Laptop Power Supply

Hello again, second posting for the day.... continuing my email purge.

Hopefully someone will find this useful

I'll be in Orange, the user will be in Blue. This posting starts just after the received email. 

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Two years ago I built a foam cutter out of a laptop power supply that is still in use today at my old school.

That leads me to my next question.  I am attempting to build a more permanent and robust version and I have come across some new materials that I would be grateful if you could offer me some advice.

I have an older power supply from a computer which tells me it can put out several different supplies depending on the connectors I choose.  Before I give you that info, I also want to try to use copper wire from an old IDE cable which I believe to be 30ga.and will run about 12 inches.

No no no, don't use copper. Copper wire has very little resistance and will short circuit your power supply. You're going to want to stick with NiChrome Wire or Stainless Steel wire (fishing leader wire). Some people will use guitar strings, but I almost exclusively suggest NiChrome Wire. 

Jacobs Online ( Link ) has so many different options for nichrome wire and he is VERY reasonable on prices. I usually suggest 24 gauge for straight wire cutters, and 20 gauge for rigid wire (bendable) cutters.

Anything smaller than 26 gauge won't really be able to withstand the force of cutting foam. Anything larger than 18 gauge is too big and unnecessary.

The power supply says I can do 5v @ 15A, 12v @ 12A, 5vfp @ 2A, and 3.3v @ 10A.  I was thinking about using the 12v @12A because that is pretty similar to what I was running off the laptop power which was 18v at about 3A with NiCr 40ga. at about 10 inches.

The 5v 15a terminal can be used if you want to have a rigid wire cutter using 18 or 20 gauge. With large gauge wire you want low voltage and high amperage.

For your standard table top cutter or straight wire hand held cutter go with the 12v terminal as long as your length is over 10-12". If your cutter is less than 6" go with the 5v terminal. 

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The user ended up using a different power supply, and here is his response.

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I used an old wall wart transformer that was 15v @ 500mA and I'm using a 9" - 38 gauge nichrome.  And, so far, it's holding up quite well.  I then wanted to try slightly higher production (thin foam slices) and so I strung up a total of 4 wires, and now after 5 passes, the transformer is heating up a little bit.  If I wanted to to more wires, that would equal less resistance, right?  So, that means I would need a higher amperage transformer, correct?  Do you think I should just hook up my 350w power supply and see what happens?

I'm pretty much only doing thin foam slicing for now.  However, because of the amount of donated foam I'm getting, I was thing of building a jigsaw like cutter for the art teacher in my building.  Or, if I could run two cutters off of one power supply, that would even be better yet.

Let me know your thoughts.  I'm not afraid to get a little crazy.  Already popped the breaker a couple times when I tried to wire up a switch and pilot light.  Still can't seem to get the pilot to stay lit while the cutter is on...

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I used an old wall wart transformer that was 15v @ 500mA and I'm using a 9" - 38 gauge nichrome.  And, so far, it's holding up quite well.  I then wanted to try slightly higher production (thin foam slices) and so I strung up a total of 4 wires, and now after 5 passes, the transformer is heating up a little bit.  If I wanted to to more wires, that would equal less resistance, right? 

Sort of… It depends on how you have your circuit arranged. Check out the Resistance Page ( Link ), but more specifically the Circuit Theory Page ( Link ). With multiple wire foam cutters, you have to determine whether you're attaching your wires in series or parallel. The circuit theory page discusses the effects series/parallel arrangement has on resistance, voltage and amperage. 

Long story short, yea, if you're wires are in parallel it dramatically reduces the resistance of your overall circuit. That will in turn draw an incredible amount of amperage from your power supply UNLESS you find out a way to reduce voltage. You can always put your cutter wires in parallel, but then toss a resistor in series to reduce the current flow. 

If they're placed in series, it basically just mimics one long wire and one additive resistance. SO you'll have to boost voltage to achieve a cutting temperature. 

So, that means I would need a higher amperage transformer, correct?  Do you think I should just hook up my 350w power supply and see what happens?

The 0.5amp (500mA) max output of your 15v power supply really won't be able to handle much of a load, so definitely, use your 350w power supply. Again, you may only have to use the 5v terminal if in parallel, but the 12v terminal if in series. 

I'm pretty much only doing thin foam slicing for now.  However, because of the amount of donated foam I'm getting, I was thing of building a jigsaw like cutter for the art teacher in my building.  Or, if I could run two cutters off of one power supply, that would even be better yet.

Definitely. Just think of each cutter as their own resistance circuit. So two FULL cutters placed in series or two FULL cutters placed in parallel. The same electrical properties apply as if you were using two wires in series/parallel.

Let me know your thoughts.  I'm not afraid to get a little crazy.  Already popped the breaker a couple times when I tried to wire up a switch and pilot light.  Still can't seem to get the pilot to stay lit while the cutter is on…

Lol, thats weird… Check out my DIY power supply diagrams ( Link ). Towards the middle of the page, you can see how you're supposed to wire a switch & pilot light with your power supply/cutter. The idea is that the switch controls the electrical flow to both your power supply and the pilot light with one throw. So you just have to split the hot wire after it leaves the switch. But that's all I got for you on that… I can't imagine why you're popping a fuse unless you've accidentally crossed a hot and neutral return - that would cause the electrical flow to short circuit to your breaker box every time you throw the switch.