Power Supply for a Foam Cutter?

Marty

The small foam cutter bows you find at the local hobby shops thats used to make scenery for train sets use a simple 9v 1300ma transformer. You could find something like this at Radio Shack. I'm not sure if a bigger bow would require a larger power supply or not.

Get a regular light dimmer switch at your local electrical store. After the switch put several 20 watt bulbs in series on the HOT wire before the bow. These lights limit the current and give better adjustment of the dimmer. You want enough lights to move the correct setting for the dimmer into the middle of its adjustment range. These lights on the hot wire also make it a little safer if you happen to touch the wire close to the hot side. But never forget that the hot side is hot and can kill you if you are standing in water. On a dry floor chances are you would not feel a thing as you are not grounded. This method gets the voltage on the hot side of the wire down to less then 20 volts.

sounds like it's dangerous to me. Anytime you fool with high voltage it can kill you. Please don't be stupid. Eddie Fulmer

check out this link for a homemade power supply for a foam bow. I just came from the shop cutting some foam wings. I use a 12 volt power supply and nichrome wire. I went inside the power supply and turned the voltage down some to get the desired effect. It was to high to start with. The power supply that is shown on the link below should work fine. I am planning to make one for myself. Please don't use anything directly connected to a 120 volt outlet! Power it straight from a 12 volt battery and you can get by with that. I built my own foam bow from 1/2 inch conduit and a wooden plug in the end to insulate the wire and make my connections. Eddie Fulmer

The supply I use costs less than $20 and gives 30+ volts at 5 amps DC. I ordered a 24v, 5a power transformer from

along with a 200v 25a full wave rectifier and a few 2200mfd 50v caps. You can add a dimmer between the wall and the transformer for voltage control (I think you also need a light bulb in parallel due to the inductive load of the transformer). Take the output of the transformer through the rectifier and then parallel a couple of the caps to smooth out the ripples. This will give plenty of power for .015 inconel wire at over 4 feet of cutting length.

I have also designed a good, isolated power controller that will work on the DC side using a MOSFet and 55 timer.

It gets the voltage down when under load. But open circuit I think you will still find there is 120V at the bow before you connect it. Considering you only need 15-25 mA to cause your heart fits I would say this is a plan of dubious safety.

If you were going to do this I would say connect the bulbs in series right back to the dimmer, and shunt the bulb connected to neutral with the bow. That way you have a functioning voltage divider at all times. My electronics is getting rusty with age and disuse, but I think this is the way to do it.

Mike D.

What I've been using for years is a cheap (like at Sears or whatever) 12 Volt battery charger. The kind where you can select between 12 volt@6amp, 12 volt@2amps, and 6 volt @(I forget). Hook it up direct to both sides of your bow wire as is and plug it in. It puts out the perfect amount of heat and you're not messing with high voltage although still enough to get your attention if you're not careful.

MJC

I don't think there is a safe way to do this. Transformers and rectifiers are a lot cheaper than a trip to the ER.

The main problem with using a 12v supply such as a charger is that you are limited in wire length. 12v starts to peter out at a little over 3ft of good cutting wire.

worse yet, it is not isolated from the line...so you stand the chance (probability --> 1.0) that you will take a jolt at some time. especially on a concerte slab in the garage...

Roger

M Dennett wrote:

Would you mind if I took some life insurance out on you? I think I will get lucky and collect pretty soon. Regardless of the number of bulbs, you can still have the supply voltage between the wire and ground.

Hopefully the money you save by not buying a transformer will increase your estate for your heirs....

In short Dumb, dumb, and dumb.

I whole heartly agree with W4JLE...........................

Folks.... DO NOT FOLLOW THIS EXAMPLE!!!!!!!!!!!!!!!

You could end up DEAD...!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Dan

Dan Thompson (AMA 32873, EAA 60974, WB4GUK, GROL) remove POST in address for email

(i) provided you have correctly identified live and neutral

(ii) provided the wire doesn't go open circuit.

If you don't understand the above, don't touch it :-)

I wouldn't use anything plugged into house current. I use a 12 volt car battery. With about 5 feet of lamp cord and 4 feet of stainless fishing leader, it has exactly the right amount of voltage to cut white foam. I use the same battery that I use for the flying field, which is a large riding mower battery from the auto parts store. The parts for my bow cost me less than ten dollars at the hardware store.

It's just electricity. This reminds me of the time when my father had a work buddy build him a transformer out of spare parts for his train set. When putting a train on the track I got one helluva A/C shock from the tracks. My brother didn't believe me, so I told him to touch it, after some coaxing he did but didn't get shocked. Thinking maybe I imagined it, I touched it again and Wham it did it again. So my father touched it, and nothing happened. Stupidly I touched it again, and Whammo. They thought I was kidding around, becuase it never happened to them. And then I realized I was the only one not wearing shoes. I made them take off their shoes and touch it. They believed me then. The next day at work, my father's friend came over to his office and before my father had a chance to tell him anything said that he thought he may have forgotten to ground something.

Thinking back, I can't believe how stupid I was to touch it 3 times. But the lucky thing was we were setting up the trainset for the grandkids to play with, luckily it was us who found out, not the little kids.

I had a meccano electrcmotor and I used to run it off a 12v train transformer. One time laying with it I got a huge shock. Well it felt huge.

I couldn't work it out. I disconnected everything and touched the transformer. No shock off either end. I toucjed the mecanno odel. No shock. I put it all back together - same problem.

It was many years later before my education caught up with the problem. Breaking the circuit to a highly inductive (stalled) 12V motor would cause an inductive spike of several hundred volts. In fact, its how car ignitions used to work...the primary of an ignition coil peaks at about

400V when the contact breakers open.

The secondary of course does a lot more than that :-)

To all of you who are worried about my health and life expectancy, I sure appreciate your concerns and best wishes. Please have the insurance policy made out so half goes to my heirs and I will gladly accept your kind offer.

On the other hand you should be aware that I have wired up literally thousands of electrical boxes at both 110 and 220 that were live. I have thousands of times touched live wires at 110 or higher on purpose because it was convenient and I knew I was not grounded and therefore perfectly safe. So do not expect to cash out that policy too soon. After all, it is not the voltage that kills you anyhow. As one poster pointed out it is the current that stops your heart. If I recall it only takes perhaps 20 volts to be lethal if you manage to get a really good connection to both of your hands. This is not above the open circut spikes on a lot of 12 V battery chargers. Particularly the cheapos. On the other hand, I have taken 10,000 volts off the output of a tesla coil with no harm at all. In fact the current is so low you can not even feel it as long as you jump the arc to a pair of pliers you are holding rather then directly to your skin. Do the later and you get a burn.

If you wish to add a safety device to my proposed power supply there are several things you can do. For instance simply hard wire everything with solder connections. This avoids the voltage ever being very high on the hot end of the bow. You could also put a female plug on the output of the supply and a male plug on the input to the bow. Same result. The hot side of the bow will be way below line voltage and no open circut high voltage is where you can touch it. You could also put an isolation transformer ahead of the dimmer. Then you could hang onto either output wire directly from the dimmer and stand in water all day and not have any problem. Just do not touch both outputs at the same time. Also you can do all set up with the power supply turned off and not plugged in as I do. When the drive weights for the bow are all set and you are ready to cut you first plug in the supply. Then you turn on the supply and adjust the voltage and stand back and wait until the profile is cut. Then turn off the supply and unplug it.

I would like to suggest that life is full of minor dangers. We kill some 40,000 people a year in the US in automobiles if I recall the number correctly. So I suppose we should not use a car? Well, if you do not know how to drive you probably should not try to drive a car. And if you are a total mechanical and electrical klutz you probably should not even consider building your own cutting bow either.

My supply offers some significant advantages over the fixed voltage or fixed current types. Namely it is adjustable. So it will work for a wire a foot long or six feet long. By being able to adjust the output you will find you are able to cut a smoother profile. And you will be able to change from one wire type to another when your first wire is no longer available. But to each his own. You are welcome to do it your way. And my way is perfectly safe as long as you have a modicum of understanding about how electrical devices work and how to operate them safely. If you do not please go buy a commercial rig.

Have a good day everyone.

Sure, you may be some kind of electrician god. The rest of us rely on good engineering and safety practices. $5.00 worth of electronic components makes a great variable duty cycle control for the low voltage side of a decent transformer and rectifier. The components are also compatible with the Hobby CNC board.