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Variable power supply? Well not yet! Help

Discussion in 'Electronic Design' started by [email protected], Apr 24, 2007.

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  1. Guest

    I have a simple 13v / 20A DC power supply I want to make variable. As
    it stands right now, it is fixed volt and I guess it draws a much
    current as the application needs. The supply is used normally for
    powering RC car battery chargers. I want to use it to cut foam with a
    hot wire. I want to vary voltage to get the proper heat in the
    wire.
    I don't think it would be as simple as attaching a 5K pot to the
    output........ would someone be so kind as to offer an easy solution?
    In my mind, I would like to build a simple divice to attach to the DC
    power supply in series to the wire. Simply plug it in and have output
    connecters on the down side of the variable deivice. Most, if not all
    of the hobby circuits use the variable pot on to feed a voltage
    regulator. I don't have the luxury to do that since the power supply
    is already built and I don't want to mess it up.

    Here is a link to something similar.

    http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEG98&P=7


    Thanks in advance.
    v/r
    Jerry L
     
  2. bart

    bart Guest

    I've made several high current supplies using an LM317 & 2N3055 pass
    transistors.
    You'll need several heat sinks for the pass transistors & it needs a
    big thermal runaway resistor off each transistor also.
    Lots of mech shit on your pristine supply.
    Google " lm317 high current".

    If you're only using it to heat nichrome wire..you might consider just
    using a lamp dimmer & an isolation transformer.
     
  3. You don't need much to heat up wire. I think maybe 10W is a good estimate
    for your application.

    If you take about 1 foot of AWG 30 which has a resistance of about 0.10320
    ohms then that means you have to run 1A through it to get that.

    The issue is that your appling 13V to it and so it will actually draw about
    125A. Now if you had a 1V source then you could do it.

    (Although I'm neglecting that the resistance will increase when it is
    heated... maybe by a factor of 10(happens atleast with a lightbulb))

    So ultimate what you have to have is some type of conductor that has a large
    resistance.

    if you can get something that has about 10ohm then that means you can draw
    about 1.3A through it and dissipate 17W.

    Before you worry about making it variable you might want to actually get
    something to work. Adding in a series resistor isn't going to cut it. If
    you find a 5 ohm pot using the 10 ohm wire above then you have 10 to 15
    ohms... this gives you a dissipation from 7.5W to 16.9W but the pot will
    need to dissipate about 5W(3.76 to be exact).

    A light bulb filament is about 10 ohms cold and 100 ohms hot. The above
    does not take into account this change.

    Normal coper wire isn't going to work because your voltage is to high
    producing extreme currents. So either you need to reduce the voltage(A 1VDC
    20A source would work nice) or increase the resistance.

    A better way would be to use AC and an SCR. Its very easy to control the
    power into the load and vary it.


    Your best bet is to find some high resistance conducting wire that is made
    for that sort of stuff. Normal wire isn't going to work. I think what is
    commonly used for this sorta stuff is nichrome:

    http://www.heatersplus.com/nichrome.htm


    Once you find some thing to use, if you haven't already, and are still
    confused then post what you have.

    For some high power pots,

    http://www.potentiometers.com/SeriesMG.cfm


    So if you had a nominal Rw ohm hot wire and Rp ohm pot then

    When Rp is all the way up,

    I = V/(Rw + Rp)
    Pw = I^2*Rw
    Pp = I^2*Rp

    and when Rp is off,

    I = V/Rw
    Pw = I^2*Rw
    Pp = 0

    You can use these formulas to find out the max current drawn(V/Rw) and the
    max dissipations in the devices(I^2*Rw for the hot wire and I^2*Rp for the
    pot). These all should be less than the max ratings. You can also use them
    to solve for Rw and Rp given whatever power you you want to dissipate in the
    wire.

    Note that when Rw goes up because it gets hot it will decrease all the
    values.

    Incase you are confused then what I would do is get the nichrome AWG 31
    which has a resistance of about 8.523 and one of those 10ohm 12.5W pots.

    This means you have approximately what I gave above. If you want to have
    more power then all you have to do is use a larger wire size like AWG 29...
    but this will increase the current and might burn up the pot(shouldn't as
    its about a factor of 2 which I think gives a pot dissipation of about 10W).

    In any case that one link seems to give some examples. I'm kinda rambling
    now ;/

    Jon
     
  4. Jon

    Jon Guest

    If all you need to do is cut foam, try a transformer fed by a Variac.
    You don't need AC
     
  5. Jon

    Jon Guest

    Oops!. I meant to say "You don't need DC". Sorry.
     
  6. Rich Grise

    Rich Grise Guest

    I've cut styrofoam with a piece of .005" (#36) nichrome wire, that
    reads about 7 ohms per foot. I used a 7.5V, 1A wall-wart, and just
    held alligator clips in my hands, and it worked astonishingly well.

    You might try a simple (read: crude) current regulator circuit:

    +13V ------+----------------+
    | |
    | [nichrome wire, ~5 ohms]
    [10K] |
    | / c
    [5K pot]<--[1K]---| NPN power (2N3055,TIP33,etc.)
    | > e
    | |
    | [1 ohm]
    | |
    +13 V RET -+----------------+

    But, of course, all of the usual disclaimers apply - I don't guarantee
    this circuit, it has no protection, etc, etc, etc - I can only say it's
    worked for me, but I used care while using the circuit.

    You'll probably want to heatsink the transistor.

    Good Luck!
    Rich
     
  7. Guest

    Thanks for the inputs.

    It sounds like all I need to do is take the output from the DC power
    supply to an LM317 that is controlled by a variable resistor. I could
    then put a meter on the downside of the LM317 to monitor output
    voltage.

    Could this cause "feedback" to the power supply and damage it?
     
  8. Not really. It would work but the 317 is rated at a max of 1.5 amps with
    heat sink. Depending on what current you need it could be to much. The 317
    will dissipate whatever voltage you are dropping across it times the current
    your using. IN any case the 317 is a voltage regulator and it is something
    you don't need. Even if you used an external transistor its overkill(The DC
    power supply probably already has the same circuitry in it for regulation
    anyways).

    Once you work out the details you need then you can just use either
    selectable resistors(Not sure the technical term but you can just use a
    rotary switch to insert resistors to increase the total resistance) or a
    power transistor(easiest way I think).

    The power transistor essentially just sits between the power supply and the
    hot wire and you can control the amount of current flowing into it by
    adjusting the base of the transistor. You'll still need to know the maximum
    current needed to heat up the wire to around what you'll be using.
     
  9. Guest

    Thanks for the replies.

    I guess what I need to do is take the 12V DC from the regulated power
    supply to a small circuit with a LM317 controlled by a variable
    resistor. That would allow me to put a meter on the output side to
    monitor the V to the heating wire.

    Could this create any kind of "feedback" to the power supply that
    might damage it?

    v/r
    Jerry
     
  10. Tam/WB2TT

    Tam/WB2TT Guest

    Along these lines he could hang a 1K potentiometer across the PS output, and
    use its variable output to feed a Darlington emitter follower (connected to
    a heat sink). With a 2 or 3 W pot, he could go as low as 100 Ohms and get a
    bit more linearity.

    Tam
     
  11. kell

    kell Guest

    Please don't use linear voltage regulation; it's inefficient and
    unnecessary.
    That would create lots of waste heat. You have a constant load so you
    could accomplish the same thing simply by having a chunky rheostat or
    fixed power resistor in series. You want to control a heating
    element, which simlifies things; you don't need voltage regulation per
    se, just some way to increase and decrease power. You have a constant
    load.

    I would use pulse width modulation for the power control. PWM control
    varies the power in your heating element by turning the power source
    on and off rapidly at a variable duty cycle. It takes time for a
    heating element to cool down between on/off cycles, so you can use a
    relativeley low frequency.
    You can build a fairly simple circuit with a duty cycle adjustable
    from zero (off) to 100% (continuously on) using a comparator and a few
    discrete components. I'm a hobbyist and was able to build a pwm
    heater control based on advice I got on the electronics newsgroups:
    http://groups.google.com/group/sci....c8614be7c8a?lnk=st&q=&rnum=2#4da90c8614be7c8a
    Any jelly bean comparator similar to the 2903 would work.
    I found nichrome wire at Surplus Sales of Nebraska.
     
  12. kell

    kell Guest

    I've been having trouble getting things posted, so if this is a double
    post forgive me.


    Please don't use linear voltage regulation; it's inefficient and
    unnecessary.
    That would create lots of waste heat. You have a constant load so you
    could accomplish the same thing simply by having a chunky power
    resistor in series with the load. You want to control a heating
    element, which simlifies things; you don't need voltage regulation per
    se, just some way to increase and decrease power. You have a constant
    load.

    I would use pulse width modulation for the power control. PWM control
    varies the power in your heating element by turning the power source
    on and off rapidly at a variable duty cycle. It takes time for a
    heating element to cool down between on/off cycles, so you can use a
    relativeley low frequency.
    You can build a fairly simple circuit with a duty cycle adjustable
    from zero (off) to 100% (continuously on) using a comparator and a few
    discrete components. I'm a hobbyist and was able to build a pwm
    heater control based on advice I got on the electronics newsgroups:
    http://groups.google.com/group/sci....c8614be7c8a?lnk=st&q=&rnum=2#4da90c8614be7c8a
    Any jelly bean comparator similar to the 2903 would work.
    I found nichrome wire at Surplus Sales of Nebraska.
     
  13. whit3rd

    whit3rd Guest

    The power per inch dissipated by a hot wire can be adjusted easily by
    (1) using a different wire diameter (fatter = hotter) or
    (2) using a different wire length (shorter = hotter)

    So if you start with a long wire that doesn't get hot enough,
    you can adjust by clipwire shunt that bridges part of your
    hot wire. If the apparatus seems too bulky, buy some thinner
    wire.

    Electronic controls are also possible, with some amusing
    variants (like using the temperature-coefficient-of-resistance of
    the wire to make a thermostat). Easiest is just to use a
    variac (variable autotransformer) on the input of your DC
    supply (this will work best if the DC supply is unregulated).
     
  14. Fred Bloggs

    Fred Bloggs Guest

    Build this, it is no more complicated than any add-on to your existing
    supply and is tailored for the job:
    http://www.nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm
     
  15. Guest

    Depends on what you want to do, but there are inexpensive ones already
    made with variable power supplies and all the tools required for
    cutting styrofoam..look at http://www.hotwirefoamfactory.com
     
  16. Guest

    True, but the idea was to have a little fun building something myself
    ( and hopefully learn something along the way). I was looking at
    several different designs but they basically have a bridge rectifier
    converting AC to DC then add the variable on top. I already have DC
    so I just wanted to add to it. It seemed the most logical thing was
    to build a "plug in" type device to put between the DC supply and the
    wire. The supply I have is a regulated 13V/20A supply. That is a
    pretty strong current. I didn't want to snap the wire with too much V
    and As.
    The wire I'm using is SS.
     
  17. rebel

    rebel Guest

    and now the key bit of information ...
    Your heating effect will be I*I*R and by not using nichrome (or other
    high-resistance) wire you are limiting your options somewhat. One problem with
    hot-wire foam cutters - and yes, i have successfully built some - is that the
    foam cutting pulls quite lot of heat off the wire and cools it. So you get a
    slowing of the cutting action and the cut doesn't remain clean unless you are
    actually controlling the temperature, which is quite difficult.

    For low-resistance wire types, you need to focus on two things.

    1. Use as thin a wire as you can, consistent with mechanical strength, to
    achieve a sensible resistance figure. If the wire becomes ductile at operating
    temperature, the wire is unsuitable.

    2. Use a current-control approach. How complicated that gets will depend on
    the wire.
     
  18. jasen

    jasen Guest

    12V should be fine with stainless steel (I've done that before),
    if it gets too hot use a longer piece.

    A regulator isn't needed.

    That stuff increases in resistance as it heats up so don't be fooled by its
    cold resistance

    Assuming the wire you have is about 0.5mm diameter about 1m is probably a
    good length for a first experiment, you'll probably want to go shorter in
    the end.

    If you want to regulate the temperature of the wire regulate the voltage.

    Bye.
    Jasen
     
  19. kell

    kell Guest

    If you use a wire with a significant temperature coefficient of
    resistance, you can use the resistance of the wire itself as a sensor
    to determine temperature. Use a Wheatstone bridge with the heater
    wire itself serving as the "unknown," or variable resistance.
    Use a comparator triggering a one-shot so that each time the
    resistance in the bridge reaches the point of balance, the one-shot
    will cause a brief lapse in the power applied to the bridge, turning
    it off for 100 milliseconds or some amount of time which works best as
    you may determine by experiment.

    While the wire remains below a certain temperature power remains on.
    As the wire gets hotter its resistance increases; at a certain
    temperature the resistance in the heater will will cause the bridge to
    balance -- the comparator will trip the one-shot, turning off the
    power for an instant, just long enough to let the wire cool a few
    degress. This cycle will repeat itself, and the temperature of the
    heater wire will cycle in a narrow range.
     
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