Wednesday, August 3, 2011

Small scale cutters

Hello everyone!

This is a question I receive with some regularity, and figured it not only applied to small scale cutters but also to larger scale projects. The process is the same, but this is a bit more specific. The contents of this blog entry come directly from the email I replied to the user. But when I get an email from y'all regarding "How To" on these custom projects, this email exemplifies my exact thought process and how I'd tackle a project such as this. The blue text is the question from the user. I hope some of you find it useful and can use this blog entry as a stepping stone to your own cutting project! 

Remember these two things as we go through the email :
1.) Voltage from a power supply is constantly output, and the amperage pulled is dependent upon the resistance of your circuit. This is how you use ohms law (Amperage = Voltage / Resistance)
More about this on the electrical theory page

2.) The amperage rating on a power supply is the MAXIMUM safe output amperage.

I'm trying to make a small hot wire cutter that uses only 2.75" of 21awg ni-chrome wire.

21ga is 0.83 ohms per foot, making your cutter a theoretical resistance of 0.19ohms - we'll round to 0.2ohms
Keep in mind the other electrical hook ups in your cutting device will have some resistance, so purchasing or using a multimeter to test the actual resistance might help with choosing a power supply.

21g requires approximately 3-4 amps to heat to a cutting temperature (400-600°F) for most commercially available foams.

This will be important as these values help you choose a power supply

Check out the NiChrome Page for more data

I used 4-D batteries at first, thinking that this will at least get me going, and sure enough it cuts the foam. But I want a continuous power supply and stop using the batteries. Calculating this small a piece of wire has been difficult for me. I tried a DC power adapter that was putting out 6v (similar to the 4 batteries in series) and is rated with 3.3 amps. This would give me about 2 small cuts and then "cut out", no more heat..?

IF you desire battery power in the future, here is my section on battery usage

The reason I included the battery section is because the functionality of a solid state power supply (DC power adapter) are very similar to that of batteries.  Basically, the batteries have a fixed output voltage, and their output amperage is dependent upon the circuit resistance, but the true output current is limited by the battery's internal resistance (which limits the outflow of current). This is why many people can get a small cutter to work with batteries, but not with DC power supply of similar voltage.  

When you hook up a 6v power supply to your device, you need to remember that the amperage pulled from the power supply (and/or battery) is a function of resistance and voltage

Ohms Law
Amperage = Voltage / Resistance
Amperage = 6v / 0.2ohms
Amperage = 30 amps

Remembering that your power supply is only rated safely at 3.3 amps, you have effectively "short circuited" your power supply by attempting to draw 30 amps from that unit. It sounds like your power supply has a built in over current protection, or OCP. 


If your power supply did NOT have an OCP protection, it would have either burned out the internal circuitry OR if given enough time unprotected it would have probably set on fire or melted, lol

When you use batteries, they PHYSICALLY only allow so much output amperage from each battery, dependent upon a few internal factors. Granted, if you over load (short circuit) a battery it will heat up quicker and eventually die quicker as a result. But batteries basically are their OWN OCP just based on the nature of batteries, their chemical make up, and internal metals. Kinda cool.. check out that battery link for more (above).

Any suggestions on how I can make this small cutter work?

No problem at all. Basically you work ohms law backwards to determine your power supply needs OR use that calculator file on the power supply calculations page provided by Jacobs-online. But if you do the first few calculations by hand, it makes it easier to understand what you're actually calculating. 

Ohms Law
Amperage = Voltage / Resistance
using algebra
Voltage = Amperage * Resistance
Remember, your amperage is derived from your desired cutting temperature, check out the nichrome page for more data on each wire's specific requirements. 

For this set of calculations, you know you need 3-4 amps, lets just say 4 amps to make it easy

Voltage = 4a * 0.2ohms
Voltage = 0.8 volts


Voltage = 5a * 0.2ohms
Voltage = 1 volt

Generally speaking, finding a cheap 0.8v power supply is going to be tough. But I'd round to 1v just to make it easier on yourself.

Another thing to consider is that you can add a resistor to your circuit to increase the resistance and effectively allow you to use a greater range of power supplies.

The more resistance you add to your circuit, the greater the voltage requirement. That means if you add resistance, you will NOT change the amperage requirement to heat your wire, but simply a greater voltage from the power supply. Only changing your nichrome gauge or desiring a higher temperature will it affect the required amperage. So if you find a 10 volt, 5 amp power supply, you'll need to add a total of around 2 ohms but still use your cutter length of 2.75"

Lets say you WANT to use that 6v3.3a power supply you have...
Using 21ga, the 3a max might not physically heat the wire enough to cut, but bare with me for the sake of the argument. 

Ohms Law
Amperage = Voltage / Resistance
Using algebra
Resistance = Voltage / Amperage

Resistance = 6v / 3a
"They" say you should never fully use a power supply's full wattage (Volts * Amperage). That supply is about 20 watt max, and i usually drop it to about 90%, hence, 3a
Resistance = 2 ohms

So, in order to use that power supply safely and NOT activate the OCP, you need to have a 2ohm circuit. Being that you only have a 0.2ohm circuit, you need to add 1.8ohms. 

To find the resistor wattage, take joules law
Wattage = Resistance & Amperage
Wattage = 1.8o * 3a
Wattage = 5.4

You therefore require a resistor with a max of 1.8o and a minimum of 5.4 watts.

Because this is going to be tough to find, you can place a 1ohm and 0.75ohm resistor in series with each other, totaling close to 1.8, and then you add the wattage. If theyre 1ohm3w and 0.75ohm3w, they add to 6 watts which is more than enough heat dissipation. 

Check out Digikey's website, which allows you to search for resistors using a set criteria.
through-hole resistors
basically set your resistance, wattage, wire wound composition, and select "in stock" to search


You have a few options for power supplies should you want something a bit bigger and stationary

If you want a solid state (DC charger) power supply that you just plug in and go, you REALLY need a power supply that has some level of built OCP. This power supply is the ONLY one I've found on the internet yet that has it all... and believe me, it took probably 6 months of searching... and ive ONLY found it through this website

While it IS a 5v4a power supply, over-voltage AND over current protection on it. This little thing is the SHIT! I've used it on a few custom projects that I too am working on. Basically the OCP on this unit allows you to keep working with your cutter and it wont clip out or turn off because it's OCP reduces the output wattage to a safe level for the unit. From what I can figure out, it actually regulates the voltage down to create a maximum of 4amp output based on your circuit's resistance but retains the ability to reduce the current if you overload the entire power supply.

I still have to do some testing on the supply myself, but I've used it and it works for me. And for 7 bucks, it is at least worth trying out. If you happen to fry the supply, you're not out a crap ton of money. But I think you might be pleasantly surprised with this unit. IF you end up purchasing this unit, let me know how it works for you, I'd be very interested. 

If you want a variable power supply - one that allows you to vary the voltage and amperage manually, you're going to have to go with a bench top power supply. 
I have some listed on my website
I've been VERY happy with the selection at
They have the greatest selection & the lowest prices of any website ive been able to find yet. I'd probably go with a Linear Power Supply, they have a tendency to be cheaper than a Switching unit

One of the cheapest linear power supplies you'll be able to find is the 30v5a supply

Remember that with these power supplies, you can vary the voltage and maximum amperage with the dials. So I'd set the amperage to 5a, and then just gradually turn up the voltage until your wire is hot. (1-1.5v). Granted, you wont need all 30v, but it allows very low fine tuning. The other thing to consider, is should you ever want to build a larger cutter. This power supply will support very large lengths of wire (10-40 FEET depending on gauge)


Send me an email, let me know what you thought of this article? Did it help you understand how to do hand calculations? Did you learn how to retrofit a power supply? Let me know

The hot wire foam guy

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