Newbie transistor switch help

Discussion in 'The Projects Forum' started by Mutad0r, Apr 6, 2010.

  1. Mutad0r

    Thread Starter New Member

    Apr 6, 2010
    10
    0
    So basically I created this: THIS
    The regular swich one ( second set of drawings )

    My version has the switch very far away ( about 20m ) and the lamp with the battery and the transistor are closeby to eachother. I thought to use this transistor thingie version as I figured I wouldn't lose any amps (or whatevers) but when the button is pressed, the lamp is only faintly lit, as if the transistor didn't do any difference whatsoever... anyway... help... or show me a circuit that would work for me; a lamp that lights when I press a button about 20m away..

    Thank you.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What transistor are you using?

    What is the lamp's voltage and wattage rating?

    Battery voltage and AH rating?

    What gauge wire are you using?
     
  3. flat5

    Active Member

    Nov 13, 2008
    403
    17
    You may have damaged the transistor because of too much current going into the base of the transistor.

    Have you tried to operate the lamp without the transistor? If the wire size and type is adequate
    you won't need the transistor.
    What wattage and voltage spec. is the lamp?
    What transistor are you using?
    What voltage is powering the circuit?
    What current is the power supply capable of providing?

    edit: I write so slow someone always beats me to the draw :)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Flat5,
    No, not too much base current. That circuit is simply a poor design. The transistor will never become saturated, as I explained in another thread (forum feedback).

    As base current increases, collector current increases until Vce is around 0.7v, which is around the Vf of the BE junction. At that point, the transistor is "stuck" with a Vbe of around 0.7v, and high power dissipation will be the result.

    Incandescent lamps have very high inrush current until the filament gets up to temperature. This exacerbates the problem, as the lamp will seem like a dead short.

    The base needs a current-limited supply that is 1/10 the collector current.

    Using a lamp rated for 1/10 the wattage of the main lamp as a base current limiter would be a very effective match.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Here is what I was talking about in my last post:

    [​IMG]

    L2, being rated for 1/10 the wattage of L1, makes a very good match to ensure that the transistor is saturated during start-up, and base current is reduced as the lamps warm up.
     
  6. Mutad0r

    Thread Starter New Member

    Apr 6, 2010
    10
    0
    First off, I DID damage the transistor, as in it does not work anymore.
    The transistors I'm in posession of are all of russian origin ( and old, but I got a box full of them ), so I don't know any specs of the transistors I have, even the internet doesnt.

    The wire I'm currently using is a one used to connect speakers to wherever the music comes. can't tell much more about it. it's not too thick though. I'm not a wire expert.

    don't know the wattage nor the volts for the lamp, not written on it.

    THe power supply is.... a hacked laptop battery... I hacked a laptop battery and it had 6 smaller ones in it. 4 could not hold a charge, so I trashed those... so, I'm using 2 of those and my ohmeter ( or whatever it's spelled like ) says that it's a wee bit over 7 volts, not sure about amps.


    ... and the hell is saturation?
    All you proffessionals make me feel like a reaaaaaaal newbie -.-
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    OK, so basically we are as clueless as you are about what you're dealing with - except you have a battery that puts out 7v.

    With BJT's (bipolar junction transistors), "saturation" means that at a certain point, increasing the current through the base will not result in an increase of current flow through the collector.

    As a general "rule of thumb", to ensure that a transistor is saturated, Ib (current through the base) is 1/10 the desired Ic (current through the collector).

    Sometimes, you can "get by" using 1/20th or 1/30th Ic for Ib, but if you want to guarantee saturation, you go with the 1/10 Ic figure.

    At some point in time, everyone was a "n00b", and were pretty clueless about electricity/electronics. For those of us who have been in the field for awhile, many "n00b" questions are repeated over and over again - but we try hard to remain patient.

    You might try your experiment again, using bulbs with a known voltage and wattage rating - and a transistor with known parameters, such as a 2N2222.

    Right now, you have so many unknowns that it will be difficult to make progress.
     
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