Power supply for a foam cutter

Discussion in 'The Projects Forum' started by PeterChatterton, Jun 27, 2010.

  1. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    I'm building a power supply based on the following two sites:
    http://www.aircraftspruce.com/catalog/cmpages/hotwirekit.php
    http://www.instructables.com/id/Hot.../Wiring-up-the-transformer-and-dimmer-switch/
    but I want to add a fuse just in case the output wires are shorted out. This would be different from the normal fuse in power supply construction which is a slow blow on the primary.

    The transformer is rated at 2A
    http://www.hammondmfg.com/266.htm
    so I'm guessing that if I put a 2A fuse in the xfmr output I should be okay.

    I've got some fast acting 2A automotive blade fuses (ATO2BP) at the top of:
    http://www.littelfuse.com/data/en/Data_Sheets/aftermarket-5.pdf

    Does this sound okay?

    Thanks,
    Peter
     
  2. sceadwian

    New Member

    Jun 1, 2009
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    A fast acting 2A fuse is going to trip if you load it with 2A continuously, so it depends on what the load (your wire) is actually going to place on the secondary. Why do you want to fuse the secondary, the transformer itself will limit the current? For basic safety the primary fuse is fine.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    You can never have too many fuses....well you COULD but c'mon.

    What your going to want to know is, what amperage does the fuse cutter require to operate? If it is 1 to 1.5 amps, then a 2A will be fine. And you can use the littelfuses.

    It is not a requirement, but, better safe than sorry. It gives you extra protection just in case the primary fuse is faulty or fails closed.
     
  4. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    Thanks for the replies.

    I didn't make it clear that at this time I'm not thinking of have a fuse in the primary, altho I've got nothing against putting one in. Any idea of what it should be rated for?

    Peter
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    I just read up on the links you provided, why are you using a dual secondary transformer? You should pick something in the 166 series it'll cost less.

    One thing you have to know about fuses, especially fast acting fuses is the amperage they're rated for is when they're going to trip, so as retched was saying if you're only using 1 or 1.5 amps of current on the secondary those fuses will be fine. If your foam cutter actually uses 2 amps it'll blow after 10-30 seconds depending on the ambient temperature.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you used the transformer shown in the first link, and it is rated for 2 amp, I would use a 3 or 4 amp 120v fuse. Plus you have a circuit breaker connected to the wall outlet, so you should be safe.
     
  7. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    "why are you using a dual secondary transformer? You should pick something in the 166 series it'll cost less."

    -- because that's all they had in stock.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I'm assuming you're using this transformer:
    266L24, 48VA, 24V @ 2A (series center tapped), 12V @ 4A (parallel)

    If your mains voltage is 120, use a 1/2A to 1A fuse rated for >=125v on the primary side.
    If your mains voltage is 220, use a 1/4A to 1/2A fuse rated for 250v or higher on the primary side.

    Wire nuts are OK if you are going to place your project inside a box. I prefer using solder and a couple of layers of heat-shrink tubing; as you know it's not going to come off if properly applied.

    Do NOT use "electrical tape" aka "electrician's tape" - the adhesive will eventually get gummy and fall off, leaving bare wires exposed.
     
  9. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    I added a fast-blow inline 2A fuse at one of the outputs of the transformer and hooked it up to the foam cutter which is 3 feet of what might be nichrome wire of about 8mm diameter, with a resistance of 1 or 2 ohms.

    The problem is that the output voltage goes steadily to 2VAC then jumps to about 5V, presumably when the fuse blows -- but at no point does the wire get warm.

    I'm measuring the voltage at the secondary in ACV with an antique Micronta 10K ohms/V multi-tester. I do have a more recent digital meter but it's missing the ground cable right now.

    Thanks again,
    Peter
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    8mm? That is way to thick. Thats a 1/4 inch. Are you talking about .8mm?
     
  11. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    You got it!
     
  12. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    I should have mentioned that, at the max setting of the dimmer switch, I do get 24VAC at the secondary. This tells me at least that it's wired up okay.

    Maybe it's the dimmer switch? I bought it new, it's a Leviton RP106-1L at:
    www.leviton.com/OA_HTML/ibcGetAttac...Vd7hA1ZA&label=IBE&appName=IBE&minisite=10026

    I have an old 12.6V 1.2A transformer I could scavenge, maybe that would make it easier to work with.

    Peter
     
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
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    For making free-hand cutters, short lengths of 0.8mm and even larger wire may be used. However, for a bow, you might want to try something even smaller, if you can find it. At least that will tell you whether it gets hot and can be controlled. Once you are over that hurdle, then all you need to do is find the right size wire for your supply.

    John
     
  14. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    You're referring to a smaller diameter wire, right?

    What if I get a shorter piece? This is what I was using with the battery charger, wrapping bits of wire around a foot of the bow. A new output for the power supply will just take a head-phone jack and some wire.

    But the reason I just wanted to heat-up only a foot of wire was because the batt. charger was only rated for 12V 2A and I didn't think it would heat any more. This 24V 2A transformer is a different kind of animal completely, it doesn't seem right to have to go this route.

    Peter
     
  15. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    For another post, I dug out a short length of .005 in NiCr and tested cutting blue board and white bead board at 700 mA; good clean cuts. About 1.1 ohms per cm hot. For a ft would need 24V
     
  16. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    I made some new cables and separated the outputs by about 9" which gave about 0.5ohms (after correction).

    There was some warming with 2ACV (on the digital meter) but not much more than I could touch. When I turned it up to 3VAC it blew the 2A fuse as you would expect.

    According to Wiretronc, 20 gauge (AWG) nichrome has about 0.64 ohms/foot so this is probably what I've got.

    Peter
     
  17. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    I'm thinking the fuse shouldn't be there at all, after all it's not in the other cutters that I've looked at.

    Anyway, I'm going to try taking it out -- keeping a nose out for burning insulation, as well as a fire extinguisher.

    Peter
     
  18. PeterChatterton

    Thread Starter New Member

    Jun 27, 2010
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    Anyway, just what does the 2A rating of the transformer actually mean?
     
  19. BMorse

    Senior Member

    Sep 26, 2009
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  20. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The 2A rating referrs to the secondary winding.
     
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