Nichrome Wire variable PSU ???

Discussion in 'General Electronics Chat' started by CD-RW, Jun 8, 2011.

  1. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    I'm going to build a polystyrene foam cutting table, based on this design here:

    http://www.youtube.com/watch?v=tiitiKyu_Iw&NR=1&feature=fvwp

    I have ordered 2 metres of nichrome wire with the following specs:

    NiChrome Wire swg 32 / Resistance Wire
    Length:
    200cm / 2 meter
    Diameter:
    0.274mm
    Resistance:
    18.99 Ohm/m
    Superficial area:
    8.608 cm3/m
    Operating Temperature:
    1100 C
    Melting point (approx.):
    1390 C
    Composition: Nickel 60%, Chromium 15%
    Manganese 1.5%, Silicon 1.5%, Iron balanced,

    Suitable for making heating elements and
    many other applications ( Foam cutters etc.)

    http://cgi.ebay.co.uk/2-0-meter-NiC...al_Components_Supplies_ET&hash=item19baa01e2d

    I already have a 12v dc car boost-starter/charger that puts out 25A on 12vdc continuous. (400A for 5 seconds boost starter mode.) I would like to use the 12vdc charger as a 0-12vdc variable power supply, to enable me to select the optimum voltage for powering the nichrome wire.

    Is there an easy way to turn my 25A 12vdc charger into a variable power supply please?

    I remember the scalextric model cars that had a 240vac to 12vdc PSU. Each car on the track had a wire-wound hand controller for controlling the speed of the cars. They did get a bit warm with constant use.

    Would this type of controller work with the nichrome wire at all?

    I don't want to build a fancy variable DC controller with lots of components.

    I was wondering if a wire wound Pot would work with this setup?

     
  2. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    Hi all.

    I might use a spare ATX power supply, and then run the 12vdc through an adjustable voltage regulator, like the LM317:

    http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/1/LM317.shtml

    The LM317 is an adjustable 3−terminal positive voltage regulator
    capable of supplying in excess of 1.5 A over an output voltage range of
    1.2 V to 37 V. This voltage regulator is exceptionally easy to use and
    requires only two external resistors to set the output voltage. Further, it
    employs internal current limiting, thermal shutdown and safe area
    compensation, making it essentially blow−out proof.

    Features
    • Output Current in Excess of 1.5 A
    • Output Adjustable between 1.2 V and 37 V
    • Internal Thermal Overload Protection

    :D hehehe
     
  3. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    How much current do you need for the nichrome wire?
     
  4. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    Hi Tom.

    The wire is rated at ~19 ohms a metre. So I might use 30cm.

    19/100 = 0.19 ohms per cm
    0.19 ohms x 30cm = 5.7 ohms

    I only need enough volts to make the wire hot enough to cut through some polystyrene. It doesn't need to glow read hot :eek:

    So I reckon an adjustable voltage regulator that can handle the current will be fine, running off the stabilized ATX PSU?
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    5.7 ohms off 12V will draw 2.1 amps... too much for an LM317T

    You could try using the 3.3V rail.
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    5.7 ohms will only draw 2.1 amps if you turn the regulator up to 12 volts

    There are higher current regulators like LM338
     
  7. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    There's a guy on youtube that has already done this. He converted an ATX PSU into what looks like a variable current and variable voltage DC PSU.

    http://www.youtube.com/watch?v=Ypk_5y0Qc0I

    Simple regulated workbech PSU from computer ATX PSU.
    Output 5....15V (0...15V) and about 20A max.
    Only about one hour of soldering and PSU is ready :) You need to buy only one component- variable resistor. See my web page for more wide output range models.

    Website: http://www.vabolis.lt

    I cannot find his instructions on the website!

    This would make a good post for a seperate project: 'Build your own variable 0-15vdc 20A PSU' if we can find the instructions. There must be lots of other members on the forum that want something like this - especially at up to 20A.
     
  8. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    5.7 ohms off 12V will draw 2.1 amps... too much for an LM317T

    You could try using the 3.3V rail.

    Or if you combine the 5V and 3.3V rails you get 1.7V.

    Be careful with sinking current into an SMPS. To keep the supply happy, you should connect a load to the rail which is sinking current; that way, it will be able to regulate itself. An 8.2 ohm / 25W resistor will work, although it will get warm.
     
  9. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Rather than regulating the voltage, I'd just go with the 12v supply and PWM the ground side of the nichrome wire return.

    Your current requirement is going to change from when the wire is in free air or cutting through foam due to the power dissipation difference.

    You might need some kind of feedback circuit to detect the change in resistance of the wire as it heats and cools. As the temp of the wire increases, the resistance will likely increase.
     
  11. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    Thanks for your reply SgtWookie. I don't expect to be using this for a long period of time. Just for occasional cutting and shaping of polystyrene. The PWM (pulsed DC?) sounds a good idea, but if the voltage drops on this I'll just turn the Pot up a bit higher.

    I will be needing a pulsed DC PSU for another (overunity) project later, so that is something definately on my TODO list.
     
    Last edited: Jun 10, 2011
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Just a caution - we don't support discussions on "overunity" topics - they are the modern-day snake oil.
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Find the ideal temperature then adjust the wire to length. Alternately, although expensive and power wasting, a wirewound pot will work given the correct ohmage range and wattage rating.
     
  14. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Replying to post #9:

    I would go to www.mouser.com and use their search engine. Get to "voltage regulators" and there will be filter choices. From that you can sus out what current ranges are available.

    There are several good sites to look at, but I'm familiar with Mouser.
     
  15. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    Ok thanks for that #12. Thanks for all the other replies as well!

    That mouser site is brilliant! So many different components to choose from.

    Power Management ICs (96,762) Wow!

    I'll probably post some pictures of the finished project here later on.
     
    Last edited: Jun 10, 2011
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