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yanpsq (yet another newbie power supply question)

Discussion in 'Electronic Basics' started by Rusty Wright, Nov 9, 2004.

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  1. Rusty Wright

    Rusty Wright Guest

    I have a solderless breadboard, multimeter, etc. on order, getting
    ready to dink around with some simple stuff to learn electronics.

    I don't understand how to set up the power to the breadboard, for
    example, if I'm using a 9v battery and I need 5v. On the surface it
    seems like I could use a simple voltage divider, with 100 ohm and 80
    ohm resistors in series which would give me 4v across the 100 ohm
    resistor and 5v across the 80 ohm resistor. But then when I start to
    connect things they'll be in parallel with the 80 ohm resistor and
    that will lower the resistance and change the voltage drop. So that
    doesn't seem like a good plan.

    Or what if I want to use some wall wart I found in the dumpster? I
    have one that's 20v.

    Or should I be thinking about using a voltage regulator like the
    LM2940T?

    Confused and lost in newbie land,
    Rusty
     
  2. Yes, you should invest in a voltage regulator. But an adjustable one
    will be more versatile. Look for an LM317. It can be programmed with
    a pair of resistors for any voltage between 1.2 volts and 2 volts less
    than the input.

    Check the data sheet for the capacitors required to guarantee
    stability and for the formulas for the resistors.
    http://www.national.com/ds/LM/LM117.pdf
     
  3. No - you'll get the higher voltage across the higher value resistor -
    remember Ohm's Law, and the fact that the current is the same in both
    resistors.
    Yes, it isn't :)
    It would be best to use a wall wart and a voltage regulator -
    otherwise, you'll go through a lot of batteries (and with batteries,
    you'll still need a voltage regulator.)

    You should look for a wall wart that will produce 3 - 4 volts above
    the regulated voltage you want - otherwise, the regulator will get hot
    dissipating all the extra voltage.

    Even better that a random wall-wart and regulator would be a regulated
    5 volt power supply

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  4. BobGardner

    BobGardner Guest

    Or what if I want to use some wall wart I found in the dumpster? I
    ========================
    Everybody builds a couple of power supplies at some time. The 20V wart would
    run a 5v regulator no prob. BUT... just go buy a regulated 5v supply... use
    this to run your projects. You test equipment needs to be working so you dont
    need to fiddle with that to get the other stuff working.
     
  5. Rusty Wright

    Rusty Wright Guest

    Thanks for all the help. We're moving our offices to another building
    and people are throwing lots of stuff out; I now have several wall
    warts and some are even 5v. I got two that were for external scsi
    drives and use that round Mac ADB connector (which I'll just cut off)
    and they put out both 5v and 12v. I'm ready to start cooking. Now if
    only my breadboard and stuff would arrive...

    Here's another question; for these dual output wall warts, what's the
    best way to figure out which wires are which? I.e., is it safe to
    just put the leads of my meter on 2 wires and see what it says? Or
    should I put something like a 1k a resistor in series or parallel?
    Should I expect it to have separate ground wires for each voltage or
    one that's common?
     
  6. Good thinking, the 1k resistor. But even better is knowledge about how a
    multimeter works and how to use it.

    A voltmeter needs to have a very high inner resistance, so you can
    measure voltages without influencing the circuit you are masuring.

    An amperemeter needs to have a very low inner resistance, because you use
    it by inserting it into a circuit and masure the current throughthat part
    of the circuit.

    A resistance-meter works in a third way, it puts a voltage, usually
    200mV, over a component, and measures how much current it can push
    through it. That is how it finds out how much resistance the component
    does against the current.

    For voltmeters and ampmeters there is also a choice of DC and AC
    measurement.

    A multimeter combine all these instruments into one unit.

    If you happen to set the MM to current and connect it to the mains it
    will be destroyed, or at least you will have to replace the fuse inside
    the MM. Work only with battery voltages until you know well how the MM
    works. Read a manual on the usage of a multimeter carefully.

    You can find out what function the output connections of a wallwart have
    by plugging it in an carefully explore what voltages, both AC and DC,
    there are on every connection.

    With the wallwart disconnected from the mains you can test what
    resistance there is between each possible pair of outputs. This will tell
    you which wires are in contact with each other.
     
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