MOSFET Advice Needed

Discussion in 'General Electronics Chat' started by Derek Emerson, Jul 12, 2017.

  1. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    Hi All

    I'm working on a little project that I need a little help on.

    I have an RF receiver that has a relay on, it's triggered from a key fob similar to a car alarm fob. The relay activates 2 electro magnets. The project requires that the whole thing operates totally silently. As you can imagine, the relay makes a noise.

    I want to replace the relay with a MOSFET but am unsure of the best way to do this. Will I need some other components to protect the mosfet from load dump or other such issue?

    Any help would be greatly appreciated.

    I'll list the part number of the relay when I get back in the house.

    If you need any other info then just let me know what you need.

    Regards

    Nelix
     
  2. EM Fields

    Active Member

    Jun 8, 2016
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    1. Receiver's operating voltage?
    2. Relay coil voltage
    3. Relay action: Form "A"? form "B"? form "C"? Latch? SET/RESET with transmitter actuation? Other?
    4. Relay contact ratings?
    5. Relay contact isolation needed?
    6. Automotive?
    7. schematics, please, or links.
     
    Last edited: Jul 12, 2017
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
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    MOSFETs are usually a little easier than BJTs for this sort of application.

    Just remember they're voltage controlled instead of current controlled - you need at least VGSthr on the gate, a few volts extra is well worth the effort. You may need to use a logic level part, but anything over about 6V drive and it shouldn't be critical.

    You obviously need current and voltage rating that's man enough to do the job. If the load is inductive; you'll need some form of clamp to protect the semiconductor from back emf.

    There's not that many pitfalls.
     
  4. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    1. 12v
    2. 12v
    3. When the transmitter is pressed the relay latches which energises the magnets, when the button is pressed again they turn off and he magnets go off, no sure about form A,B!c
    4. 10A
    5. Don't know
    6. Not an automotive application
    7. I don't have any schematics but can provide links to the relay

    http://tutorial.cytron.com.my/2012/...-of-a-relay-without-reference-to-datasheet-2/

    I'm powering this from a 5v power bank, the board on the left of e picture steps the voltage up to 12v, I have a couple of diodes on the output, I can provide pics if needed.

    I have a couple of these lying in my box of bits, they seem a good match, maybe a little over spec'd:

    http://uk.rs-online.com/web/p/mosfe...3743D35343039373737267374613D3534303937373726
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    15,689
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    If under 200ma for the electro-magnets you could use a 2n7000.
    If more power, a IRL530 for logic level operation.
    Max.
     
  6. ElectroHydraulicEng

    New Member

    Mar 31, 2017
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    1
    Have you considered just using a solid state relay - no noise.
     
  7. ian field

    AAC Fanatic!

    Oct 27, 2012
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    The 2V VGSthr spec should be fine. The MOSFETs off a scrap PC motherboard are also a good choice.

    I frequently use TO220 MOSFETs when a smaller device would do - its purely a matter of what's abundant in the junk box.

    If your circuit can drive anywhere near 12V, you should study the data sheet carefully - typical power MOSFETs can handle 18V on the gate, but some of the lower VGSthr types have a lower voltage limit.
     
  8. ANIRUDHRA BUDHATHOKI

    New Member

    Jul 12, 2017
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    HLW
    PLS PROVIDE ME IDEA..HOW CAN WE CREATE PROJECT
     
  9. EM Fields

    Active Member

    Jun 8, 2016
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    Your "VGSthr" is the voltage which, when applied between the gate and the source, establishes the point where the channel just starts to conduct, and has little to to do with Vgs(on), which is when the channel is fully enhanced.
     
    Last edited: Jul 12, 2017
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    They have the convenience of a guaranteed input voltage range and threshold.

    MOSFET VGSthr can be a little ambiguous.
     
  11. EM Fields

    Active Member

    Jun 8, 2016
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  12. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    The magnets are 600mA each
     
    Last edited: Jul 12, 2017
  13. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    0
    You'll have to be a little more 'layman' with info/instructions as I am only an amateur with a basic knowledge of electronics.

    So would the Mosfets I already have be a straight replacement with no other components?
     
  14. ian field

    AAC Fanatic!

    Oct 27, 2012
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    A MOSFET is pretty easy, but a few rules you have to follow.

    A SSR is less guessing game but cost more.
     
  15. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    The data sheet for the mosfet shows VGS(th) - Gate Threshold Voltage as 2-4, does this mean it won't work?
     
  16. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    I was hoping to use what I already have.
     
  17. ian field

    AAC Fanatic!

    Oct 27, 2012
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    If you were driving it with 5V TTL - probably not. They never quite switch rail to rail. MOS or CMOS usually does a little better.

    The VGSthr is often a range of values - if you supply the highest value, its guaranteed to meet the headline RDSon. If you only supply the lower value - it might not.

    If you over specify the MOSFET, it might switch adequately if the gate drive is a little short of ideal - in most cases its better to give it a volt or two more than spec.
     
  18. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
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    I'm driving it with 12 volt, I'm using 5v power bank going into a boost converter that steps it up to 12v. The 12v then supplies the receiver board and magnets.

    I'll try and draw a diagram and post it.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    That's a common misconception.
    The Vgs(thr) is the point where it just starts to conduct current (typically a few hundred μA, as indicated in the data sheet) not where it meets the Rds(on) spec., so a value of 2V-4V means it may only be slightly on for a Vgs of 5V and the Rds will be quite high.
    Usually a Vgs(th) of 2v-4V means the Rds(on) is specified at a Vgs of 10V.

    You need to look in the data sheet where the Rds(on) is specified to determine the Vgs needed to achieve the lowest Rds(on).
     
  20. ian field

    AAC Fanatic!

    Oct 27, 2012
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    A regular power MOSFET is usually happy with about 6 - 8V on the gate. Presumably a logic level type should work with the attenuated Vcc you get from TTL - but don't bet the farm on it.

    If you're switching DC - getting hot is either a MOSFET not man enough for the job, or not enough gate voltage.
     
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