Switch (2A) using 3.3V as input

Discussion in 'General Electronics Chat' started by alan1975, Mar 6, 2012.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
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    Hello,

    I need to switch on or off device (2A) 4-5v using 3.5V as logic input.

    I have LM339 i drive it using 4 battery nimh, 4.8V, but i allow to discharge that device to 4V.

    LM339 output (as comparator) is (VCC - 1.5v) = that's mean if i input 4.8v i get about 3.3V.

    I try connect that to gate BUZ11 N-FEET transistor, but without success.

    Problem is i can get only small amount of current, under 0.5A by BUZ11 !
    When i attach gate to VCC 4.8 i can get without problem 2A.

    In my opinion (but i can be wrong) BUZ11, want about 5v as input... but i can't get it from lm339.

    Can You recomend me soemthing what i can do ?

    What about Thyristor ?

    Do You know any, what will allow me constant 2A, and will be full open using 3.3 - 2.5 V ?

    Regards
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Either use a logic-level mosfet or use a normal or darlington bipolar transistor instead.
     
  3. alan1975

    Thread Starter Member

    Jan 1, 2012
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    What about voltage drop ?

    You see, i'm afraid of lose 1V just for using transistor... I's a battery circuit, that's the main problem.

    I don't want also relay, because of noise (click, click, click, click)...

    Regards
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You sure about that?
    LM339 IS a comparator and has open collector output, so the output can go as high as Vcc.

    Try looking for some logic level mosfet, there should be some small voltage ones that can switch 3A with 3.3V or even less voltage on gate. I am using irlml6402 smd mosfet for some leds, it has 0.065Ω on resistance which yields 0.5W dissipation, so if you used few of them in parallel you should be safe. Or some similar larger package.
     
  5. alan1975

    Thread Starter Member

    Jan 1, 2012
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    0

    You have right!
    I mount bad op-amp, i install lm324n... that's why i had (-1.5V)

    I'm going to order LM339, i think this will fix issue!

    I'm also thinking about this Mosfet IRLZ44N

    It's says here :
    http://www.irf.com/product-info/datasheets/data/irlz44n.pdf

    Logic-Level Gate Drive

    I just don't understand how read gate voltage what will open it fully.


    Regards
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    First, the Vgs parameter on page 2 gives you a hint on how much it needs to start openening a little. Then from fig.3 on page 3 you should see that for example with 3V Vgs it has on-resistance of about 1Ω, and with 4V around 0.4Ω.
     
    alan1975 likes this.
  7. alan1975

    Thread Starter Member

    Jan 1, 2012
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    Thank You.

    I will order them.

    Next update, when arrive...

    Regards
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Rds(on) is much lower than that. See the attached snippet from the datasheet.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Ok lets compare the two, Rds is stated as 0.035Ω for Vgs 4V.
    Yet in fig. 3 for 4V Vgs the current is 60A for 25V, which means 0.42Ω resistance.
    Why is there such a huge difference?
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It's because Vds is 25V. The transistor is in the saturation region, like this:
    [​IMG]


    Look at Fig. 1 in the datasheet. When Vgs=4V and Vds=0.1V, Id≈4.4A, so Rds≈23mΩ.
     
  11. alan1975

    Thread Starter Member

    Jan 1, 2012
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    Hello again,

    After week, i got my LM339N.
    I'm trying wire it up but i can't get it to work...

    I wire it up just like on picture but won't work.

    I power Vcc with about 5v to pin 3 and pin 12 to GND.

    Inverting with 0.3V and Non inverting with 3.3V.
    To test i switch them together..

    Once i got less than 0.5V at output, another 0v.

    Right now i check 3 different LM339N IC, and i can't get them to work...

    Is this my fault ?

    I attach picture, how i wire it up.


    Regards
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The comparator has open collector ouptuts, so you need to connect a resistor from the output to the + rail to be able to see any output.
     
  13. alan1975

    Thread Starter Member

    Jan 1, 2012
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    0
    Hmm, i think You have right.

    But how to do this ?

    It's impossible to get output 0 - Vcc ?

    I try 10k, but i can't switch 0-Vcc :/

    I have N-Chanell mosfeet -> STP40NF03

    http://www.alldatasheet.com/view.jsp?Searchword=STP40NF03

    How get that resistor value ?

    I just want turn this on / off, when one voltage will be greater than another.

    Regards
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If the ouptut doesn´t go high with a resistor between output and Vcc something has to be wrong elsewhere.
     
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