Tuesday, January 13, 2015

Plug Power Supplies

I can't believe I've never posted this question before... I receive this email probably once a week and finally realized I don't have the response written anywhere... Arg...

Almost everyone who emails me wants to use a salvaged plug power supply (cell phone charger, small electronic power supply, etc). The problem with plug supplies is that they only output ONE voltage and their amperage is often capped rather low (usually 1.0-1.5 amps). When used with nichrome wire to cut foam, most will either clip off or burn up as the unit overheats from over current (short circuiting).

If you're like most of us, you're building your own hot wire foam cutters to save money. While you can save a ridiculous amount of money by making hand cutters and table cutters for both straight and bendable wire cutting, I highly recommend spending a little money on a good bench top power supply like this one (link). It will last you darn near forever and can be used for almost every hobby foam cutter.

If you want to save additional money on a power supply you can build your own from scratch. Check out my DIY Power Supply which uses a step down AC transformer from Jacobs-Online. And no, a door bell transformer really won't work either, unless you want to frustrate yourself.

Between the Jacobs' step down transformer or the Circuit Specialists cheap bench top supply, you'll be purchasing or making a power supply well beyond that of commercially available hot wire foam cutter power supplies at a fraction of the cost.

Check out the email exchange below, and I'll post as additional replies come along.


Friend, what am I doing wrong?

After watching a number of Youtube videos on building foam cutters and  reading several different blog articles, I built the table in the "Full Table".  

I purchased the Nichrome 60, 26 gage wire from Jacobs, and soldered the bottom to the transformer cable (Bottom connection photo), and since I want to cut interior holes in foam, I made a "soldered hook" out of the other end of the transformer cable (Top connection photo) and a loop in the Nichrome to attach it (spring tension tight--Top connection photo).

I used an old transformer that I had that is rated 12 volts (as recommended--see photos), but when turned on, it does not heat the wire.  

It is possible that the top loop connection is not completing the circuit, but I suspect the problem is that the transformer is putting out only 1250mA LPS.

The cable is only 8 inches long.  

What do I need to heat it to cutting temperature?


My pleasure to help, and having worked people through this exact problem multiple times, I see your issue straight away.

Long story short, your problem is not in the design nor construction of your table top cutter. It looks like you have effectively isolated your circuit and it does not appear to be grounding anywhere. (I couldn’t tell the exact connection of the lower attachment on the eye hook, but I’m assuming you have the connection wire soldered in place too.)

The issue lies within your power supply but first here is a summary of your circuit ::
26 gauge NiCr Wire - 2.67 ohms per foot
8” nichrome length = 1.78 ohms
Output Voltage = 12 volts
Max Power Supply Output Amperage = 1.25 amps

First off, I usually tell people that while 400°F can cut foam but I recommend shooting for 600-800°F just to make it a fast cut. For 26ga NiCr Wire, you will need 2.1 - 2.6 amps to reach 600-800°F respectively.

Secondly, you need to keep in mind that plug supplies like you have output a CONSTANT 12 volts, and the amperage is a function of your circuits resistance. Meaning.. You are applying 12 volts to your 1.78 ohm circuit.. and with Ohms law you can calculate that yields 6.74 amps!

Your power supply can only output 1.25amps (or 1250 mA), and cannot handle 6.74 amps. The supply likely has built in over-current-protection or OCP which I discuss here (link). Chances are the power supply is saving itself and simply shutting off, not allowing any current to flow through it.  Regardless, even if it was allowed the 1.25 amps to flow, that wouldn’t be enough heat to cut.

On the Introduction page, at the very bottom of the page I have a quick reference cheat sheet for gauges, amperages, and lengths. It’s a quick’n’dirty scale to let you know if your power supply can match up.

Also, check out our Power Supply Calculation Page which also has a walk-through video on how to calculate your power supply requirements for your project. 

For this cutter, you will be best served by using a variable voltage power supply or bench top power supply. That way you can adjust/fine tune the voltage to achieve an optimal cutting temperature for your application. With you’re current set up, you’re going to need around 3.8-4.6 volts, which will draw 2.1-2.6 amps from your power supply. Granted, you might want your wire hotter than that though as these theoretical calculations do not take into account heat lost to the environment, additional resistance from your electrical attachment, nor heat consumed during cutting. 

It is more likely that you’ll require upwards of 6 volts and 3.4 amps. 

Circuit Specialists have a very cheap bench top power supply. It is a 30 volt 10 amp (link), variable voltage power supply. You honestly can’t get this kind of power for that little price. This supply also allows you to manipulate either voltage or amperage, which is great. But remember, power supplies ONLY apply voltage to a circuit and then draws amperage based on your circuit’s resistance. So by “controlling” amperage, you are setting what your desired output amperage is and the machine is calculating your necessary voltage to achieve that amperage based on your circuit. So don’t let that dial confuse you. But the supply is a really nice, great functions, and for such a cheap price (comparable models for 2-3 times that).

If you want to build your own power supply, check out this page (link). To answer a question ahead of time, a door bell transformer will not work for you either. 

So I hope that explained everything for you! Let me know if you have any additional questions. 

Thursday, September 4, 2014

Power Supply Convo...

I guess I'm not sure if I should keep posting these emails or not.. The questions seem to be common and I get them regularly. And I suppose not everyone reads the blog or news section, lol... BUT maybe it will help someone searching the web if the happen upon it. SO, here's another PS question.

We need to cut PU Foam having a density of 6 to 7 kg/cu.mtr by using a straight
hot wire cutter.
As per suggestions, we have worked out to use  a  26 gauge NiCr wire of 2 feet length for a straight wire cutter using a power supply of 12 volt 2 amp.

I think you should anticipate at least 4 amps.. The 2 amp mark is really a minimum when it comes to cutting foam. Also, try to find a power supply that allows you to adjust the voltage.

When you start to cut foam, heat exchanges from the wire to your substrate (PUFoam). As the wire cools, it takes time to heat back up to a cutting temperature. In most cases what happens is your cutting speed slows dramatically, and most often will lead to wire breakage as people push harder to cut faster. 

With 3-4 amps you have a hotter wire, that can replenish heat faster, and cut faster without compromising your wire. Ideally the substrate (PUFoam) never actually touches the wire, it is the heat radiating around the wire (the kerf) that actually cuts the foam. 

Please let us know if we need output voltage as 12v DC or 12v AC.

AC versus DC, doesn’t matter. It is just important that you have enough current. Try to find a 24v4a variable power supply, and you should be all set!

Depending on where you’re at in the world, this power supply should suit all your needs and is rated for international use. 

Thanks for your guidance and are sure that will positively be of immense use to me.

I would like to tell you the exact use of hot wire foam cutter which I want to use for commercial
purposes to cut low density (ranging  from 6 to 7 kg/cu.mtr) PU Foam sheets of thickness varying from 2.5 mm to 6 mm stacked to a height of about 18 inches (i.e. say 100 sheets of 4.50 mm piled over one another) and cutting to smaller size sheets of size 8 X 18 inches from bigger sheets of size 32 x 72 inches.

So please suggest what type of nichrome wire and power supply would be required for a straight hot wire foam cutter so that large volumes of the above sheets can be cut.

It looks like you’re cutting wire will need to be 36” wide to make a comfortable cut. You always want to give a little extra cutting line to make it easier on yourself, but should be able to get everything to line up if you have a standard cutting jig or a stop-board. But I’m sure you can figure that out. 

Polyurethane foam is a little funny when it comes to cutting. Because it is an open cell foam, the little pockets of air between the foam matrix will actually suck up a little heat during your cut. Interestingly enough, the air acts like a little insulator during the cutting process and actually slows things down further.

At cutting lengths over 24” I tell people it is a good idea to go with 24 gauge nichrome wire. It is a little more stiff and is a little stronger than 26 gauge wire. Plus, the voltage/amperage requirements of 24 gauge is a little easier to deal with than 26 gauge nichrome wire. 

One draw back with nichrome wire at longer distances is that it may be hard to control the wire expansion and line “sag.” I’ve had some wavy cuts with larger cutters, but you can help prevent this by making the wire quite tight with a spring and use a higher temperature. A higher temperature helps less PU foam touch the wire during cutting, and the tight spring helps prevent line sag.

One other thought would be for you to use/try Rene Wire (link). Rene is sort of a “top of the line” cutting wire. It is highly resistant to breaking, won’t have as much line sag, and cuts well for both hobby and industrial purposes. For power supply calculations, 27 gauge Rene wire is similar to 28 gauge Nichrome wire.

If you want to make your own power supply, I would recommend the XFR3018E transformer from Jacobs-online. You’ll have to have a bit of electrical ability, and wire in a dimmer switch. I have a walk-through tutorial where I use a different transformer, but the images are essentially the same. 

Saturday, August 30, 2014


Just another question on AC or DC power supplies. Seems to be common.

when we say power supply requirement is 12 volt 2 amp .does this mean that we need a power supply to convert 230 volt ac supply to 12 volt dc or ac.

For Nichrome Wire, it doesn’t matter if it is AC or DC. You just want to make sure your power supply has enough power (wattage) to handle the circuit’s current (amperage). I usually tell people that 2 amp is kind of your minimum. If you can find a 4 amp power supply you’ll be able to have a wider range of wire temperatures. 

Also, remember that the voltage is constant and amperage is a function of your wires resistance. So depending on your circuit, and depending on your voltage setting, you may only require 2.5 amps to heat rather than the full power supply’s 4 amps… Just remember voltage is constant, amperage is variable based on wire/circuit resistance

Monday, August 25, 2014

Kanthal Wire

I am interested to know whether it is possible to use round Kanthal 0.32mm wire as opposed to Nichrome wire in a HWFC?

I don’t see why not? My experience is in Nichrome wire and a little in stainless steel. But when building a simple cutter, you only really need a resistance wire and the alloy shouldn’t really matter. 

I have used and recommend NiChrome because its electrical properties are known and easy to calculate. Most of the other alloys have resistance information but not necessarily amperage-to-heat tables. 

Using Ohms law, with a given wire’s resistance you should be able to calculate an amperage based on applied voltage. So you can see if your power supply has enough juice to handle the circuit. 

Aside from that, I did find a website that has some reference data ( here ).  Whenever people ask about novel wire or applications, I always encourage them to give it a try knowing that you can always fall back on nichrome if you need to!

Thank for the quick reply. I have done a lot of research in constructing a HWFC's, and of all the websites and YT videos no one seems to have tried Kanthal. So being a bit of an experimenter, I have ordered some Kanthal from a supplier on eBay.
 Also I would like to use a 12v 6a supply apapter and regulate the dc output (as opposed to regulating  the ac input) with a high amperage dimmer switch that I also saw on eBay.... similar to eBay item number  191295309081 
Thanks for the the info from your website, it has been most useful.
Perhaps I will make a video of the HWFC I build and post it on YT.
Yes, I will let you know the outcome of using Kanthal when I finish constructing my HWFC.

All that sounds great, I don’t have any additional input just some thoughts.  

In general, you never really want to dim the primary side (AC input) of a transformer. Especially if it is an AC-to-DC power supply. It can have unreliable but also potentially adverse effects on the electrical set up. Even with your AC step down transformers (AC-AC) you want to dim the secondary windings (output side) of the power supply. 

As for that DC dimmer, that is cheap! I’ll have to save that information. I had a user email me some information on a DC dimmer, but they were hard to find. I don’t know why I never thought of a LED strip dimmer…! Thanks for the information. 

I think your set up is going to work well. Having the 6a output on your power supply should be more than enough to amperage to power almost any circuit. You’ll just have to watch out for your circuit’s overall resistance though, as the 12v might not be enough pressure gradient to push current though high resistance circuits. But as always, give it a crack and see if it works!

I read your email with great interest - thanks for sending it.
I received the Kanthal wire yesterday. I got the 0.32mm 28 AWG Kanthal A1 gauge wire.
From this website:
It states that Kanthal A1 for 28 AWG is 0.439 ohms per inch. Now, if I construct a HWFC with about a 24 inch length of wire this should give a resistance of 10.536 ohms (24in x 0.439 ohms). Which if I use a 12v supply the wire should draw (presumably) a maximum of 1.14 Amps (12v ÷ 10.536 ohms) and also presumably the wire should be glowing. Please let me know if my calculations are incorrect or I've overlooked somthing. :)

I am still waiting for the dimmer switch and adaptor, hopefully these should arrive soon then I can start construction.
I'll let you know what happens...... success or failure!! :D

All your calculations look correct to me, I’m just not sure about the amperage-to-heat ratio. In the world of NiChrome 1.14 amps really isn’t that much at all. But, if a low amperage like 1.14 can heat up Kanthal enough to glow… it will be a great low amperage alternative to NiChrome. Now you’ve peaked my interest, lol, let me know how it goes.

Thursday, June 12, 2014

DIY Power Supply

Hey All -

Every few weeks or so, I'll get an email asking about the DIY power supply video and information I provide on my website (here).

When Jacobs-Online originally sold their step down transformers, they only had a single input on the primary windings. A few months back (maybe a year), they changed the primary windings to allow for both 120 and 220v input. Thus making the power supplies rated for international use.

My video only has a walk through for the old style transformer, but the website has all the diagrams necessary to wire the transformer into either 120v input or 220v input.

I am currently in the process of updating my video to account for this change, which will also include all the new diagrams I have posted on the DIY diagram page.

I hope this clarifies any questions, although I have a feeling I'll still receive emails until I get the video done... :P

Saturday, April 19, 2014

Cold wire...

I recently purchased a Mastech Hy30005D power supply for a Hotwire foam cutter. This unit is working well for lighter 27 gauge wire.  However, I need to use heavier 20 gauge wire for detailed shapes, and this unit seems to be under powered for the task. Could you please guide me in a selection?  Funds are by no means unlimited, but a need a reliable unit that can provide ample heat to cut at a good pace.,

Just for reference, I'm sure this is the unit you purchased? Link

This unit has a max output of 5amps, which is going to be the source of your problems. 

Let's quick reference the nichrome data chart found on my website - Link

As you'll note, for 20 gauge NiChrome wire, 5 amps will generate just shy of 600°F. But for 27 gauge wire, 5 amps will generate just shy of 1800°F!

The thing that I've found is wire temperature and cutting speed are related, but often two different things… 

Most styrofoam products will melt at around 400-600°F, so theoretically a 600°F cutting line should suffice for cutting purposes. But remember, heat has to LEAVE your wire, and enter the sytrofoam to melt and cut the substrate. So your wire has to heat BACK UP in order to cut the next section of foam. Often what people will notice is when the wire FIRST touches the styrofoam it will cut REALLY well, then it will slow down as you get more into the cut.  This is because the heat has left your wire, and the wire is having a hard time heating back up to match your cutting speed. 

The fix is that you need to send more current through your wire. You need to have a hotter over-all wire, to increase your cutting speed. The heating-cooling-heating process will find a happy medium, you just have to find out what that is…

For 20 gauge nichrome, 5 amps will work.. albeit a little slow. For some people, considering the cost, a little slower cut might be acceptable if you have a working product as a trade off.  

From a power supply, I think you would want to have the ability to increase your amperage to 7-8 amps, just so you can play around with your final cutting speed. Here are a few options
30v10a - Link - $94
30v10a - Link - $170
18v10a - Link - $140

Another option would be to select a variac, which is an AC transformer… higher voltage, so be cautious during use
130v20a - Link - $109
130v30a - Link - $134

For the money, I'd select the $94 unit from circuit specialists. It has both a higher voltage (good for small wire) and a higher amperage (good for large wire). 

Friday, April 18, 2014

Tattoo Power Supply Freak Out

I am building a hotwire table. I'm using my tattoo power supply as a source. It's output is fine (3-15-V Dc).Even the clip cord reads the same . When I connect it to the table my voltage goes crazy! So the problem has to be in my table components. Is a "stove" Bolt necessary? Or will any bolt do?

Any bolt will do fine, but I agree the problem is not necessarily with your power supply.

So… let's see if I can keep this simple. 

Any time a power supply is placed under an electrical load (your cutter) you will see a small voltage drop at any attached reader. This is a normal result of electrical resistance, current output, etc. 

But your problem is MOST likely related to the maximum output amperage/wattage of your power supply. If the numbers are going all crazy once you hook up your supply, then I'd wager a guess you're "clipping" out the power supply as an over current protection. Or, if there is no OCP, then it could just be feeding back, overloading your power supply and it's shorting out. 

Here is a document I have on OCP - Link

While you mentioned your power supply is 3-15v DC, that is not the most important part. You need to determine the maximum safe output amperage and/or the max output wattage. THAT is what tells you the capacity of your power supply. In MOST circumstances tattoo power supplies max out around 2.0 - 2.5 amps, but usually 2.0. Some are only 1.5 amps. The issue is that most hot wire foam cutters, depending on what wire gauge you use, require about 2.5-3.5 amps to heat effectively.

Relating all this back to your cutter.

Your cutter has a defined resistance, and when an electrical pressure (voltage) is placed on that circuit (cutter) it will generate a current (amperage). If your resistance is too low, and your voltage is too high, it will allow too much current to flow; and that flowing current may be higher than what your power supply can handle. 

Your solution is to use a smaller gauge nichrome wire (which has a higher resistance) or include a resistor in your circuit. I have a tutorial video on making your own resistor out of aluminum wire. 

Check out my Electrical Calculations Videos ( link ) which should be a really good walk through from beginning to end.