VRE < 1Volt?

Discussion in 'The Projects Forum' started by rougie, Oct 16, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Hello,

    I have been told that the voltage across an emitter feedback resistor should be at least 1V. Please view the following attachment which shows an Re resistor of 15 ohms with around 0.16 Volts across it. Ic can vary from 0 to approximately 11 ma while the base voltage will vary from 1V to 0.5VDC.
    Most of the rest of the voltage drop will be across the ce junction.

    The thing is that I have swapped transistors with different betas ex:

    100, 112, 155 and 165 and the only difference I would get for Ic was a 100 to maybe 500ua amps...
    which really doesn't change the Ic current much.

    In this case, is having less than 1 Volt across the Re acceptable?
    Or am I running after trouble if I keep it this way?

    All help really appreciated!
    r
     
    Last edited: Oct 16, 2012
  2. crutschow

    Expert

    Mar 14, 2008
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    It depends upon what type of circuit you are trying to build.
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
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    When the base is at 1 volt, then I can drive the led with
    around 11ma and as the base ramps down to 0.5vdc
    the led is driven proportionally with a downwards current
    until led goes off hence 0 ma.

    The 1.0 volt input actually comes from the circuit I posted in my last thread.
    In any case, I have provided the full circuit here...

    I don't see the harm for Vre being less than 1V since t2 is saturated when T1's Vc is 1.0V.
    Ib T2 = 1/10th of Ic T2!!!


    thanks for all feedback.
     
    Last edited: Oct 16, 2012
  4. Audioguru

    New Member

    Dec 20, 2007
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    You were lucky to find transistors with the same base-emitter voltage. But they are all different as shown on the datasheet.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Rougie, you are obsessed with trying to drive a nail, and the only tool you will use is a screwdriver. When you want to drive a nail, use a hammer.
    If you study a little, you will understand the technology behind the hammer, then you can use it successfully.
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    But if I raise Vre to 1V, Ic of T2 will be less than 11 ma!!!!

    confused!
     
  7. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    believe me, I am trying!
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    But your latest circuit does not have an Re so we don't know what you are talking about.
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Audioguru,

    T2 has an Re! The 15 ohms!

    See diagram here in this attachment!

    I have a circuit using two transistors. As it is, Vc of circuit T1 outputs a 1.0V. As Vs ramps up from 2.0VDC to 3.3VDC, Vc of T1 will ramp down from 1V to 0.5 Volts. When the base of T2 sees 1.0 V, the infra-red led has 22 ma (measured 0.31V as VRe) going through it (led fully on). When T2 sees 0.5 VDC at its base the infra-red led will draw 0ma (led fully off).

    The circuit works very well here.... I am just wondering if the fact that Vre (the 15 ohms) is not 1V, given the actual circuit, do you guys see a problem with this ??
     
    Last edited: Oct 16, 2012
  10. Audioguru

    New Member

    Dec 20, 2007
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    I think your problem is that the supply voltage and the variable VS voltage are so low as to be very close to the 0.5V to 0.8V base-emitter voltage of a transistor (every transistor is different).

    Try your circuit with increased supply and control voltages of 10V or 15V (then decrease the currents the same).
     
  11. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Yes but the circuit works very well. I don't see any problems... If you see a problem, I would like to know what it is so I can correct it! All I am asking is if it is okay that Vre is less than 1V?

    If I have 1V at the base of T2, then its 1V - (aprox: 0.7) = 0.3VDC. This gives me an Led current is about 20ma. (Earlier I *measured* 11ma but as we said, measuring is wrong due to internal meter resistance).

    Others form other forums see nothing wrong with a circuit like this... I am still pretty new to this... so I am just insecure and would like to know if the 1 Volt drop across Re dogma is really required in my circuit.

    The circuit works... I am just seeking an approval!

    thanks for your reply!
    r
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    If you are making only ONE circuit then yours works so it is OK if you do not change its temperature.
    But if you make many of them then it only works properly when the base-emitter voltage of the transistor is close to yours.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Rougie, I think you have been misled about the voltage across the emitter resistor being 1V. That is for a specific circuit and application where a certain amount of negative feedback is desired.

    In your case, your concern is setting the current drawn by the LED. Any resistor in series with the LED and transistor is there to limit the maximum LED current. Adjust the resistor accordingly and do not be obsessed with the voltage across the emitter resistor.
     
  14. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Thank you so much Mr. Chips
    r
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    I have not read through the complete thread but normally one would connect the emitter of the NPN BJT to ground and put the current limiting resistor in series with the LED on the collector.
     
  16. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Well, it might be several circuits. But Audioguru, I have here about a dozen or so pn2222 in my stash... I swapped them in and out of my circuit (for T2) and they all seem to have approximately 0.6 to 0.7 Vbe's.

    Are you saying that I might get a batch with Vbe's of 0.2V or 0.3V or 0.8V or 0.9V ???

    Also, these transistors are pretty volatile even when we use collector feedback configurations... I have swapped transistors in and out for T1 and Vc is never perfectly 1.00VDC!

    In any case for this reason, I planned for a compensation action by varying Vs so Ic of T2 will always come close to 12 ma regardless of the Vbe or T1 discrepancies.

    I am starting to understand that the whole circuit is relative to Vs. I could probably also vary R2 of T1....

    Thanks for your help!
     
    Last edited: Oct 17, 2012
  17. MrChips

    Moderator

    Oct 2, 2009
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    Is your goal to keep the current at a fixed value? Then you need a constant current source circuit.
     
  18. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    2
    Hi Mr. Chips,

    Yes, I see what you are saying. But about a month and a half ago, when I started transistors, many fellows instructed me that if I want to make several circuits like this, the beta would vary form one transistor to the other and one way to stabilize this was to use one of the negative feedback configurations such as emitter, collector or voltage divider biases. For T2, I selected the emitter feedback bias configuration.

    Now, if I remove the Re resistor in T2 and put it at the collector, isn't that going to remove the stability that the negative feedback emitter bias resistor would provide? And as you mentioned, why is it that my circuit (more precisely T2) would not fall into the criteria where it would be a specific circuit and application where a certain amount of negative feedback is desired. A little confused on this.

    Nope, Ic of T2, will always ramp up from 0 ma to approximately 12 ma and then start over at 0 ma and ramp up again to approximately 12 ma... and always continue this way!

    The whole idea here, is that Vs ramps down from 3.3V to 2.0V, respectively, Vc of T1 will ramp from approximately 0.5VDC to 1VDC. However the 0.5VDC at Vc of T1 will obviously be governed by (Vbe+Vre) of T2. So it might end up that Vc of T1 may have ramp values of perhaps 0.6V to 1VDC or 0.64 to 1VDC ... depending on what Vbe+Vre is for that transistor !
    Now, when the Vc ramping signal of T1 is felt at the base of T2, the led's current will continuously ramp from approximately 0 ma to 12 ma.

    The circuit works really well, its just that this is pretty much my first project I am doing with transistors and its pretty stressful cause I am afraid I am doing something that may fail in the future!

    Your help is much appreciated... thanks
    r
     
    Last edited: Oct 17, 2012
  19. Audioguru

    New Member

    Dec 20, 2007
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    Correct. But the OP is a nOOb who knows nuttin' and puts resistors any old place.
    Maybe he wants the LED to fade in and out instead of switching abruptly on and off.
     
  20. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    I used the word "ramp" over and over... I don't know how else to explain it to you anymore!


    And from now on I am sure you can be a little less insulting with your choice of words... Do we understand each other?

    r
     
    Last edited: Oct 17, 2012
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