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.