Help with mosfet and transformer

Discussion in 'General Electronics Chat' started by elimist, Dec 27, 2010.

  1. elimist

    Thread Starter New Member

    Dec 27, 2010
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    hello,

    I am trying to make an arduino control a solid state relay which switches a 120vac load. I tried connecting a digital output pin and ground on the arduino to the the input terminals of the relay marked + and - without anything connected to the output terminals and the input status light was really dim. connecting the input terminals to a wall adapter works so I think I need a transistor. do you think this mosfet will work?

    I always thought that transformers only work on AC but when I tried simulating a transformer on this circuit simulator, it seems to work with DC. why is that?
    [​IMG]
     
  2. shteii01

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    Feb 19, 2010
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  3. elimist

    Thread Starter New Member

    Dec 27, 2010
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    what?

    I don't need to convert anything from AC to DC
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Transformers work with CHANGES in magnetic flux.

    When DC is first applied to a transformer a magnetic field builds up.

    As soon as the field reaches steady state, the mutual induction that impresses a voltage on the secondary dies away.

    The transformer action with DC you are seeing is simply the initial inrush of current and the associated buildup of magnetic flux. During this very short time at power on, the magnetic field is changing and the transformer is transforming. It only lasts a very short time.
     
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  5. retched

    AAC Fanatic!

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    If you CHANGE the DC, with PWM or pulsing, you can use a transformer with DC.
     
  6. ashish19183

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    Sep 16, 2009
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    relay coil consumes current which should be provided from driver circuit

    u can use transistor to boost current and choose transistor which have current

    rating required by relay.

    now Transformer only works on AC and if you have DC then use a PWM signal

    to generate step down voltage.

    if ur specification is unique share with us so i can give u answer with calculation.
     
  7. retched

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    A relay coil is used as an electromagnet to move a ferrous metal contact.

    The DC induces a magnetic field that draws the contacts close. PHYSICALLY moving the contacts, like you using a hand held permanent magnet to draw a paperclip to the magnet.

    In a transformer, there are 2 coils.

    In order for the first coil to transfer the energy to the second, flux lines MUST BE BROKEN.

    By pulsing the coil, and the nature of a coil to RESIST CHANGE, the magnetic field grows and shrinks when pulsed. This causes MOVING FLUX LINES that are continually causing change in the secondary of the transformer.
     
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  8. elimist

    Thread Starter New Member

    Dec 27, 2010
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    Thanks, I think I understand how the transformer works with ac now. The mosfets I mentioned in my first question arrived today and my circuit worked, but I have a few questions about mosfets:

    1. Do mosfets work in a darlington pair like bipolar transistors?

    2. Sometimes I see mosfets on the internet with ratings like 600v 36A in a to-220 package. I am a little skeptical because the AC outlets here are only rated at 120v 20A. Do these mosfets really work ?
     
  9. ashish19183

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    Sep 16, 2009
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    600V 36A is the Max. rating of MOSFET that means it can work up to this

    voltage and current, ur voltage is coming under this so it surly gonna work....
     
  10. elimist

    Thread Starter New Member

    Dec 27, 2010
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    If the tiny leads on the transistor can work at 36A, why are the home AC plugs so big and bulky? Is it because house wiring is alternating current?
     
  11. retched

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    Its all about wattage.

    10A at 12v is WAAAYY different than 10A at 240v.

    Also, you have to see HOW LONG the mosfet can handle those voltages/currents.


    You may only be able to pulse them at 5uS pulses at those specs.

    Also, heat sinking will come into play. READ and KNOW the datasheet well before using them.
     
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  12. edgetrigger

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    Dec 19, 2010
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    pls check the attachment!
     
  13. elimist

    Thread Starter New Member

    Dec 27, 2010
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    I looked at the datasheet again and most of the graphs do say 250us pulsed.

    what attachment?
     
  14. retched

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    I see no attachment.
     
  15. edgetrigger

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    Dec 19, 2010
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    use a 12 volt relay with a tansistor for you switch ac loads. Also you could use a triac (bta12/600) triggered by an optocoupler like moc3010 and swithc ac loads. Mount the triac on small heatsink.
     
  16. elimist

    Thread Starter New Member

    Dec 27, 2010
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    @edgetrigger thanks, but I already got my solid state relay to work.

    I made a darlington pair with mosfets but it is not working like it did with bipolar transistors. Why is that?
    [​IMG]
     
  17. edgetrigger

    Member

    Dec 19, 2010
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    Darligton pair is used to get a high current gain, meaning a small current controlling a large current.
    Mosfet is a voltage controlled device not a current controlled one. You can’t make a darlington pair using mosfets. If at all you want two stage you can build it ( i’ll tell you how) but it will defeat the very purpose of using Mosfets.
     
  18. elimist

    Thread Starter New Member

    Dec 27, 2010
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    why does it defeat the purpose of using mosfets? I thought that I can use a circuit similar to this to control a power mosfet that needs a high Vgs through a smaller signal mosfet using a small control signal(such as the 5v from my arduino).
     
  19. edgetrigger

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    Dec 19, 2010
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    Firstly mosfets are used for mid - high frequency swithcing applications. that doesn't mean that they can't be used for swithcing a relay. But it will be like using a spatula to eat instead of a spoon. So our spoon a Bipolar Juntion Transistor( either single or darlington) is enough to swith the relay. Also interfacing TTL 5volts is very easy with a BJT. So if you want to just switch a relay please use a transistor instead of a MOSFET.

    If you want it completely semiconductor then you could use a triac in place of the relay. So you could easily switch on/off ac loads.
     
  20. elimist

    Thread Starter New Member

    Dec 27, 2010
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    I think I should have started a new thread. My questions about using a mosfet in a Darlington pair are unrelated to my relay question. I want to control a power mosfet using 5v, but the power mosfet needs more than 5v for saturation. I thought that I could use another mosfet to first amplify the control voltage.
     
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