Calculator Question

Happy Halloween!

I just wanted to post a quick question I had from a user that used the Jacob's Online Calculator, found on the website. This is a copy/pasted message from the email I sent back. His questions are in blue, my responses in black.

Question

Please help me to understand your calculator.

Here is a link to the main help page, which may answer some of your questions. 

http://hotwirefoamcutterinfo.com/Hot_Wire_Foam_Cutter_Info/JacobsHelp.html

I need the tempereture to be 160 degree celsius.
The length should be around 60cm.
The gage should be 32 because i am using it as a heater.
The diameter is 0.203 mm the volts would be 12 volts .
if I punch this on your calculator 

the result show that I need a power that is =+/- 7 Watt with the current of +/- 0.5 amps . Is this correct ?

So if I go to a shop for my particular application I need to buy a power supply that has 7 watt ,12 volts,0.5 amp.
Is this correct?

You're pretty much correct, yes.

Check out the electrical theory page to reference what I am about to discuss. 

http://hotwirefoamcutterinfo.com/Hot_Wire_Foam_Cutter_Info/__ETheory.html

Just remember that the VOLTAGE needs to be similar between what you calculated and what is on the power supply. From the power supply, the output voltage is always constant. But the amperage or current flow is a function of the resistance of your circuit. (See Ohms law).  The listed amperage on the power supply is the maximum safe output amperage for that power supply. 

Think of it this way. You need to find a power supply that is 12 volts, but has an amperage rating HIGHER than 0.5 amps. A 12v 2a power supply would work great, because you're always putting out 12 volts, but your circuit will only draw 0.5 amps and it's rated for a maximum of 2 amps.

If you reduce the length of your wire, or increase the gauge, you will reduce the resistance of your circuit. Lower resistance, when you apply the 12v from your power supply, will draw a greater current from the power supply. 

But a long-story-short, find a 12v power supply, and an amperage rating of anything higher than 0.5 amps, id probably say 1 amp or more. 

As for wattage, it is similar. The listed wattage is the maximum safe wattage output from the power supply. 

Also,

Watts = Volts * Current

...

Current = Watts / Volts

If you find a power supply that has just voltage and wattage, just divide the wattage by the voltage and you'll see the maximum safe output current. 


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Okay everyone, as promised just a quick note. Take care and don't eat all your candy at once!


-carlo