Hello, I'm trying to make a hot wire cutter but the wire isn't getting hot at all. I've tried using several plug in power adaptors (outputs: 5vdc/1amp, 12vdc/2amp, 12vdc/3.52amp). I am using a .011 guitar string that is 3" long. When I put a meter on it I'm getting 12 vdc but the amperage is jumping around - it never stays consistent so I'm guessing this is why. This particular power adapter is the 12vdc/3.52amp taken from an old laptop. I'm not sure what I am doing wrong here since all my research suggests the 2+ amps should work.

 

I forgot to add that I was able to get the wire hot by using my car battery charger set to 2 amps. I'm not sure why this worked but not the power adaptors that put out the same voltage.

 

It sounds like your power supplies do not have a high enough current rating. I would expect you need way more that 2 Amps.

 

It sounds like your power supplies do not have a high enough current rating. I would expect you need way more that 2 Amps.

Hello dendad, thanks for your reply. That does make sense but how come the car battery charger set to 2 amps works?

 

Did you measure the battery charger current? It may be supplying greater than 2Amps. First thing you need to do is find the actual current needed to heat the wire. Do you have a variable power supply that you can test it with?

 

A switch mode power supply will go into self-defense mode when overloaded. If the current demand is too great, it will reduce output voltage and even shut down completely.

Your "all over the place" readings may be the result of the startup surge current required. When the wire is cool, its resistance is at its lowest, requiring the most current until it heats up (resistance increases, so current demand drops). A linear power supply can handle the initial surge current. A switching power supply will shut down (briefly) if the surge current is too high, then try again. And again. And again....

This phenomenon is often seen when attempting to operate a motor from too small a supply. The motor starts to spin up, then shuts down before reaching speed.

 

Did you measure the battery charger current? It may be supplying greater than 2Amps. First thing you need to do is find the actual current needed to heat the wire. Do you have a variable power supply that you can test it with?

I never verified the amps coming out of the car charger. I'll do this tomorrow. That would help explain things though.

 

A switch mode power supply will go into self-defense mode when overloaded. If the current demand is too great, it will reduce output voltage and even shut down completely.

Your "all over the place" readings may be the result of the startup surge current required. When the wire is cool, its resistance is at its lowest, requiring the most current until it heats up (resistance increases, so current demand drops). A linear power supply can handle the initial surge current. A switching power supply will shut down (briefly) if the surge current is too high, then try again. And again. And again....

This phenomenon is often seen when attempting to operate a motor from too small a supply. The motor starts to spin up, then shuts down before reaching speed.

The first two power adapters (5vdc/1amp, 12vdc/2amp) were "switch adaptors". The laptop one could be but it does not say it on it. Can I use the car charger without killing it?

 

A laptop charger is almost certainly switch-mode.

The car battery charger should be fine –:they're built to take abuse.

 

Variac, and a microwave transformer converted into high current. I can drive a "short" to 800+ amps.
Using some math and physics you can calculate the watts (and amps) needed to get the wire to proper temp.

 

I agree with the Microwave-Oven-Transformer ( MOT ),
with a custom-wound-Secondary, ( very easy to do ),
and a simple Current-Regulator-Circuit.

This will give bullet-proof-service, and will provide consistent and predictable performance.

A "Store-Bought" Toroidal-Transformer would be icing-on-the-cake.
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A piece of steel wire 3" inches long will have a very low resistance, it will require a high current at low voltage to heat it - this is not going to match up well with any readily available power supply.

The suggestion of hacking a microwave oven transformer is a good start, you can adjust the number of turns to get the perfect heat output.
You would start by destroying and removing the (lethal) secondary winding, replace it with a few turns of #12 insulated wire and try it.

Add a Variac to the input of the transformer to adjust the power level if you can afford one.

 

Back in my foam cutting days I used an old (old!*) four station variable output model train transformer as the power supply. It weighed at least 15 lbs. The "cutting wire" was nichrome, about 7 inches long, and fairly thin -- I'm guessing around 24 ga.

*That thing was an antique then, probably worth some real money these days...

 

To everybody recommending a custom-wound microwave oven transformer with orvwithout variac, please go back and read post #2. The foam cutter works when powered by car battery charger set to two amps. This means that at perhaps 15 -16 volts or less at around 2 amps is sufficient to make the foam cutter work.

A more stout 12v or 15v switch mode or a beefy linear supply or even a 12VAC output transformer should do the trick.

 

Measure the voltage across your cutter wire when powered by the battery charger. I think you will be surprised. The difference is that it is trying to control current, while the other supplies are trying to control voltage.

 

Seldom is a Hot-Wire-Foam-Cutter used with a single Wire-Gauge, or Wire-Length.
Therefore, a large Transformer will most likely be the best all-around choice,
since it can accommodate virtually any Wire-Length or Gauge that might be used in the future.

Computer-Power-Supplies are just not gonna cut it, ( pun intended ).
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Seldom is a Hot-Wire-Foam-Cutter used with a single Wire-Gauge, or Wire-Length.
Therefore, a large Transformer will most likely be the best all-around choice,
since it can accommodate virtually any Wire-Length or Gauge that might be used in the future.

Computer-Power-Supplies are just not gonna cut it, ( pun intended ).
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Allow a little in the budget for a fan to blow away the fumes or better yet, vacuum them away. They are not good stuff. "Outdoors" with a modest breeze works too; just position the work "downwind".

 

I agree with the Microwave-Oven-Transformer ( MOT ),
with a custom-wound-Secondary, ( very easy to do ),

I did a MOT project a few years ago, wound a secondary with just two turns of #0 or 00 (forget) fine-strand low-V cable and then put some lugs on their ends. Cooking things is easy.

To everybody recommending a custom-wound microwave oven transformer with or without variac, please go back and read post #2. The foam cutter works when powered by car battery charger set to two amps. This means that at perhaps 15 -16 volts or less at around 2 amps is sufficient to make the foam cutter work.

A more stout 12v or 15v switch mode or a beefy linear supply or even a 12VAC output transformer should do the trick.

If it's 6ohm wire, 12v, that's just 24watts. My bench power supply is rated 30v 5A, it's a basic linear power supply, so any basic ~150w supply should work as long as it's rated at least 2amp.

I was just noting variac and mot, because it's a very useful tool.

 

I did a MOT project a few years ago, wound a secondary with just two turns of #0 or 00 (forget) fine-strand low-V cable and then put some lugs on their ends. Cooking things is easy.


If it's 6ohm wire, 12v, that's just 24watts. My bench power supply is rated 30v 5A, it's a basic linear power supply, so any basic ~150w supply should work as long as it's rated at least 2amp.

I was just noting variac and mot, because it's a very useful tool.

Where did you get 6 Ohms? that sounds rather high for 3 inches of a guitar string.

 

3" of 0.011" steel wire is aprox 0.825 ohms.

2 amps provides 3.3 Watts of heating power output.
That's at 1.65 volts.

Of course the resistance changes a bunch with temperature.

Long story short, it's going to take more than 3.3 watts to make a decent hot wire cutter.

From what I have seen, you should plan for about 30 watts per foot as a starting point.