Can BJT s and Mosfet s use multiple / different voltages

Discussion in 'General Electronics Chat' started by RandyFL, Jun 29, 2015.

  1. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    I have a 555 wired in astable mode and powered by 9 volts and was wondering if a 2N3055 or similar BJT could used at a different larger battery...
    kinda like a the way this arduino is wired... according to this tutorial...

    http://blog.lib.umn.edu/ali/2010ema8600/tutorials/

    thanx in advance
     
  2. MikeML

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    Oct 2, 2009
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    Sure. The 555 output will swing from ~1V to ~8V when powered with 9V, which is more than enough to drive the gate of NFET. The 0V side of the 9V supply must be tied to the 0V side of the 12V supply, however.

    912.gif
     
    Last edited: Jun 29, 2015
  3. RandyFL

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    Aug 28, 2014
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    but can that be done with
    1. a BDX53c and
    2. can it be on a different breadboard
    thanx in advance

    All the best

    PS what was Nfet's number specifically ( I am looking for 8 amp or higher )
     
    Last edited: Jun 29, 2015
  4. MikeML

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    Yes, provided you put a 1KΩ resistor between pin 3 of the 555 and the base of the Darlington.
     
  5. RandyFL

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    Aug 28, 2014
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    thinking out loud here...
    2 breadboards one with a 9 volt attached to the 555 and the other breadboard with a BDX53c connected to a 12 volt 220 CCA ( lawnmower battery ) as long as their tied to 0V side... it will work...?

    PS
    Could you put another drawing in... with two different breadboards or separately...
    or however that would work... thanx in advance
     
    Last edited: Jun 30, 2015
  6. Alec_t

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    Sep 17, 2013
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    Should do. Bear in mind that if the switched load is at all inductive (e.g. relay, solenoid, motor) you will need a reverse-biased catching diode across the load.
     
  7. RandyFL

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    Aug 28, 2014
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    Thank you.
    I think He drew that in His original drawing...

    All the Best
     
  8. dl324

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    Mar 30, 2015
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    True for bipolar transistors, but MOSFETs have a parasitic diode between the S/D. If that diode can handle the kick back, an external snubber isn't required.
     
    Last edited: Jun 30, 2015
  9. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    MikeML,
    I don't think you have to re draw it...
    I'll buy a 12 volt rechargeable battery tomorrow. And test a BDX53c on a different board ( or barrier strip ) with different voltages...

    12 volts, 3 ohms, 4 amps and 48 watts ----> test

    I might have to put the heat sink on the BDX53c ( I don't need the mica... on just one heat sink....? only if more than one BDX53c together on a single heat sink....correct? )

    Cheers
     
    Last edited: Jun 30, 2015
  10. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Test 1. The PN2222a on a 2nd breadboard with a 470 resistor ( and a Led ) worked with 12 volts...It was a simple test to see if would work and it did... thanx

    On the next test... A BDX53c with a 3 ohm power resistor 100 watt ( will a Led in the same position work - curious ) I will measure the amperage with an ammeter...

    Which brings me to my next question...
     
  11. RandyFL

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    Aug 28, 2014
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    My next question is...
    whats the difference between bjt and fets and which are better...
     
  12. RandyFL

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    Aug 28, 2014
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    Can a transistor be used as a relay



    He had the collector part backwards...but the schematic did work.
    Was it really used as a relay... or was this a fluke ( not the voltmeter :) )...

    cheers
     
  13. MikeML

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    Oct 2, 2009
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    A relay has isolation from input (coil) to contacts. A transistor does not; the drive circuit must be connected to the output circuit, somehow.

    Both a relay and a transistor can have power gain, meaning a small amount of input power can switch much more power...
     
  14. dl324

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    BJT's are current controlled devices, MOSFETs are voltage controlled devices (sometimes visualized as a voltage controlled resistance). The term FET includes many families of devices that have different characteristics. For example, JFETs have a conduction channel formed with no bias applied and you bias them to turn off. MOSFETs have no channel under no-bias and you provide bias to form one. JFETs and MOSFETs both have a high input impedance, but for JFETs it's a PN junction and for MOSFETs it's a capacitance.

    The "better" device depends on application, designer preference, cost, part availability...
     
  15. dl324

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    It depends on what you mean by "used as" and it depends on how the relay or transistor was being used.
     
  16. RandyFL

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    Aug 28, 2014
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    I have a hard time wrapping my head around this ( your ) statement... i.e. the water analogy... the water in this example is the volts and the pressure is the amperage...its like saying there's water in the pipe buts its not moving ( FET ) or there's 100 amps but the volts is only 3 ( BJT )... how can one exist without the other. If you had stated they both ( relatively speaking ) do they same thing but the cost is different that I could understand...ones for current and one is for voltage ( I have to ponder that over the 4th july ) and have a nice 4th of July ( my wife is back from the hospital ).

    All the Best
     
  17. dl324

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    It should be pressure is volts and water is current.

    If you use that analogy, it can still apply to both BJT's and MOSFETs. Both can be thought of as valves that control water flow in a pipe; it's just that the actual control mechanisms are different.

    For a MOSFET, the valve is controlled by voltage. On an NMOS device, applying a positive voltage equal to the threshold voltage starts opening the valve. When you apply Vgs(max), the valve is completely opened and you have the lowest resistance Rds(on).

    For an NPN transistor, you inject enough current into the base, usually through a current limiting resistor, and the transistor starts conducting when the BE junction is forward biased sufficiently (0.6V or so). If you inject enough current, the transistor will saturate and you get max current. Here the pipe/valve analogy breaks down.
     
    Last edited: Jul 3, 2015
  18. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Volts is the pressure of the water and Amperage is the volume of water that flows past a fixed point in a fixed amount of time...
    Are we in agreement.... :)

    Cheers
     
  19. dl324

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    Yes.

    How's your Wife doing?
     
  20. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    After 17 days in the hospital ( she had emergency surgery and then a clot appeared in her left from the ankle to the groin )she is very weak and has a swollen left leg ( clot ). She has been on a heavy dose of levenox ( at the hospital )and cumadin at home... she has to go to a cumadin clinic for a week and hopefully lots of bed rest... and since I'm recovering from a foot surgery in May we make quite the pair...
    Thank you very much for inquiring... means a lot.

    So do you think the guy in the video actually used it as a relay or just thought He did...

    Cheers
     
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