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how to get more amps to my lamps?

Discussion in 'Electronic Basics' started by Alex Bryant, Aug 31, 2004.

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  1. Alex Bryant

    Alex Bryant Guest

    I have a series of mini-bulbs (#44 lamps, like the kind used in
    pinball machines) underneath a platform in my project. My
    microcontroller hits various transistors in order to get the 6 volts
    to the appropriate lamp. Everything works, but the lamps that are
    farthest away from the power source get their current from small wires
    about 6 feet long, and hardly glow at all. I need all these lamps to
    glow their brightest, to be seen through a series of clear plastics.
    Because of the way I've built this whole thing, I can't get the power
    source (4 AA batteries) much closer to the bulbs. Would thicker wire
    be the answer?

  2. How thin is the wire? I think you have something else wrong.

    The resistance of a #44 lamp, when illuminated, is about 25 ohms. Your wire
    would have to be microscopically thin to have a resistance comparable to
    that in a 6-foot run. Check voltages at both ends.
  3. Alex Bryant

    Alex Bryant Guest

    Well, it's not microscopically thin, but it's close...standard red
    Radio Shack project wire. I'm on the road at the moment and can't
    check the voltage, but maybe I'm implementing the transistors
    wrong...Would the rating of the resistor going to the base of my
    transistor, affect how much current goes out the emitter to the lamps?

  4. John Fields

    John Fields Guest


    Depending on how much current comes out of your micro's IOs when
    they're feeding the base of the transistor, figure the drop across the
    base resistor plus about 1.2V and subtract that from the supply
    voltage. What's left over will be the voltage into the lamp(s).

    What you should do is use the transistor for a low side switch, like

    The value of R will depend on the gain of the transistor and how much
    current your micro can source, but for a #44, which takes 250 mA at
    6.3V and a transistor with a beta of at least 100 with 250mA going
    through it, your IO will need to supply 2.5mA, so that's about 2000
    ohms. Regardless, you should make sure your transistor goes into
    saturation so the power it dissipates (which will be 'stolen' from the
    lamp) won't cause excessive dimming. Or, use a logic level MOSFET and
    forget about having to worry about base current at all.

    The other problem might be that you're wiring your lamps like this:

    +6V-----+- - - - - -+
    | |
    [LAMP 1] [LAMP n
    | |
    GND-----+- - - - - -+

    When, if you're using thin wire, you should be wiring them like this:

    +6v------------+ |
    | |
    +6V--+ | |
    | | |
    [LAMP 1] [LAMP 2] [LAMP n
    | | |
    D | |
    IO>-G | |
    S | |
    | | |
    GND>-+ | |
    D |
    IO>-----------G |
    S |
    | |
    GND>-----------+ |
    That way, the voltage drop in the wires caused by the current draw of
    each lamp will be restricted to its own loop and won't affect the
    other lamps.
  5. Quite possibly. Can you post the circuit diagram on your web site and refer
    us to it?

    Also, your wire is nowhere near thin enough for its resistance to have an
    appreciable effect.
  6. Alex Bryant

    Alex Bryant Guest

    I have posted a schematic of this setup at:

    Please let me know if there is a better way to set this I'm a
    novice, any feedback would be appreciated!

  7. John Fields

    John Fields Guest

    1. Make sure the transistors can take 300mA of collector current
    without coming out of saturation.

    2. Connect one side of each the lamps to +6V.

    3. Connect the other side of each lamp to the COLLECTOR of its

    4. Connect all of the transistor EMITTERS to -6V.

    5. Replace the 10K resistors with 330 ohm resistors.

    That should do it
  8. Please let me know if there is a better way to set this I'm a
    Put the lamps in the collector lead rather than the emitter lead of the

    As it is, you are using the outputs as emitter follower voltage regulators.
    The lamps do not get full voltage. They get 0.6V less than the output
    voltage of the PIC.
  9. Rich Grise

    Rich Grise Guest

    And invert the logic of your outputs.
  10. Rich Grise

    Rich Grise Guest

    Wrong - never mind.
  11. Alex Bryant

    Alex Bryant Guest

    I now run the lamps in the collector lead, BEFORE the transistors in
    the circuit path, and have replaced the random resistors that were in
    the breadboard with new 220 Ohm resistors. Also, I noticed that some
    of my transistors simply didn't work...I bought a brand new pack from
    Rad Shack and replaced the transistors for the lamps that weren't
    lighting, and they all started working like a charm. My 6V lamps are
    now getting over 5.5V, which is plenty for me.
    This is just a proof-of-concept version (translation: very
    sloppy, no points for neatness, just make it work), so my next step
    will be to clean up all that wiring, eliminate some of it, hopefully
    replace a good bit with a 40-pin ribbon cable.
    I still have a few weeks till the 'fun' phase of the project
    begins. I don't know whether anyone's interested, but I'll post some
    pictures, once I've brought together the functionality from the
    different design phases.

    Thanks to everyone for the help and advice....

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