Trying to understand a mosfet

Discussion in 'General Electronics Chat' started by Hextejas, Dec 25, 2017.

  1. Hextejas

    Thread Starter Member

    Sep 29, 2017
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    This is what I am about to breadboard. It shows 2 V sources and I am trying to understand the intent.
    1) Is the VGs a variable voltage source for the gate ?
    2) Is VDs the load, maybe a rechargeable battery ?
    3) what's gonna happen when the load fills up, should this need something to turn off the mosfet ?
    4) To my ignorant mind, S and D seem to be misnomers. S =Source=where stuff comes from, and D=Drain=where=stuff goes.
    I think stuff is current so why does current, Id, look like it is flowing into the Drain ?

    Thanks
    amp86.gif
     
  2. wayneh

    Expert

    Sep 9, 2010
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    1) Yes, as a way to show the behavior. It’s not something you would normally do.
    2). It’s the power source and there is no load or current limiting as shown. Definitely not something you would really do!
    3) This is a paper circuit and not real.
    4) it’s correct, and yeah, it seems goofy at first. We usually orient a MOSFET this way so that the body diode does not conduct.
     
  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I'd say that a better representation of the figure on the left would look like this:

    amp86.gif
    Notice that the N-Mosfet is located on the "low side" of the load.
     
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  4. Hextejas

    Thread Starter Member

    Sep 29, 2017
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    Suppose that I would want to put a 12v fan or light as the load, would it look the same ?
    It would be nice to be able to see something happen.
    Ok more confusion.
    I put an auto lamp as the load, runs on 6v. And I am using 2 voltage sources.
    9vdc for the gate and 2v for the source. I thought that the mosfet would amplify the 2v enough so that it would light the lamp.
    Plus I now have 2, V-, and I don't know how to hook them together or even if I am supposed to.
    Lamp stays off.
    Help !
     
    Last edited: Dec 25, 2017
  5. cmartinez

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    Yes it would look almost the same... for inductive loads (such as a fan) a snubber diode is recommended across the load to prevent damaging the Mosfet due to voltage transients. For resistive loads (such as a light) it's not really necessary, but I like to install the diode anyway because there's no knowing if you could accidentally disconnect the light while the mosfet is still on... which could also cause transients that could damage the Mosfet.
     
  6. Audioguru

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    Dec 20, 2007
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    Please draw the schematic and attach it. We need to know the positive and negative of the batteries and if the Mosfet is N-channel or is P-channel and its part number. The Mosfet is used as a switch and does not amplify.
    Will the 6V lamp survive 7V?
     
  7. Hextejas

    Thread Starter Member

    Sep 29, 2017
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    Mosfet is IRFZ44N, npn
    Lamp will survive 7v
    The schematic is what shows above though I substituted the lamp for the VDs battery.
    That is interesting about the mosfet not being an amplifier. I knew that they were used for switching but thought that they amplified based upon gate voltage. So just leave it on all the time .
    Lots to learn
     
  8. Danko

    Active Member

    Nov 22, 2017
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    See on MOSFET as on potentiometer controlled by voltage. Higher gate voltage - higher wiper. Lower voltage - lower wiper.
    R-by-voltage.png
     
  9. Audioguru

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    Since you replaced the powerful Vds battery with a passive light bulb then there is no current available to flow through the light bulb.
    A Mosfet or transistor is not a generator. It simply modulates the voltage or current applied to its load.
    An amplifier modulates the voltage and current from its power supply so that a small input level creates a larger output level.
     
  10. Hextejas

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    Sep 29, 2017
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    Danko, is that going to work if V1 is 2V ?
     
  11. cmartinez

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    But mosfets can be used as amplifiers alright... just in a different configuration than transistors, that's all.

    You see, transistors are controlled through the current passing through their base, whereas mosfets are controlled through the voltage applied to their gates.
     
  12. Hextejas

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    Sep 29, 2017
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    I said that wrong Audio. I am away from my bench so can't get my schematic.. I left the 2V battery in with the + connected to the source and the --- connected to the --- of the other battery. I really don't have a clue as to what to do with 2, V---.
     
  13. Danko

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    Nov 22, 2017
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    Yes. It works. You have current through lamp, but from 2V it is low current, not enough to light lamp.
     
  14. Hextejas

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    Sep 29, 2017
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  15. BobTPH

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    Jun 5, 2013
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    You do not understand what amplification means. It does not mean it will turn a 2V supply into 5V. An amplifier requires a supply voltage somewhat larger than the maximum voltage it can output. For example, an audio amplifier run from 12V might give you an output of 10V peak to peak at max, which is only 3.5V RMS. A bridged amplifier from 12V can get close to 24V peak to peak which is about 8.5V RMS.

    What a MOSFET does, when in the saturation region, is limits the current going through it to a specific value based on the voltage on the gate. Below the threshold voltage, there is no (or very little current). At the threshold voltage there is about 1/4 of a mA. At higher voltages, it might pass 1 or 2, or even 100A, depending on the specific MOSFET you are using.

    Bob
     
  16. Audioguru

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    a 2N7000 Mosfet has a "typical" transfer characteristic graph that shows when its gate-source voltage increases 2V then its drain-source current increases 3 times. But some 2N7000 Mosfets have less gain and others have more. Then you use Ohm's Law to design a circuit that amplifies a signal.
     
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  17. Hextejas

    Thread Starter Member

    Sep 29, 2017
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    You sure got that right .
     
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