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Transistor and Voltage Comparator Question

Discussion in 'General Electronics Discussion' started by necrow, Nov 14, 2012.

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  1. necrow


    Nov 14, 2012
    For my project, I need to evaluate two voltages and output the higher one. What I was originally thinking was using an LM741 (I have them on hand so this would be ideal) with a reference voltage set and another voltage (that changes from time to time) as the other voltage. I would also connect a forward facing diode after the output so the negative (low) output of the LM741 when the "wrong" voltage is higher would be 0. I would connect the ouput to a transistor that would fully saturate when the voltage was high and also to a not gate and connect the correct input voltages. Does this make sense in theory?

    Also, I'm having some trouble saturating the transistor I'm using. I don't have it on me but I've heard reports that it's a PNP not an NPN so I'll check that ASAP. If, though, I was using a transistor like this one: for example, what would I need to do to saturate it? I'm a bit confused reading the specs and applying them to my application. What current would I need to apply to the base, and is there a certain minimum or maximum amount of current I need to the collector?

    Any replies are greatly appreciated!! If the circuit description is too confusing I could attempt to draw it out for you guys.

    Also, if there is an easier way of doing this, please let me know!

    Edit: I could also use an LM339 and connect one comparator with the reference voltage as the actual reference and then another comparator with the reference voltage switched, and then connect a transistor to each so one would be activated depending on which voltage was higher. Yes?
    Last edited: Nov 14, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It would be easier if you showed us your schematic, and also if you gave us the full question from your assignment rather than paraphrasing the bits you think are important.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This could be a starting point:
    There may be much simpler solutions.

    Note that this circuit demonstrates the principle. You will have to be able to explain how it works and what every component does.
    You may also have to adapt the component values.

    Note also that this specific example works for positive voltages only.


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