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Improvised power resistor

Discussion in 'Electronic Design' started by Thomas Carlan, May 27, 2007.

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  1. I know it is possible to make an improvised capacitor by using layers
    of mylar sheet (or glass) and metal foil.

    Is there any equivalent solution for a power resistor in the 20-50W
    range?

    Thomas Carlan
     
  2. Charles

    Charles Guest

    You can wind nichrome wire around a ceramic core.
     
  3. D from BC

    D from BC Guest

    I'm curious if anyone will mention how a power mosfet can be turned
    into an active resistor..
    D from BC
     
  4. Bob

    Bob Guest

    This would be more appropriate in sci.electronics.basics

    Depends what resistance you want and how much inductance
    and temperature drift you can tolerate.

    100meters of thin insulated wired on a drum typically has a resistance
    of
    a few tens of ohms but has signfiicant inductance. You can usually get
    access to both ends of a wire on a drum to connect drums in series or
    parallel.

    The lead from a graphite pencil is a few ohms end to end.

    Adam
     
  5. Depending on the value, salt water in a ceramic mug. Power capacity is
    strictly a function of how hot you will let the water get and the thermal
    transfer characteristics of the ceramic.

    Jim
     
  6. Sure 20-50W lightbulb.

    (So now 'what resistance', also note a lightbulb when cold has a 10 times lower
    resistance then when warm).

    Or you can wind your own resistors using special wire with a low temp coefficient.
     
  7. John

    John Guest

    I'm curious if anyone will mention how a power mosfet can be turned
    Probably the best way is to do a web search for "electronic load".
    Lots of info and schematics out there.

    -- remove SPAMMENOT for e-mail responses --
     
  8. Robert

    Robert Guest

    I heard of one guy who took one of those hollow tubular resistors and ran
    Surgical Tubing down the center of it then circulated water through the
    tubing and back to a large reservoir kept cold.

    Don't know how well it'd work.

    Robert
     
  9. Any kind of metal will work as a (generally low-value) resistor. Steel
    strapping, for example, but the resistance won't be very constant with
    temperature.


    Best regards,
    Spehro Pefhany
     
  10. MooseFET

    MooseFET Guest

    A long length of hookup wire.
    A light bulb.
    A couple of wires in a bucket of water.
     
  11. ^
    salt or hard


    Jim
     
  12. Robert Baer

    Robert Baer Guest

    Metal ribbon.
     
  13. Winfield

    Winfield Guest

    It's useful to put some of the story here on s.e.d.
    First, we'll make the MOSFET's current follow Ohm's
    law for a resistor, I = V/R, using a current-sense
    resistor Rs, and a resistive divider, R1/(R1 + R2).

    --+--------------+- V
    | |
    R2 |
    | __ ||-'
    +-----|+ \__||-,
    | ,--|-_/ |
    | |___________|
    R1 |
    | Rs
    | |
    --+--------------+---

    If we select Rs so that R = Rs (1 + R2/R1), and also
    R1>>Rs, we'll have made a nice active power resistor.
    It's necessary to add an RC to the opamp output, to
    prevent oscillation, as we've written about here many
    times before, but that's the basic idea. If the R1 R2
    voltage divider ratio involves a multiplying DAC, it's
    easy to create a digitally-programmable power resistor.
     
  14. MooseFET

    MooseFET Guest

    It doesn't matter much so long as it isn't super pure.
     
  15. Yes.

    Stainless steel safety tie wire works well, if you don't
    need very high resistance values. Supporting the wire so it
    can heat air or water without shorting is more trouble than
    just making resistance.
    e.g.
    http://www.stainless-rebar.com/american.htm

    What range of resistance are you needing, and energy at what
    frequency do you need it to absorb?
     
  16. D from BC

    D from BC Guest

    I've been using a 50W tubular resistor as a test load recently..
    I got it dissipating 100Watts..so I keep it in a bucket of water...
    D from BC
     
  17. D from BC

    D from BC Guest

    Cool! Looks like a great way to dodge buying power resistors.
    D from BC
     
  18. D from BC

    D from BC Guest

    I liked all the suggests posted..wires, ribbons, ionic liquid, bulbs
    ...etc..
    But I gotta add one obvious wacky suggestion. (IFAIK..not posted
    yet..)
    Using lots and lots and lots of 1/4W resistors together!!!

    D from BC
     
  19. Robert

    Robert Guest

    How do you insulate the electrical connections from the Water?

    Robert
     
  20. D from BC

    D from BC Guest

    Pure water is a poor conductor. All my connections to the power
    resistor are bare..
    I put it in a plastic bucket.
    The only thing bad that happened was a little rust and there's steam.
    D from BC
     
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