide a sketch of a typical soldering operation in action and indicate the materials and approximate dimensions? How much power is required will depend on things like thermal masses, heat conduction, resistivities, melting temperatures, and so on.

Just for the sake of providing a datapoint, the ubiquitous Weller D550 soldering gun has two power settings, 200 and 260 Watts.

6. Mar 1, 2017

### Nidum

Resistance soldering has a specific meaning . It is where a current is passed directly through the components to be joined . Similar in principle to spot welding except that temperatures are lower and solder is used .

Used to be crude electrically - just lots of amps DC or AC .

More sophisticated systems now in use in industry use high frequency AC , pulsing and feedback control .

Last edited: Mar 2, 2017
7. Mar 1, 2017

### JoeSalerno

I looked into the isolation transformer and that seems like a great first step towards safety. As for materials and wattage, irons similar to the kind I am trying to make are in the 200-250 watt range. I planned on running regular wire from the power source and joining stainless steel wire to that with a highly temperature resistant connector. The stainless steel wires would be exposed very close to eachother sort of like tweezers. For the type of soldering being done, nothing too big or industrial. This will be mainly for home use and would work with (hopefully) atleast a few different sizes of home use solder. By including an isolation transformer at the start of the circuit, what else would be needed electronic component-wise?

8. Mar 1, 2017

9. Mar 1, 2017

### JoeSalerno

In the model that you shared the power supply was able to output a LOT of current. If that would be possible with the isolation transformer that'd be great. http://model-railroad-hobbyist.com/node/22204 This is more along the lines of what I am trying to produce. A tweezer like assembly that produces positive current on one end and negative on the other. The solder would be the bridge that completes the circuit. I see that the power supply this guy made is somewhat dangerous, so hopefully I can get some help on that end. For the most part, I'm looking to make a safe version of what he did. Making the iron isn't the problem, but if somebody knows how to provide the safe power from an isolation transformer, I'm very open to new ideas and suggestions.

10. Mar 2, 2017

### JoeSalerno

11. Mar 2, 2017

### CWatters

I think 3W would be too small unless you are joining really tiny parts. Most small soldering irons are 15W to 25W and I have one that's 150W. However they also rely on the thermal mass of the bit. I note that unit I referred to above generates >40W.

12. Mar 2, 2017

### JoeSalerno

I would like as many watts as I can get, but the issue for me is size. I'm trying for something about the size of the power brick on a laptop charger. Being able to replicate the 40W would be great great, but that transformer's simply too big. I was thinking that I could set up a circuit that goes from the wall, to a transformer, to a rectifier if necessary to get it to DC, and then through a potentiometer to make the current changeable. My big problem now is finding the right transformer. I don't care if i have to buy and scrap a charger, but I'd prefer if I could just use a transformer by itself. If anyone has a link for a transformer that would work size and wattage wise, I'd be able to start ordering and building!