Transistor Trouble: base-emitter voltage ~0

Discussion in 'General Electronics Chat' started by abelcorver, Apr 29, 2010.

  1. abelcorver

    Thread Starter New Member

    Apr 10, 2010
    14
    0
    Hey everyone,
    I'm very new to electronics... Some time ago I posted some questions here regarding transistors... I got my basic flip-flop to work, however, I want to make a circuit which will allow me to toggle the two LEDs, just by pushing a button one time.

    However, once I press the button, the base-emitter potential difference is ~0, or at least very (i.e. too) low for my circuit...

    [​IMG]

    [The brown dots indicate some spots where the voltages are all the same (~700 mV)]

    Here is the Yenka file, in case that'll help:
    http://www.abelcorver.com/files/forums/problem_0.yka

    Any help is greatly appreciated! Thank you! :D

    Regards,
    Abel.
     
  2. wannaBinventor

    Member

    Apr 8, 2010
    179
    4
    Not completely sure.... I'm a newb myself, but I think you are getting 0 volts at that one emitter because there is no load between it and ground.
     
  3. abelcorver

    Thread Starter New Member

    Apr 10, 2010
    14
    0
    Thanks wannaBinventor. Yeah, true, sorry if I didn't explain my problem properly, but the problem with that transistor is the (24-0) mV base voltage, which isn't enough to make the transistor conducting.

    The other transistors even have a base-emitter voltage of 0! :confused:
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    You don't have a resistor in line with the base of the lower left transistor.

    The B-E diode effect is then causing the 700mV reading.

    Likewise the upper right transistor (of the lower half).
     
  5. abelcorver

    Thread Starter New Member

    Apr 10, 2010
    14
    0
    Aah thanks!! I seems to work great now! :D
    I still don't really understand all this though.
    Wasn't there a 10 kOhm resistor on the left in the very top row of resistors? So electricity had to pass through that one, right?
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56

    The top right transistor is connected to the LED and has the 10K resistor at its base to turn it on.

    But the lower transistor has it's base conected to the base of the top transistor, and has it's emitter conected to neg. supply, so the base emitter of the bottom transistor is acting like a diode SHUNTING the led and transoitor at the top so the top transistor has little current to its base as well as steering current away from the LED itself, because of the shunting effect.

    When that transistor is conducting it should have a voltage drop at its base of (Vled + Vbe)..respect to ground.
     
  7. abelcorver

    Thread Starter New Member

    Apr 10, 2010
    14
    0
    Hey,
    thanks for your explanation. I understand the problem now.
    So, the circuit seems to work, well, at least once every 10-20 times
    I press the button :p So there's still something wrong, and I don't understand what it is. There's probably something wrong with the resistance values. (I pretty much suck at dealing with resistance values -.-' :) )

    Here's the new Yenka file:
    http://www.abelcorver.com/files/forums/problem_1.yka

    Regards,
    Abel.
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    Just looking at the top portion of your circuit alone, which is the flip flop part.

    FF1.jpg

    You have your feedback (cross coupling), that will produce the state changes.

    Q3 cutsoff Q1 and Q5 cutsoff Q2, for the output portion, and Q3 and Q5 crosscouple to eachother for the state change.

    However Q2 base voltage comes thru R2, but also you have Q3's base voltage coming thru R2 as well, this is not very stable, because the base current of Q2 thru R2, could be enough to upset the base voltage of Q3, allowing it to conduct less than whats needed to ensure complete cutoff of Q1 and Q5.

    So the base currents of both Q1 and Q2 determine to what extent the base voltages would be to Q3 and Q5.

    Better to isolate the base voltages of the seperate transistors thru diode steering networks.

    Also the outputs on the emitters are not as stable for proper operation as well.

    A flip flop is ususaly designed as two inverters crosscoupled, with the output connected to the collectors to get full supply voltage to the leds, (outputs)
     
  9. abelcorver

    Thread Starter New Member

    Apr 10, 2010
    14
    0
    Hey,
    sorry for the late reply, but school kept me quite busy the past few days. Thanks a lot for your detailed help!

    I think I understand what you're saying, but I'm not sure what you mean with "diode steering networks". I also don't understand why the emitters aren't stable.

    I'll be on holiday until next monday, so I won't be able to reply until then.

    Thanks a lot in advance!!

    Regards,
    Abel.
     
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