# [SOLVED] Need help with simple project using resistive wire



## stevehammy (Mar 7, 2013)

I am designing a device that works on the same principal as a soldering gun, but it is much smaller in size and power. I would like it to be powered by a plug-in dc power supply (wall wart).

The purpose of the device is to melt holes through thin plastic material. Basically it is a small loop of resitive wire on the end of a tube about the size of a pencil, which, when dc voltage is applied will cause the wire to glow red hot. The resistive wire is 29 gauge, 1 inch in length overall, and has a resistance of about 4 ohms.

I experimented with AA batteries and found that 5 volts is about right to make the loop of wire glow red hot. The voltage on the finished product needs to be adjustable from zero to full power by the user in order to fine tune the temperature of the wire. Also there needs to be a couple of LEDs which are are always at full power, and an on/off switch. So I envision this to be a box with a jack for dc input from the plug in adapter, a dial to adjust the temperature, an output jack for the resistive wire, two LEDs on top and an on/off switch.

My questions are:
-What voltage and amperage rating should the wall wart be?
-What is the best way to provide adjustment of the resistive wire from zero to full power.
-Will the two LEDs be a problem - can they run on 5 volts?

Bit of a newbie here... thanks for reading and I greatly appreciate any help.
Steve


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## octaneman (May 13, 2009)

*Re: Need help with simple project using resistive wire*

Hi stevehammy 

Building your project will require you to make a few calculations first before you put things together. 

Looking at your design specs you are applying 5V D/C to a 4 ohm wire to get the result you want. By applying ohms law (V= I R) the current running through the wire is 1.25 amps and the power will be at 6.25 watts ( P= V*I). This is just a rough estimate in selecting the wall wart, you still need to add on the rest of your project's power requirements before making a final determination. 


In your design you indicated that you also want a potentiometer, a switch and some LED's. LED's come in many shapes, sizes, colors and ratings. A typical led runs on 3.2 volts @ 20mA. Setting up your LED's in series or parallel will play a crucial role in your overall power requirements, so you have to decide how want to set up your array. 

There are plenty of calculators on line that will help you. LED series parallel array wizard or you can just download one from here: http://www.techsupportforum.com/forums/f278/technicians-tool-box-557191.html

Note : You will need to consider adding a voltage regulator (LM7805) to your design and some capacitors to keep the power output stable. 

To adjust the resistive requirements from zero to full power is done through a potentiometer. But, selecting the right one for your project will depend on the size of your enclosure and the total current running through your circuit.


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## stevehammy (Mar 7, 2013)

Thanks for the helpful comment and links to the LED calculator Octaneman. I ended up using an LM2596 DC BUCK Module with a 6V 2.5 amp wall wart. So far it is working out well.


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## gcavan (Aug 13, 2009)

I have to question your original premise. Do you actually require the wire to be "glowing hot". Few plastics require temperatures over 150°C to melt, and many melt below 120°. Using a high-temp tool such as you are proposing will burn its way through the work instead of melting it, and in doing so emit some very noxious fumes.


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