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Old Sep 04, 2012, 11:30 AM
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richie967's Avatar
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Hot Wire Foam cutter power supply

Hi All, looking at building a hot wire foam cutter to cut 1 meter wing panels. I do not want to build my own power supply and would prefer to use something 'off the shelf' as i'm not too confident in my electrical ability.

I have found this

Fusion PS200 Adj Power Supply [FS-PS200ADJ] 40.84

The PS200 Adjustable is a regulated 5~15V power supply with current limiting from 0~15A. With an LCD readout showing the voltage and current being supplied, it can also be used to charge Lead Acid batteries.

Features
Input Voltage: 220~240V AC
Output Voltage: 5~15V DC Regulated
Current Rating: 0~15 Amps
Output Connections: 4mm Banana Sockets / Clamps

Will it be okay to use? I have used the foam calculator and have seen that 12v and 3A is needed for this length, but the advantage of voltage and amps adjustment is that i could use the power supply for different size cutters if needed. Do you think this power supply would work, its quite cheap @ ~ 40 delivered.

I have noticed it could output 15v @ 15A, is this a bit dangerous? Ideally i would like one that is quite safe if i turn the dials the wrong way by accident!

And does anyone use any type of fuse to prevent melting the internals of the power supply?

planning on using guitar string top-e as advised,
Any help greatly appreciated

R
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Old Sep 04, 2012, 01:19 PM
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Hi
I have just used my power supply to cut my wings i used for infomation a watt meter in line so i could see how much power was required.
75 watts for a 39" bow cuts a treat
Iain
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Old Sep 04, 2012, 03:59 PM
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Here is a thread that might help with some of your questions....
http://www.rcgroups.com/forums/showthread.php?t=1625684
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Old Sep 05, 2012, 10:47 AM
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I use the wire cutting PS feature of my iCharger 106. Works great! Also works as a pretty good battery charger too
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Old Sep 05, 2012, 01:42 PM
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I was wonder if this would work as a hot wire cutter.

I recently upgraded the powersupply and graphics in my desktop and converted the old 300W powersupply to a bench top supply. I can supply 300w of power at 5V, 12V +, and 12V -. I can supply 24amps through the 5V+ line, 8 amps 12V+, and 2.8amps through the 12V- line. This is all internally regulated. There is a way I can make it an adjustable power source but, alas I am not terribly electronical building inclined and it involves building a regulating board.
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Old Sep 05, 2012, 02:04 PM
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Quote:
Originally Posted by outdoorsjd View Post
I was wonder if this would work as a hot wire cutter.

I recently upgraded the powersupply and graphics in my desktop and converted the old 300W powersupply to a bench top supply. I can supply 300w of power at 5V, 12V +, and 12V -. I can supply 24amps through the 5V+ line, 8 amps 12V+, and 2.8amps through the 12V- line. This is all internally regulated. There is a way I can make it an adjustable power source but, alas I am not terribly electronical building inclined and it involves building a regulating board.
+1 being I use to repair computers, I had tons of power supplies around but never realized they could be used for this application.

http://www.wikihow.com/Convert-a-Com...b-Power-Supply
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Old Sep 05, 2012, 02:27 PM
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trying to get a use out of the old supply if I can. If you have an old enough of supply you can also have a 5V - in addition to 5V+ 12V+ 12V- lines. Heck for as much as I am into it I'll probably just hook a wire up and see what happens, if it overloads it the failsafe in the powersupply will shut the power supply down. I primarily made it to use for HHO projects but figured this would also be a great use for it.
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Old Sep 05, 2012, 05:04 PM
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As another thread mentioned, put a brushed-motor ESC on the output of the computer power supply and control it with a servo tester ~$4.95US. That will allow trimming of the (12 volt) actual output to the power/heat required.

Note - if the supply is for an ATX the power-ready line needs to go to ground.
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Old Sep 05, 2012, 07:33 PM
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Not a solution for Richie, but may be useful to others:

(Richie, see the edit in my next post for answers to your questions)

I have a cheaper suggestion than your proposed power supply, that uses pre-built components, and you can re-use them for other purposes, too.

Get a 12 volt battery charger -- that preferably can supply 6 or more amps continuously. It MUST be a transformer type old fashioned battery charger NOT a "computerized" or regulated modern type. You want the type that had a built in meter, not LED lights.

Then get a Harbor Freight Router Speed controller. These are regularly on sale for about 20 bucks or less. That's it besides a bow, and you don't have to wire anything.

Just plug the charger into the router speed controller. Now you can adjust the charger's output with the router speed controller.

The other advantage of this system is that the HF speed controller comes built in with a fuse, cord socket three way switch for full power, variable power and off. These features are often missing from homemade setups, and with the box usually cost more as components than the HF unit does complete.

You can also use these things individually to charge a battery, or add variable speed to a router, as originally intended!

ps that setup would work well with stainless steel wire leader material of about .023" diameter and a 36 inch bow.

If you want a different material, diameter, and or bow length, you can use my hot wire calculator to figure out what will work. If you can't use the calculator, I'll be glad to help figure out what you need on that thread. As mentioned earlier by CayminLast it's at:

http://www.rcgroups.com/forums/showthread.php?t=1625684
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Last edited by vtdiy; Sep 05, 2012 at 08:29 PM. Reason: Misread original post. see better answer below
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Old Sep 05, 2012, 07:45 PM
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EDIT: Just noticed that you are in GB (no Harbor Freight) and also had already used the calculator to determine specs for a 1 meter bow. And that the power supply seems reasonable to you in cost.

So to answer your questions:

1.) The capability to supply more amps than you need is fine. That is normal and desirable. You actually don't want a power supply that is operating at the edge of its current supplying capability.

2.) There is no danger from electrocution on the output terminals on a responsibly designed undamaged 15 volt power supply.

3.) In a low voltage supply like this one you can size your hot wire thin enough that it couldn't draw more than say 3.5 - 4 amps maximum at 15 volts. It will still get quite hot at the maximum setting, and you would normally cut a little cooler than that. But it won't be able to draw 15 amps, because the resistance of the wire is too high to allow it. For best protection you want to be cutting just a little under max range available, rather than say cutting at 25% of full range. Sizing your wire properly will do that.

4.) A fuse normally protects currents above the output maximum (15 amps in this case) or the mains current above a value the manufacturer deems damaging to the unit. That would not protect against fire if an improperly sized wire was connected to it that drew less than 15 amps.

The main danger for a hot wire setup that is properly designed and sized to the bow would be simply leaving the bow at a maximum setting in contact with something flammable, or shorting the hot wire terminals and creating a lot of heat with a shorter or lower resistance wire than you normally use.


I do need to put in a disclaimer here however. Operating a power supply and hot wire cutter must be at your own risk. You need to be responsible for determining the safety of any rig you put together and operate. I may not have covered all of the potential issues here. Just trying to provide information I'm aware of. I'm sure others will chime in if I've missed something.
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Old Sep 06, 2012, 06:17 AM
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Thanks all for your help everyone...

Quote:
Originally Posted by vtdiy View Post
EDIT: Just noticed that you are in GB (no Harbor Freight) and also had already used the calculator to determine specs for a 1 meter bow. And that the power supply seems reasonable to you in cost.

So to answer your questions:

1.) The capability to supply more amps than you need is fine. That is normal and desirable. You actually don't want a power supply that is operating at the edge of its current supplying capability.

2.) There is no danger from electrocution on the output terminals on a responsibly designed undamaged 15 volt power supply.

3.) In a low voltage supply like this one you can size your hot wire thin enough that it couldn't draw more than say 3.5 - 4 amps maximum at 15 volts. It will still get quite hot at the maximum setting, and you would normally cut a little cooler than that. But it won't be able to draw 15 amps, because the resistance of the wire is too high to allow it. For best protection you want to be cutting just a little under max range available, rather than say cutting at 25% of full range. Sizing your wire properly will do that.

4.) A fuse normally protects currents above the output maximum (15 amps in this case) or the mains current above a value the manufacturer deems damaging to the unit. That would not protect against fire if an improperly sized wire was connected to it that drew less than 15 amps.

The main danger for a hot wire setup that is properly designed and sized to the bow would be simply leaving the bow at a maximum setting in contact with something flammable, or shorting the hot wire terminals and creating a lot of heat with a shorter or lower resistance wire than you normally use.

I do need to put in a disclaimer here however. Operating a power supply and hot wire cutter must be at your own risk. You need to be responsible for determining the safety of any rig you put together and operate. I may not have covered all of the potential issues here. Just trying to provide information I'm aware of. I'm sure others will chime in if I've missed something.
@vtdiy, Thank you for the information, it is really useful, I have just bought that power supply, It should arrive tomorrow, I have used the hot wire foam calculator,

I downloaded your Spreadsheet calculator and put in these settings:

1200mm desired wire length
12v (power supply does 0-15v, but seems people use 12 so went with this.)
3 amps.
0.5 watts per inch (went in the middle, the guide in the spreadhseet says that 0.4-0.6 is desireable)

It says the best wire diameter to use for this is 0.52mm, but the closed i could find is 0.6mm, will that be okay?
The Calculator says that I need a wire with a resistivity of 700 - how do i find out if the wire I have is that spec?

I bought 2 meters of 24 SWG/ 0.6mm Nichrome (Nickel/Chrome) Wire from ebay. And i have bought a single guitar machine head to use to tighten it up to get the right tension. I plan on doing cuts of 1000mm max. but thought i'd make the wire 1200mm (~47inches) long.

SPECS of wire i bought
Resistance: 4.4 Ohms per Metre

Approximate Chemical Composition
Ni80%
Cr20%

DENSITY
8.31 g/cm3

ELECTRICAL RESISTIVITY
AT 20 oC (microhm • cm) - 108
AT 20 oC (ohm. Circ. Mil/ft) - 650

MAXIMUM OPERATING TEMPERATURE
300 oC
572 oF

MELTING POINT
1400 oC
2550 oF

COEFFICIENT OF EXPANSION
12.5 m/m C
(20 - 100C)

I dont quite understand what resistance is, or how it relates to the power supply and wire. Therefore I won't plug it in or test it until I completely understand it. I am glad that the power supply won't kill me! (I do appreciate the disclaimer thought! ). I may buy a fire extinguisher for electrical fires. I have bought an inline fuse holder, It can use normal household plug fuses. I guess i dont want to fry the power supply, what fuse should i use if I intend on 3a and 12v? Or is a fuse un necessary?

Regarding safety, I didnt want to manufacture my own power supply as I will be cutting the wings in my lounge, as i live in an apartment. I am going to get a fire extinguisher just in case. I will also be making a holder for the bow. So there is no danger of it touching anything flammable when electrified.

How do i get a wire that can draw more than 15 amps? I haven't seen an amp rating on the wire spec

Am i right in saying that the watts needed is 3a x 12v = 36 watts, therefore I need 36 watts to cut 1meter?

Apologies for tall the questions - But thanks for the input everyone, its useful to me. I'll update the thread with pictures of my finished result when ready!

Rich
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Old Sep 06, 2012, 06:24 AM
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Cardiff, UK
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Nice Finish

Quote:
Originally Posted by The SKY MAN View Post
Hi
I have just used my power supply to cut my wings i used for infomation a watt meter in line so i could see how much power was required.
75 watts for a 39" bow cuts a treat
Iain
If I can cut something half as good as that, I will be a happy man! Are you going to cover them with anything? Are you cutting EPP or EPS?
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Old Sep 06, 2012, 07:50 AM
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Wow, lots of questions! Hard to know where to begin. In fact this is all very simple, and I think your concerns will evaporate as soon as you begin cutting wings. People do it every day. But it is always good to understand how things work -- and that can be part of the fun itself. So I will try to answer your questions as best I can.

First of all, the calculator I came up with was based on data supplied by forum readers measuring their actual cutting specs, not theory. So I asked people in that thread to measure their amperage, and voltage while cutting, and send in their wire type and diameter and length. From that data, the calculator was constructed. Therefore it is very practical. It relates directly to cutting foam for wings and other modeling activity, using widely available materials and power supplies. I've seen another calculator online that was constructed from theory, and it is very limited, and yet complicated by comparison.

Because my calculator depends on people sending in data, it doesn't cover everything so far. Nichrome wire is one area where no one has sent in any cutting data, so we have estimate that. Once you make your bow, please let me know how it works, and provide some data, and we'll update the calculator with that, okay?

I'm going to break this up into several shorter posts rather than one long one. So that's the first point.
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Old Sep 06, 2012, 08:19 AM
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Southern Vermont
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Quote:
Originally Posted by richie967 View Post
I downloaded your Spreadsheet calculator and put in these settings:

1200mm desired wire length
12v (power supply does 0-15v, but seems people use 12 so went with this.)
3 amps.
0.5 watts per inch (went in the middle, the guide in the spreadhseet says that 0.4-0.6 is desireable)

It says the best wire diameter to use for this is 0.52mm, but the closed i could find is 0.6mm, will that be okay?

The Calculator says that I need a wire with a resistivity of 700 - how do i find out if the wire I have is that spec?
Rich
Okay just a few notes on using the calculator.

In General: you enter figures in the white blanks, and read calculated results in the yellow blanks. Click anywhere in the spreadsheet to recalculate results after changing something.

1.) Always put in the actual heated cutting length of your wire, not the overall bow length. You want to put in 1 meter (40 inches) if that is your heated length, not 1200 mm. Unless that is what the actual heated length is. The heated length is the distance between electrical contacts.

2.) Put in the rated maximum nominal voltage of your power supply unless you have some specific reason not to. It's okay that you put in 12 V but it will be better to put in 15 V. (note: voltages over 35 are dangerous never use any power supply with higher voltage outputs for wire cutting.)

3.) Put in the maximum rated current available from your power supply. In this case it is 15 amps. The calculator won't force you to use all 15 amps, it will simply check to make sure that the wire you choose won't exceed the rated output of your power supply. That's the only reason it asks for this figure.

4.) I usually cut at .4 to .6 watts per inch, but personal preferences vary. People have reported cutting at 1.5 watts per inch and running the wire at red heat. Personally I can't imagine this. I don't like the idea of running that hot, and their wire probably doesn't last too long! But to give yourself some adjustment range, you might choose say 0.6.

5.) You didn't mention the adjustment figure called "Setting" on the spreadsheet. What this means is that for the cutting heat you have chosen, where on your power supply's settings should this "normal" cutting heat be? In the middle, or closer to the maximum setting?

In other words, if you have a knob to adjust your supply, at what position of the knob do you want your "normal" cutting heat (chosen in the last step) to be? If you are concerned about safety -- put this fairly high, like say 80% or more. By setting this high, you can't dial in much more heat by accident than the usual cutting heat.

Fixed power supplies can't adjust so they should set this at 100% unless there is a specific reason not to.

5.) Resistivity is something you fill in. There is no suggested single figure for this. It merely means how much resistance a material has for its thickness in wire form. In other words an insulator, like plastic, has more resistivity than a metal like copper. And stainless steel has more resistivity than regular steel. There is a short table of resistivity included on the spreadsheet for reference purposes (the grayed area). If you are using stainless steel wire you enter 700. But that is just for stainless. All of these figures were derrived from actual user input, So they are practical figures. Unfortunately we don't have a value for nichrome in the table yet. But we can approximate that from your specs.

Things like "guitar string" are difficult to pin down, because strings may be made from different materials. I think we got one data point for guitar string, and that one appeared to rank somewhere around steel's resistivity (low) , rather than stainless steel (higher). Anyway, I'll give you an estimate for nichrome shortly, from your data of the wire you bought and you can try that in the calculator.

phew, enough for one post........


Below is a picture of the spreadsheet as an example. It does not have your figures in it. It's just an example of what it looks like for people who are trying to follow along here.


]
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Old Sep 06, 2012, 08:38 AM
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Setting for power supply....

Regarding the Setting question you asked...

Quote:
5.) You didn't mention the adjustment figure called "Setting" on the spreadsheet. What this means is that for the cutting heat you have chosen, where on your power supply's settings should this "normal" cutting heat be? In the middle, or closer to the maximum setting?

In other words, if you have a knob to adjust your supply, at what position of the knob do you want your "normal" cutting heat (chosen in the last step) to be? If you are concerned about safety -- put this fairly high, like say 80% or more. By setting this high, you can't dial in much more heat by accident than the usual cutting heat.

Fixed power supplies can't adjust so they should set this at 100% unless there is a specific reason not to.
The Power Supply does not have a single setting 0 -100%. It has 2 dials, one for volts, and one for amps - therefore, would 50% be 7.5v and 7.5a?
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