Calculations

Electrical Power

 
Power Supply Rating

But when choosing a power supply there are a few things you should consider.

It is STRONGLY recommended that you purchase a power supply that is variable voltage, rather than trying to use a steady-state fixed voltage power supply.

Do you want to be able to regulate the heat?

Or maybe you just want a quick plug and play with one basic power output?

Do you want to be able to have a switch to turn the power supply on/off, or just unplug?

Do you want to have a power supply that you can interchange between multiple cutters?

While many other combinations are possible, for the application of determining the power supply requirements the Voltage radio button need to be selected to determine this unknown value. In this scenario the desired temperature, gauge of wire, and cutting length are required.

Select the temperature you require, which should be no less than 600°F.
By changing this slider you will notice that it affects the amperage requirement at the bottom. You will also be able to find the amperage requirements on the NiChrome Data Page
Next determine the gauge of wire you are using for your cutter. Remember 24-26 gauge for typical straight line cutters, and above 20 gauge for rigid wire cutters.
By changing this slider you will notice that it affects the amperage requirement and ohms per foot at the bottom
Establish your cutting length
If you have your cutter built but do not have NiChrome on hand, you may use string as a mock-up to determine approximate length. 
If the calculator slider does not provide the length you require, check the radio buttons at the bottom left to change the range

With these three variables selected, the calculator will provide you with your power supply voltage and amperage requirements. Considering other components to your hot wire cutter, you may want to alter your power supply requirements by +/- 5%.

My recommendation is that you purchase a variable voltage power supply with an amperage rating no less than 20% greater than what is calculated. This should ensure that your power supply can handle the current your circuit will draw. Also, being a variable voltage power supply you can specify the exact voltage calculated for your circuit. 

When you use a steady-state fixed voltage power supply you will need to keep a very close eye on your cutting length and/or circuit resistance. But you can visit the Battery Power Section for more discussion on this topic. 
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In this section I will be referencing an online calculator used to help determine the values of a necessary power supply for any given NiChrome Wire circuit. At the bottom of the page you can learn how to manually calculate your requirements, but this online calculator is SO fantastic it is all you need!!

If needed, this calculator will help you determine the nichrome wire size, length and power supply requirements for your specific application. But if you’ve taken my recommendations on the Introduction Page should already know the wire size but maybe not the length.

When using this calculator, you will need to select the radio button to correspond with the variable you wish to calculate.  Then, move the sliders on each of the other known variables to complete this calculation.  The location of the slider on the variable you are solving for, will have no bearing on its calculated value.

Basically, alter three values and it calculates the selected fourth.

You will also notice on the bottom bar, the calculator will determine your amperage & wattage requirements, in addition to theoretical circuit resistance and wire ohms per foot of the wire gauge you’ve chosen.

Because of the resolution of the sliders, you may not be able to get the exact number you desire. Set the sliders to the closest number and your results will be within the tolerance of the experimental values which are around +/- 5%.

Your power supply can be determined by the watts shown and the amps. You should choose a power supply that has an amperage requirement 20% larger to provide a good safety factor and prevent damage to your power supply. While this may not always be necessary, just monitor your power supply to make sure it doesn’t get too hot while cutting.

The output current rating (Amps or Miliamps) and total watts (or VA) are the power supply specifications that should be considered and cross referenced for this purpose. For more discussion regarding this topic, please see our Electrical Theory Section for additional information.

Please visit this help page for more explanation of this calculator.

Determining Power

Supply Requirements

First, understand your basic electrical circuit.

A DC power supply will be placed in series with your nichrome wire.

It will be important to note that the difference between series & parallel in case you choose to add more switches to your circuit. But you may visit the Resistance Section to learn more on this subject!

If you have not yet built your device, then you should estimate your cutting wire length and cross reference the NiChrome Wire Data Sheets.

If you have built your device, it is best to attach a multimeter to physicall measure the over all resistance.

STEP 1

Before you decide to determine the requirements of your power supply, it is absolutely PARAMOUNT that you know the resistance of your circuit!

STEP 2

With your now known resistance, calculate the Voltage Requirement (Vp)

Follow these steps if you intend on manually calculating your electrical circuit properties as well as requirements for your power supply.


Visit the Resistance Calculation Page and the Circuit Theory Page for more a more in depth discussion.

MANUAL

CALCULATIONS

Click the image for a downloadable .pdf file.

Here is a much larger chart that has a whole range of NiChrome Wire gauges as well as the amperage rating for desired temperatures and the associated voltage requirements for defined lengths of wire.

This should get you close, if you want a quick’n’dirty guide for choosing a power supply for a variety of gauges, lengths and temperatures.

Summary
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This tutorial is a walk through for all the information on the remainder of this page.

Power Supply Calculations