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low volt drop tranny

Discussion in 'Misc Electronics' started by exxos, Jul 30, 2003.

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  1. exxos

    exxos Guest

    Hi all,

    Does anyone know of any transistors which have little voltage drop across
    them ? I am currently using a TIP132 ( 5A job ) on a 12V feed though after
    it switches on I only get around 10.5volt out max. I wonder if there are any
    better versions out there which less volt-drop across them ??

    TIA,
    Chris
     
  2. scada

    scada Guest

    Use a MOSFET
     
  3. Jim Thompson

    Jim Thompson Guest

    TIP132 is an NPN Darlington so you're basically seeing a drop equal to
    two Vbe's.

    A PNP Darlington would improve to a Vbe plus a Vsat, but as "Exxos"
    pointed out a MOSFET is your best bet.

    ...Jim Thompson
     
  4. exxos

    exxos Guest

    already tried, it seems you need a gate voltage of say 10volts to get a
    conduction votlage of just 8 volts across it.
     
  5. Jim Thompson

    Jim Thompson Guest

    Use a P-Channel device. Do you have a hang-up for N-type devices ?:)

    ...Jim Thompson
     
  6. Boris Mohar

    Boris Mohar Guest

    Than you are doing it wrong. Gate voltage with respect to what?
    What is "conduction voltage" ? You can get a mosfet with few milliohms
    of on resistance. Just how much voltage drop will that cause?
     
  7. CWatters

    CWatters Guest

    If the TIP132 is a darlington then the loss you are seeing is because the
    emmitter voltage must be less than 2 x 0.7 =1.4V below the base or it turns
    off.

    If you change the TIP132 to a PNP device you should get that "drop out" down
    (to about 1V if I remember correctly =Vbe+Vsat). If you use a PNP device the
    you also need to invert the signal driving the base (perhaps using a small
    NPN transistor).

    A P-Channel FET would reduce it further. You need one with a gate threshold
    (Vgt) of around 6V or less and an ON-Resistance of...

    Ron=Von/5A eg a 0.5V drop requires Ron < 100mOhms and a 0.05V drop <
    10mOhms

    You could also use an N-Channel FET and a charge pump to produce a gate
    voltage of more than 12V + Vgt but that's getting messy.
    Colin
     
  8. exxos

    exxos Guest


    With a 12VDC input to gate and drain you only get 9VDC output from a W20NB50
    ( only use as it was the only fet I had to hand )

    chris
     
  9. exxos

    exxos Guest


    I didn't need a darlington so I used a TIP41 which has the volt drop down to
    about 0.5volts, a lot beter. I've not had much luck with the fets so far. A
    pump could work though as you say its getting messy. I wonder if theres any
    reasonable germanium trannys still out there, 0.5 drop for a TIP41 is near
    enough I guess,

    cheers,
    Chris
     
  10. CWatters

    CWatters Guest

    This is the sort of circuit we are trying to get you to use (best I could
    find without drawing one myself)...

    http://www.rason.org/Projects/transwit/transwit.htm

    See Fig 1

    Note that a 2N3906 is not the right device for you as it won't take the
    power.

    Use a power transistor with a low Vcesat or better still a P type FET.

    Colin
     
  11. It _has_ to have at least a volt or more drop because it's a
    Darlington transistor! Two emitter to base junctions at .6 V each.
    So get rid of the single transistor and use two separate transistors,
    with the second transistor's base driven by the first transistor's
    emitter. Don't forget to include a current limiting resistor in
    series with the base. The first transistor's collector goes directly
    to the positive supply, _not_ to the collector of the second.

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  12. exxos

    exxos Guest


    I know this is what everyone is saying and see the diagram though I dont
    think the PNP will give any better results than the NPN im using, both still
    have volt drop. I guess we could argue over a few milivolts though I dont
    see anything speical towards a PNP. I even tried one and can't say theres
    much noticeiable difference, well apart from the circuit works backwards
    with a PNP ;-)

    Chris
     
  13. Jim Thompson

    Jim Thompson Guest

    Sigh!

    ...Jim Thompson
     
  14. Boris Mohar

    Boris Mohar Guest

    Did you conclude from that experiment that it is not worth pursuing
    using a more suitable device as others have suggested?

    Take a right hand glove and try it on your left hand. Eventually you will
    manage to cram it on but it will not feel comfortable. Now turn it inside
    out. Fits little better but still not quite right. All the fuzz is on
    the outside.

    Use a P channel mosfet.


    --

    Regards,

    Boris Mohar
    Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
    Aurora, Ontario
     
  15. John Fields

    John Fields Guest

    ---
    Since you've implied that you're using a high-side driver,

    +12
    |
    |
    C
    +12--[R]---B NPN
    E
    |
    [RL]
    |
    GND


    what that means is that as the voltage across the load increases, the
    emitter voltage will also increase and the base voltage will have to
    rise to one Vbe drop above the emitter in order to keep the transistor
    turned on. If you have a 12V supply connected to the collector and you
    drive the base to 12V, then the load _has_ to be at least one Vbe below
    12V in order to allow the transistor to be turned on. If you use a PNP,
    then you're essentially creating a low-side driver because the load will
    appear between the collector and what the PNP considers to be its
    supply, ground.

    +12
    |
    |
    E
    GND--[R]---B PNP
    C
    |
    [RL]
    |
    GND

    Now, with the PNP driven into saturation by driving its base more
    negative than its emitter, only Vce(sat) will keep the voltage across
    the load from rising to the supply voltage.


    OTOH, If you're using a low side driver:

    +12
    |
    [RL}
    |
    C
    +12--[R]---B NPN
    E
    |
    |
    GND

    then you've got other problems.
     
  16. Andre

    Andre Guest

    Cool ! :) Any chance of a schematic (or is it just a lot of 1 Amp FETs
    in parallel ?)
     
  17. CWatters

    CWatters Guest

    Not my design. You can buy them from...

    http://www.kontronik.com/

    They are used to power competition electric model aircraft.

    See the "Beat FAI 24" as an example of the top of the range.
     
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