Replacing Relays with mosfets

Discussion in 'The Projects Forum' started by ivars211, Jan 7, 2014.

  1. ivars211

    Thread Starter New Member

    May 5, 2013
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    Hi, I am quite new in electronics and I was wondering, if it's possible to replace all relays in my circuit with mosfets, everything works great with relays, but I just want it to consume less power and be quiet.
    I attached the circuit diagram below.
    Btw, my relays are rated as 10A power 25V, so something like 25V 3A would do as well.

    Thank you!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
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    Should be no reason why not, what is the nature of the relay coil circuits?
    Max.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Yes. Relays 1, 6, and 7 can be replaced by a P-MOSFET, source to +12V with drain being the output (if you have a 12V control signal). This will give an inverted control (0V will turn on the P-MOSFET). If you want 0V to turn off the switch then you can add an NPN BJT or N-MOSFET driver.

    Relays 2 ,3, 4, and 5 can be replaced by an N-MOSFET but you must move the resistor going to ground to the drain side of the N-MOSFET (which will have no effect on circuit function) with the N-MOSFET source going to ground. These will turn on with a +12V signal on the gate.

    What is the voltage level of your control signal?
     
  4. ivars211

    Thread Starter New Member

    May 5, 2013
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    Thank you for your replay.
    It's just powering higher voltage/current lamps with fewer relays.

    What I am asking is, what kind of mosfets would I need? Because I tried this circuit with TIP122 and it didn't work, it worked from the positive side (positive -> TIP122 -> load -> ground), but when added another transistor to ground, it didn't (positive -> TIP122 -> load -> TIP122 -> ground). The lamp barely lit. Relays are 5v ucoil, if that changes anything.
    Maybe I need PNP instead?

    EDIT:
    crutschow, thank you so much, I really appreciate it!
    5v for signal from arduino.
     
    Last edited: Jan 7, 2014
  5. crutschow

    Expert

    Mar 14, 2008
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    The TIP122 is an NPN darlington BJT not a MOSFET.

    For high side switching (load connected to ground) you need a PNP BJT or a P-MOSFET.

    For low side switching (switch connected to ground) you need an NPN BJT or an N-MOSFET.

    What is the maximum current the relays have to switch?
     
  6. ivars211

    Thread Starter New Member

    May 5, 2013
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    Thank you for your replay. I kind of figured that out when looked at data sheet.

    I need to power 36W lights, so it is 3A minimum, so ~5-10A range seems right for mosfet. Any suggestions for mosfets?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    It somewhat depends on where you will buy them. Do you have a supplier you use? What's your location?
     
  8. ivars211

    Thread Starter New Member

    May 5, 2013
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    Thank you so much for your help.
    I am in UK, don't have any specific supplier, mouser, ebay...
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Just go to the website for your preferred supplier and look for logic-level N-MOSFETs and P-MOSFETS with at least a 50% rating above the maximum voltage and current levels you will be operating. You will also need a transistor to drive each of the high-side P-MOSFETS such as an xN2222A and well as some miscellaneous resistors.
     
  10. ivars211

    Thread Starter New Member

    May 5, 2013
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    crutschow, I am so thankful for your time and input. I made a circuit, just to make sure I fully understand all the information you gave me.
    IRL540N data sheet
    IRFU9024NPBF data sheet
    Just want to make sure I've got the right mosfets and circuit, before buying anything.
    [​IMG]

    Thank you again!
     
  11. crutschow

    Expert

    Mar 14, 2008
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    3,220
    That looks ok and the transistors look more than adequate, but what is the purpose of the resistor in parallel with the N-MOSFETs? I realize you also had those in your original circuit but I'm confused about their purpose. Are they for dimming purposes?
     
  12. ivars211

    Thread Starter New Member

    May 5, 2013
    20
    0
    Yes, you are right, they are for dimming! I could use PWM, but then it becomes quite complicated.
    I cannot tell how thankful I am for your help!
     
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