replace relay with Mosfet transistor

Discussion in 'The Projects Forum' started by mah, Dec 21, 2014.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    i want to replace a relay to switch Ac load that will draw about 20 Amps . i used IRFP250 and bridge rectifier as in the picture . i still didn't simulate or do it . the voltage gate come from a plc (programmable logic controller) 24 Volt positive or negative .
    will this circuit need any thing else ? will the mosfet need heat sink to act as a switch ?
    if i replaced all of this with a triac will it work?
     
    Last edited: Dec 21, 2014
    Steameeey likes this.
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I would suggest buying a off the shelf solid state relay and do it the right way.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You appear to be switching an AC induction motor from DC?
    Usually no can do!
    Use an off the shelf Opto, such as Opto22 type.
    Or Triac if needed, the Opto22 gives isolation.
    Max.
     
    Last edited: Dec 21, 2014
  4. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    i don't want to use SSR because i need some thing small and cheaper . what is thee problem with this circuit?
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In your circuit the AC motor will get pulsating DC.
    Is that what you want?

    Bertus
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You show AC motor, a simple 1ph induction motor does not run on DC.
    If a Universal motor, it will run on AC/DC.
    A triac circuit should power it off AC.
    Max.
     
  7. bertus

    Administrator

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  8. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    i would use it to turn on ac heater not specifically motor but i didn't find a heater in proteus if this is the problem.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Ensure the Heater consists of an element only.
    I would still tend to use a Triac or SSR.
    Max.
     
    mah likes this.
  10. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    would you recommend a triac and the gate voltage will be +-24 v ? will it be a simple triac connected in series with the heater ? . i want the triac gate to operate on negative or positive voltage from the plc .
     
    Last edited: Dec 21, 2014
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What kind of output is that on a PLC?
    Max.
     
  12. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    it is 24 volt dc .the output differs related to programming so i want the circuit to be flexible with both positive and negative 24 v if required. positive and negative won't happen at the same time so i think i will need invert gate to operate the triac in both directions
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    But thats what you are making and a poor one at that. :oops:

    Your IRFP250 is only rated for 200 volts and your schematic shows it running from a 220 VAC power source of which already has a peak voltage of around ~320 volts.
    On top of that the device has a 30 amp peak rating of which most any DC motor that takes 20+ amps at 310 volts is likely to have a huge startup current way way past 30 amps.

    Cost and size wise an basic SSR that will exceed your current schematics capacities is a round. $5.37 including shipping and Smaller than a deck of cards.

    http://www.ebay.com/itm/250V3-32VDC...074?pt=LH_DefaultDomain_0&hash=item3ce9f7b602

    $1.02 more ($6.39) gets you a 50 amp unit from china.

    http://www.ebay.com/itm/1-Solid-Sta...370?pt=LH_DefaultDomain_0&hash=item3a8f20bc22

    Any random turn on SSR can do phase angle shift voltage/power control just by varying which part of each half of the cycle it turns on at. Highly effective and very reliable.
     
    Last edited: Dec 21, 2014
  14. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    but this SSR working on just positive 3-32 v but i need it to be flexible with also negative input .
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Umm?????????????? What???????????:eek:

    Obviously you're way in over your head.


    I am backing away now. :(
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I don't get it?
    If it is a heater element, why do you need bi-directional current control ?
    Also what is the exact nature of the PLC output that has a +- output?
    Max.
     
  17. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    the heater is working on AC so if i used just half cycle it will work with half power but i need full power. the plc output differs from plc to another, some plcs output is positive another type is negative i want the circuit to work on both of them.;)
    but what triac gate will operate on 24 volt?!
     
    Last edited: Dec 21, 2014
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A triac gate is traditionally fed off of the 110/240 AC as long as you use the appropriate gate resistor?
    Max.
     
  19. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    ok , recommend a triac for this application , i will connect the ac source in series wirh bridge rectifier and optoisolator on the gate of triac as in the last picture . so the gate will work on +ve and -ve cycles but the plc will control this on/off operation.
     
  20. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Anyone else thinking absolutely none of this adds up from one end to the other? o_O

    First it's a motor then it's a heater but a solid state relay is too expensive although they proposed cheap circuit is a solid state relay anyway but will not work for any of the parameters listed on the schematic. Next the PLC's are both power source and power sink type and the ......... Heck I give up. :confused:
     
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