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Darlington transistor suggestion ?

Discussion in 'General Electronics Discussion' started by Externet, Jun 21, 2018.

  1. Externet

    Externet

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    Aug 24, 2009
    Hi.
    Looking for the least Vce at saturation for power switching, will conduct half Ampere Ic maximum. 12Vce maximum, PNP or NPN.
    Have a bunch of old non-convincing TIP121.
    Cannot use a mosfet; gate voltage too low to drive it.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Then do not use a Darlington pair as it will always have a comparatively high Vcesat (see 'disadvantages')-

    What is your drive voltage? What is the max. drive current you can provide?
     
  3. BobK

    BobK

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    Jan 5, 2010
    Use a BJT to switch a MOSFET.

    Bob
     
  4. Externet

    Externet

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    Aug 24, 2009
    Thanks.
    There is only 2.5V, but enough mA for the base current. Will look into a higher hfe transistor.
     
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    No. hfe is AC current gain, hFE is DC current gain when the transistor is an amplifier, not a switch and is NOT saturating. Most transistors saturate as a switch fairly well when the base current is 1/10th the collector current even if the hfe and hFE are very high.
     
  6. duke37

    duke37

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    Jan 9, 2011
    You might be better using a complimentary pair (npn/pnp) instead of the normal darlington configuration Then you may be able to get down to 0.5V drop.
     
  7. BobK

    BobK

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    Why don’t you tell us what current you need at what voltage drop? There are many solutions, and you seem to have fixated one the one that will not work.

    Bob
     
  8. BobK

    BobK

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    If you use a BJT to switch a MOSFET, and the switching frequency is less tham about 10 KHz, the BJT might need only microamps of base current.

    You can also find MOSFETs that will switch at 2.5V, though they might not handle your current requirements. Bit who kniows since you haven’t told us how mich current is needed nor how much current your 2,5V signal can source.

    Bob
     
  9. Externet

    Externet

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    Aug 24, 2009
    upload_2018-6-21_9-42-29.png

    upload_2018-6-21_9-50-36.png


    Would like to know who comes up with this trifle of which is upper case, often misused by even instructors... When Gain.dc or Gain.ac would say it...

    Well, I was after around 0.3Vce sat conducting less than 0.5A to energize a circuit powered by a nearly discharged lithium cell from about 10 to 20mA base current... Is that asking too much ?
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Have a look at this MOSFET: Vgsth <= 2.3 V.

    Or use a 2-stage architecture:
    upload_2018-6-21_17-12-50.png
    The only disadvantage here is that the circut is inverting: a high level (2.5 V) at the input will turn Q1 off. If you cannot compensate for this e.g. in the driving circuit (or its software), add a third stage in front of Q2 to make the circuit non-inverting.
     
    Externet likes this.
  11. Externet

    Externet

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    Aug 24, 2009
    :) Nice find... question... what is called logic level ? Are they generalizing ? Is it 5V logic; 3.3V logic; 2.5V... logic level ? Is it left to guessing for their product ? Shouldn't it be specified like "3.3V logic" ?
    Which brings another... what about level translator ICs handlig/performing from lower voltages ?

    upload_2018-6-21_13-37-11.png

    Like a level translator IC to drive larger loads ?
     
  12. BobK

    BobK

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    Harald's circuit is a level translator. Inverting in this case.

    Bob
     
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    Instructors and books are often wrong. I find descriptions of hFE and hfe in datasheets.

    When will everybody understand that the threshold G-S voltage of a Mosfet is when it BARELY turns on with a very low current that is so low that it is almost turned off??
     

    Attached Files:

  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Should be obvious from Ids ~ (Vgs-Vth)², shouldn't it? Unfortunatley: who studies transistor model equations anymore today?

    Without equaitions: that's what the graphs are for in a datasheet.
     
  15. Audioguru

    Audioguru

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    Graphs on a datasheet are for "typical" devices that you cannot buy, you get what is available. Devices with passing but minimum spec's are much worse and probably will not work if the circuit is designed with the graph numbers.

    When I design a circuit I look at all minimum and maximum specs so that everything I make works perfectly.
     
  16. Bluejets

    Bluejets

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    Oct 5, 2014
    Perhaps use a photovoltaic opto to drive the mosfet.
     
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