Simple power supply for Hot Wire

[Wingnut31]

I want to cut some thick foam insulation (Owens Pink 5/8" thick by 4'X8') into thinner 2' X 4' sheets. I built my cutting jig. And built the circuit using a single pole dimmer switch and a 24V doorbell transformer. The wire got hot for a few minutes but than went cold. I checked the circuit and found that the transformer went dead. My wire is 23" long and not very thick.

Can anyone recommend a transformer that will work, or what I need to build a simple (LOW COST) power supply for this beast. I really want to use this foam, I hate using fanfold and the prices on depron suck. Ready board isn't thick enough to give me the dimensions I need.

Please help.

Here is a link to one transformer that looks like it may work. Another recommendations. Transformer (http://www.surplustraders.net/specialist.php?specialist=ted&stock_no=MD739&cat_no=0130S&title=12VAC%204%20AMP%20POWER%20TRANSFORMER&desc=Robust%2012VAC%2F4AMP%20power%20tr%61nsformer .%20Be%61rs%20p%2Fn%20T05452C%2FE217010.%202-5%2F8%22L%20x%202-1%2F8%22H%20x%202-1%2F2%22W.%20This%20tr%61nsformer%20comes%20comple te%20with%20line%20cord%20%61nd%20inline%20SPST%20 inline%20switch.%20The%20output%20le%61ds%20%61re% 20short%20-%201-3%2F8%22%20long%20%61s%20the%20tr%61nsformer%20w%6 1s%20origin%61lly%20intended%20for%20PC%20bo%61rd% 20mounting.%20Although%20we%20h%61ve%20not%20me%61 sured%20the%20true%20output%20we%20believe%20th%61 t%20it%20is%20c%61p%61ble%20of%2013.5VAC%2F5%20AMP S.%20We%20c%61n%20%61lso%20supply%20this%20tr%61ns former%20mounted%20on%20%61%20bo%61rd%20with%20%61 ssoci%61ted%20components%20to%20m%61ke%20%61%2012V DC%2F4Amp%20power%20supply.%20(TH751).&stock_qty=1&img_name=md739.gif&img_width=200&img_height=193&img_alt=T05452C/E217010%2012VAC/4AMP%20TRANSFORMER)

[Ron W3FJW]

What kind of wire are you using? Nichrome?

[XJet]

My hotwire setup uses a transformer that offers two 28V outputs at 5A each -- so I can parallel the windings to get 28V@10A or 56V@5A.

I use 40 inches of 0.6mm stainless steel MIG welding wire and have a 400W light dimmer wired into the primary (mains) side of the transformer.

This gives me full control over the wire-heat and usually runs at under half-power with the windings set up to deliver 56V@5A.

Being able to vary the heat of the wire makes life much easier and the stainless MIG welding wire is far tougher than the thinner nichrome wire I was using before -- allowing me to use plenty of tension so as to ensure no bowing of the wire due to drag during cuts.

[AndyKunz]

I would make those cuts on a table saw. Quicker and easier.

Andy

[Wingnut31]

Ron My wire at the moment is Steel Safety wire, it looks to be about 1/64" thick,. I have some guitar string too. Right now that is about all I have. I thought of using some thin music wire from the LHS.

X that sounds like a good setup.

Andy, I have no saw with that ability. I want to re-saw a 5/8" thick panel down to two 5/16" panels. (Not counting the wire kerf) It would be more like two 1/4 thick panels. The width and length of the panels are 2' by 4'.

[Wingnut31]

Are there any appliances I can try to scavenge one from?

[8pointroll]

I use 40 inches of 0.6mm stainless steel MIG welding wire and have a 400W light dimmer wired into the primary (mains) side of the transformer.

Does the transformer get really hot? I'm pretty sure you should not do this as your 400W lamp dimmer will effectively be converting your sine wave mains into a square wave. This is not good either for the transformer or for generating noisy harmonics.

IMHO You would be better placing the dimmer on the output from the transformer to regulate the energy getting to your cutting wire.

[8pointroll]

Ron My wire at the moment is Steel Safety wire, it looks to be about 1/64" thick,. I have some guitar string too. Right now that is about all I have. I thought of using some thin music wire from the LHS.


Now then...heres a can of worms :eek:

I don't think any of these wires sound appropriate for foam cutting. Ron suggests Nichrome wire and that is commonly used for foam cutting, it is available in a variety of diameters and therefore resistance values. (http://www.wiretron.com/nicrdat.html)
Once you know the ohms/foot value of your wire and the length of your wire you can work out what power you will need to cut foam (V=IxR).

For example
0.6mm nichrome wire is approx 22 guage, (http://www.graphicproducts.com/supplies/wire_gauge.html) this is approx 1ohm per foot @22 degrees. If you cutting wire is to be 4 feet long the resistance of your cutter will be 4 ohms. The scale from the given web page starts at 200 degrees so taking that as a maximum heat :eek: you will need to pass 2.9Amps through the wire, this will require approx 12 volts. this will mean the power dissipated in the wire will be 36 watts (W=IxV).

My first foam cutter was just a set of 7 x 2volt lead acid batteries strapped across the wire bow but having an electronic means of varying the current is very desirable, and I built myself a regulator, however a cheaper solution might be a few 50Watt half ohm resistors you can add in series with the wire to obtain the required current.

As you have already found out, if your transformer is under rated or abused by chopping its input with a light dimmer it may well give up the ghost.

Had you considered using a 12volt car battery and a few resistors instead?

[andrew b]

$RESISTORS$ ! why not wind a coil of cutter wire around something that won't burn and use a croc clip to select a "jumper" point? :p The car battery has the advantages of high current and cheapness, you will then be able to use a thicker cutter wire to match the resistance to voltage, stainless MIG wire sounds good and cheap. I messed about with thin wire using low current, then I tried thick wire, low volts and high current (up to 25 amps) works MUCH better. I found a switchable high current "overdrive" was great for cleaning the wire after use, a dull red with a wipe from a damp cloth worked great on stainless wire

[rick.benjamin]

successful foam cutting
http://www.utahflyers.org/index.php?option=com_content&task=view&id=37&Itemid=33

[XJet]

Does the transformer get really hot?
No, not even warm to the touch.

I'm pretty sure you should not do this as your 400W lamp dimmer will effectively be converting your sine wave mains into a square wave. This is not good either for the transformer or for generating noisy harmonics.
It won't be a square-wave although it will have a very fast rise-time.

In theory this could cause ringing in the transformer and spurious harmonics but in practice there have been no issues at all. I built this rig about five years ago and it's still going strong.

IMHO You would be better placing the dimmer on the output from the transformer to regulate the energy getting to your cutting wire.
The currents on the secondary side are much higher than on the primary and an "off the shelf" light-dimmer is not designed for the lower voltages so probably would not work.

On the CNC foam cutter I designed and built, I've used a FET-based PWM modulator to control the wire heat. This *does* work on the secondary side but my old manual setup still uses the light-dimmer. Simple, cheap and very effective.

[gavoss]

For a simple powersupply, go by an inexpensive 12V automotive battery charger that has a meter. Harbor Freight has them for about $30. Buy a light dimmer switch and put it in line before the transformer in the charger. Now you have a safe and regulated power supply for about $40. You can use a variac if you can find one, but they are usually more expensive than $40. gv

[poulsbobill]

For a simple powersupply, go by an inexpensive 12V automotive battery charger that has a meter. Harbor Freight has them for about $30. Buy a light dimmer switch and put it in line before the transformer in the charger. Now you have a safe and regulated power supply for about $40. You can use a variac if you can find one, but they are usually more expensive than $40. gv
Thats what i use... a car battery charger!! And my sons old guitar strings.

Bill

[Brandano]

I'd use an excess length of nichrome wire wound around a heat resistant support and crocodile clips to create a makeshift variable resistor. For example zig-zagging around two rows of nails hammered in a plank of wood. Simple, rugged, should work with any power supply suitable for wire cutting and should give enough fine control. An old wirewound speed control for RC cars would work as well (or any speed controller for DC motors, if you can generate a servo signal!) but it might not be easy to find nowadays.

[Wingnut31]

A lot of helpful info here, my bane is that I have little expertise with electronics. The charger sounds like an idea worth trying. I just spent the evening scavenging a transformer from a CD changer. I have lots of voltage options up to 21volts. I hooked it up to the wire and nothing, the transformer got hot though. I checked the amperage with the multi meter and it only read about 1.5 amps. I figured since I got only small sparks. I'll spend tomorrow reading the link you posted Rick.

I should have paid more attention in shop class.

Thanks for the help guys. I'll up date as I progress.

[Ron W3FJW]

Whatever you decide to do, keep in mind that a thicker wire is usually better as the melted foam is less likely to flow around the wire adhering to itself in spots making the cut pieces hard to separate without damage to the foam.
In addition to a hot wire foam cutter, a soldering gun with a short length of #14 wire can be used to cut out and shape small pieces of foam as the wire can be formed to complex shapes that one may want..

Edit: Make sure the wire is cool before you try & reform it. Don't ask me why I'm saying this... :mad:

[Wingnut31]

I like that idea. Ill have to try that.

[mt_100]

I use this one. http://www.charlesriverrc.org/articles/tools/joedydruliafoamcutterps.htm

Made a little box for it and use a PVC pipe bow with 40lb stainless steel fishing line, non braided. I hook a voltmeter on each end of the bow and it cuts clean at 5v. At 8v it will liquefy the foam and at 12v the wire is red hot, not recommended.

[ZAGNUT]

Does the transformer get really hot? I'm pretty sure you should not do this as your 400W lamp dimmer will effectively be converting your sine wave mains into a square wave. This is not good either for the transformer or for generating noisy harmonics.

IMHO You would be better placing the dimmer on the output from the transformer to regulate the energy getting to your cutting wire.

don't know if it has anything to do with what you're talking about but there are dimmers made for inductive loads. quite common to use these between mains and toroidal transformers for 12 volt lighting. my foam cutter is similar to xjet's but i use a 22 volt, 11 amp toroid and cut with 0.5mm nichrome.

[vintage1]

Nichrome is not necessarily the best wire to use. It has a high and thermally stable resistivity, but that's not what you want.

Stainless steel trace is often better.

As far as powering it goes, a dimmer driving a low voltage transformer THAT IS DESIGNED TO BE DIMMED is the simplest.

If you go to an electrical contractors and ask for a dimmer and dimmable LV lighting unit, they should supply you with what you want. Up to about 200W at not very much money.

[orraman]

successful foam cutting
http://www.utahflyers.org/index.php?option=com_content&task=view&id=37&Itemid=33

rick.benjamin

Thank you for that link, first class information on the

FOAM CUTTER PLANS AND INSTRUCTIONS and the
ONE PIECE JIG AND ANCHORED CUTTING WIRE DESCRIPTION.

This shows how to cut tapered wings single handed. I have also used this system to cut hydrodynamic conic sections on the planing surfaces of floats and hulls.

For those considering using a dimmer as a regulator for a normal (magnetic) transformer there is much useful information on the following link. It explains de-rating and the problems that cheap dimmers have at low operating currents and it can be seen why some DIY circuits incorporate a mains voltage filament lamp to provide the minimum load required. In the UK the common 250W 240V dimmer by MK Ltd has a minimum load rating of 40W. Some working designs do not incorporate the lamp assuming that the bow will provide the required loading.

http://www.crestron.com/downloads/pdf/tech_sales_tips/transformers-dimming_and_switching.pdf

Dave

[desire]

assuming you have a servo driver or transmitter and receiver. all you need is a brushed motor type speed control and 7-8 cell battery pack. connect your cutting wire to where the motor goes. you only need about 3 amps of current. best thing about this set up is no 120 volt power to kill yourself with.

[Wingnut31]

But what fun is life without risk? Just kidding. I'm really jealous, today I went home and saw my dad's new toy. I had a sheet of aluminum to cut parts templates, (permenant ones) and he pulled out his plasma cutter. THe cut that thing makes is so clean. I'll be getting the parts for the one Mt-100 posted, of the Battery charger. I need one anyway.

Thanks for all the in-put folks.
Brian

[Tomapowa]

Not to purposely dredge up this thread.... but I finally located an article I had posted on my club's webpage years ago.... pretty cool hot wire cutter... I built 3 for members in the past and all worked successfully... so they told me!

http://www.nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm


Post Note: Just noticed that NSRCA is moving their webstore... here's a newer version of the hot wire power supply project...

http://nsrca.us/all/building/65-powersupply.html

[arx_n_sparx]

http://www.allelectronics.com/make-a-store/category/790500/Transformers/Power-Xformers/1.html

I'd go with Tx-244 or 245. By all means use the dimmer on the mains - this is about the simplest solution.

I've built the nscra PS, and I'm not a big fan of it. From 0 - 30% of the dial, nothing happens. Then it suddenly switches on. Probably not enough voltage to trip the gate. The short bow resistors also get stinking hot if you leave it on for a while (as my Dad found out - the painful way. I mounted them just under the on/off switch [my bad] and he got burned pretty good when he went to shut it off).

I wouldn't worry about harmonics or "ringing" with either of those 2 transformers. A foot switch to turn it on and off would be a good idea though IMO.

Brad

[Joel K. Scholz]

Hi Guys. I have a 12 volt battery charger that the electronics gave up, but still produces 12 volts at 2 and 10 amps. The problem is when I hook it up to a 40" bow the wire glows red. Too Hot . I am using .030 nichrome wire. Is there a simple way to regulate this thing so I can use it. I have also a couple of old train transformers I have tried to use, but the heat is very inconsistant. It may be the rheostat on them is just worn out.

[rick.benjamin]

Is there a simple way to regulate a 12 volt battery charger?
Use a light dimmer.
Remove power before attempting this mod!
Cut the cord between the house current plug and the charger.
Insert
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100644809&N=10000003+90401

[Joel K. Scholz]

Thanks Rick

[arx_n_sparx]

Rick's post only applies *if* the electronics have truly given up. Pb chargers these days use a switch mode regulator - if you use a dimmer, the only thing that gets hot is the charger!

Brad

[slipstick]

Alternatively you can use an old brushed speed control driven by a servo tester on the output. That will work for any DC power supply even a battery charger (or of course a battery).

Steve

[Joel K. Scholz]

I have removed all electronics from the battery charger in question leaving only the switch that changes the amps from 2 to 10 volts.The specs on the transformer are:input 120volts, 60Hz, 2amps. Output 12 volts DC, 10/2 amps. My assumptiomn is that I would put the dimmer between plug and charger in this case. Will this be a "good" setup for cutting?

[arx_n_sparx]

My assumptiomn is that I would put the dimmer between plug and charger in this case. Will this be a "good" setup for cutting?
Good assumption - it'll work for shorter bows - say up to 40" long or so.

Brad