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Goofing about with another transistor question help if you can please. — Parallax Forums

Goofing about with another transistor question help if you can please.

whiteoxewhiteoxe Posts: 794
edited 2009-08-10 17:08 in BASIC Stamp
I grabed this picture, of some site and thought it would be simple to work out, however I can;t make sense. Does the second 1k resistor to 0V decrese the base current ? If it was meerly a pulldown i would have thought a 10K resistor would do thre job.
I calculatedc thbe· base current v/1k = 9mA ,
As the· base current is supposed to be 1/10 of collector i know this must be wrong as there is only a LED with a 680 Resistor drawing collector current


ANY Help Mike?· Can you show how this diagram is working for an NPN Transisstor.?

Post Edited (thewhiteoxe) : 8/10/2009 9:03:44 AM GMT
npn transistor.gif
457 x 370 - 8K

Comments

  • LeonLeon Posts: 7,620
    edited 2009-08-10 10:39
    What is the hFE of the transistor?

    Leon

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-08-10 15:47
    This circuit is a little strange in the sense of where the switch is located with reference to power.

    The way that it is, it would 'work' without the transistor and the two 1K resistors if you tied the LED cathode to GND or 0V

    Since the complete schematic is not there, it's hard to make sense of the original intentions of this circuit. The second resistor you mention does not appear to be connected to the base resistor of the NPN. If I were to guess, the Switch is just a means for turning the main power on, and you still need to supply a positive signal to the base resistor in order to turn the LED 'on'. In which case the resistor to 0V and the switch doesn't do anything other than to apply an additional load to the main power supply.

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    IC Layout Engineer
    Parallax, Inc.
  • phil kennyphil kenny Posts: 233
    edited 2009-08-10 17:08
    thewhitefox said...
    As the base current is supposed to be 1/10 of collector i know this must be
    wrong as there is only a LED with a 680 Resistor drawing collector current

    The collector current will be beta times the base as long
    as the transistor is operating in the linear region and isn't saturated.

    The base current determines the collector current in the linear region.
    Not the other way around as you have phrased it.

    In this case the collector current is (9 - 1.5 -0.5)/680 or roughly 10 ma.
    I used 1.5 volts for the LED forward drop and 0.5 volts for V_CE_sat.

    The base current will be about (9 - 0.7)/1000 or 8.3 ma.

    Therefore, the transistor is probably saturated and you can't expect
    the collector current to be 10 times the base current.

    phil

    Post Edited (phil kenny) : 8/10/2009 5:18:03 PM GMT
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