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Old Aug 20, 2006, 02:05 AM
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Train transformer questions

Looking to use one of these to power a foam cutter (small, 8-10" table cutter), I have a couple of questions for those of you that would have experience with these things.

The listings give the specs as such:
"Outputs: 0-16 VDC reversable, 18VAC constant 7VA Max"

And:
"Outputs: 0-17 VDC reversable, 20VAC constant 6VA Max"

Both use "VA" on what I assume are the amperes. Is that variable amp, or some other term? I'm not familiar with VA.

Also, I'm not sure of the age of these transformers. I think they're newer which means I will likely have to remove or somehow bypass the internal circuit breaker. Any idea what these things look like? Any tips on getting around it?

Thanks
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Old Aug 20, 2006, 04:28 AM
Just one more flight...
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Quote:
Originally Posted by Wufnu
Any tips on getting around it?
YES! I have finally found a nice way around the rheostat hot wire hassles. I originally bought the Proxxon from Thermocut… http://www.thecellarstore.com/hotwire.html

Then once the transformer went out I was able to get one from Radio Shack… this was nice and local.

BUT once the rheostat/speed control went out, the real trouble began. I of course could not find the exact replacement NOR did I have 6-8 weeks for it to be shipped to me even if I did find it…

I looked into the train transformers as well. I know guys have great success with those but good luck trying to get it figured out in under a week (can you tell I have been through hell with these things???)

So here is my RCSuperHappy soulution:

- I use the shell/platform of the origanl wire cutter (you can make your own table easy out of wood or whatever)
- I use the transformer from Radio Shack (see pic below)
- And best of all, I use a super simple "spring rheostat" Check out this site for how that works:

Spring type rheostat:
http://www.energyquest.ca.gov/projects/rheostat.html

I then opened up the underside of the table so I could have a fan blowing on the transformer, the spring rheostat, and the hot wire while I work. Just like having air intakes for your ECS, and li-pos, getting air in these things is a must if you are going to be using them hours straight like I do.

I have been through it all and am going to stick with this… Its acatuly better than the origal. I can get the wire to much cooler with this set up , which is what I need a lot of the time. Once it burns out again, I can go to Home Depot for the springs, and Radio Shack for the transformer and have it fixed today!
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Old Aug 20, 2006, 04:33 AM
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What problems did you have with the train transformers? I figured the circuit breaker would be a piece I could just solder a bypass on.
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Old Aug 20, 2006, 04:43 AM
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I didn't even get that far! I could not find them fast enough to mess with. I had to get mine fixed within a day and didn't have time to order and expierment with them. I found a bunch on line, and guys that have figured it out, but I don't specifically how they got around that.
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Old Aug 20, 2006, 04:55 AM
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To be honest, I'm intimidated by buying stock transformers and such. I'm a novice at that sort of thing, I wouldn't even know how to hook it up. That's why I was so interested in the train stuff, they're a one stop solution. I'll keep looking, but if anyone knows how to bypass the breaker let us know!
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Old Aug 20, 2006, 05:10 AM
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… Yeah… I did trip the circuit breakers in my house the first time I installed, cause I had the wires backwards… Actually I did it twice. If you can't get it let me know cause I could draw this all out and walk ya through it if you get desperate. Now that I have gone through it, its pretty simple.

I was just reading here that the train ones shut off cause they sense the heat….

http://www.spacemodeling.org/new/how_to/Foam_Cutter.htm

"Initially I used a cheap electric train transformer for a power supply, which provides excellent adjustment of the wire temperature due to its finely adjustable output voltage. This works well with only one wrinkle: it has a thermal circuit breaker built in which causes it to shut off after 15 minutes or so of continuous use. I'm now using a power supply from Jim Fackert, which can provide up to two amps regulated output for voltages between six and twelve volts."

Maybe if you took the plastic off the train controller and had the fan on it the whole time??? Might work. Just like an ESC will shut off if you don't have air flow on it...

All the articles that I Google say they get their transformers on ebay for $10. Could be worth a try.
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Old Aug 20, 2006, 05:15 AM
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That's the prices I'm looking at. They have old ones, really old ones, that do not have breakers but they don't list the specifications o.O
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Old Aug 20, 2006, 05:25 AM
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Yeah, thats what the articles where saying. Try and get the old ones that DON'T have the circut breakers.
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Old Aug 20, 2006, 08:41 AM
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Quote:
Originally Posted by Wufnu
...Both use "VA" on what I assume are the amperes. Is that variable amp, or some other term? I'm not familiar with VA.

...

Thanks
VA is the voltage times the amps or essentially watts. So with a max 6VA at 6 volts 1amp is the highest you should go with current.
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Old Aug 20, 2006, 11:27 AM
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Wufnu: First off, ditch the idea of using a train transformer. The older ones are okay (many yrs ago) but nowadays almost all of them are current limited in short circuit - which is essentially what we are doing when hot-wiring.

VA is indeed volts x amps. The thing to remember is that it's the current (amps) that heat the wire. You will need aqbout 1.5 amps for light white beadboard, and the temp/current goes up if you have to cut coloured (blue) foam. You need it a bit higher than *that* temp if you want to cut EPP.

So, we'd be pushing a transformer hard if it's rated for 2 amps. I strongly advise going double that, to a 4 amp capable transformer so that you don't let the magic smoke out.

In order to push through the current that we need, we need *pressure* - that's what voltage is - electromotive force. If you are strictly sticking to very short bows, you won't need much pressure - 12 volts will do you for about up to 2 feet in length. If you want to make longer cuts, you will need a 24V transformer. My long bow is about 4 1/2 ft, and I'm using a 24V/100VA transformer - call it rated for about 4 amps (a hair over, actually)

This is probably your best place to get a transformer for a reasonable price: http://www.allelectronics.com/cgi-bi..._Xformers.html

Don't worry about that "VCT" thing - it means volts center tapped. You won't be using the center tap. All you need then is a *good* dimmer switch or a ceiling fan speed controller on the input (120V side of the transformer). Leviton makes a dimmer rated for 600 Watts that you can snag at HD for about $10 or so.

I built my own power supply, but if I had to do it again, I'd go with the transformer and dimmer switch. It's about the easiest/cheapest way to do it.

Brad

PS - do NOT use a cheapo dimmer switch. It cannot handle the inductive load, and it will fry!
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Old Aug 20, 2006, 12:09 PM
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Go to your local computer store , ask for a used power supply , usually they will give you one for free , or very cheaply , works great , also good for charging batts.
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Old Aug 20, 2006, 05:00 PM
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They went up to like 18VDC and 20VAC, so that would be like .35A

I'm looking at a 110W O scale transformer from what appears to be from the 30-50s. $13.

I took E&M this past semester, I had just never heard "VA" before. We call them Watts o.O
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Old Aug 20, 2006, 05:36 PM
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I got a used but perfectly good G.E. variac from one of the electronics parts resale places in the back of Nuts & Volts magazine. It is somewhat overkill: cpable of hot wiring stuff for full-size planes, but as only 20 bucks. of course, it was darn heavy so shipping wass outrageous. You DO have to be careful using a variac for hot wire cutting: you can easily dial up a dangerous amount of power. Me, I keep the connection points insulated and covered, and have the dial marked so it's always kept in the "safe" range.
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Old Aug 20, 2006, 06:43 PM
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Quote:
Originally Posted by Wufnu
I took E&M this past semester, I had just never heard "VA" before. We call them Watts o.O
They are watts - until you get into power factor. Then it's VAR - volts/amps reactive
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Old Aug 30, 2006, 09:03 PM
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Quote:
Originally Posted by lake flyer
Go to your local computer store , ask for a used power supply , usually they will give you one for free , or very cheaply , works great , also good for charging batts.
Computer power supply? How would you go about using one? I have few but don't know what to do next. Do I need a dimmer switch? A PC power supply will output 12V at 20A or 5V at 10A if I remember correctly.

Thanks!
Azeiku
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