10 amp relay on pcb question

Discussion in 'The Projects Forum' started by Smothers1690, Aug 11, 2017.

  1. Smothers1690

    Thread Starter New Member

    Aug 11, 2017
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    Hi guys. New here to the forum. I'm pretty new to electronics so sorry if my terminology is bad or I'm not making any sense.
    Basically I have a project going where I will be needing a relay that can handle about 10 amps(12v D.C.) on the switch side. I'd like to power it with a standard 9v battery and mount it on a pcb board. Also it only needs to be a spst relay. My questions are
    1. Where the best place to find this relay
    2. When connecting the circuits together can I just solder a solid wire onto the back of the pcb to handle the current? What size wire would I need?
    3. I'm also looking for a small digital ammeter to put inline to measure the amperage. Anyone know where to locate this?

    Sorry if this doesn't make sense. Thanks for the help in advance
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You can find relays on EBay that should work for you. Automotive relays are cheap and should work OK at 9 volts, and the sockets for them are also cheap, may even come with the relay.

    Having a socket for the relay may do away with your need for a PCB.
     
  3. Reloadron

    Distinguished Member

    Jan 15, 2015
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    It sounds like what you are looking for is a relay with contacts rated for 10 Amps DC and a coil voltage of 9 Volts DC which can be placed on a PCB (Printed Circuit Board). There are literally dozens of manufacturers out there There is Finder 40 Series - Miniature PCB/Plug-in relays 8 - 10 - 16 A as an example and also manufacturers like Omron PCB Power Relays and the list goes on. Since I have no clue where you are located a supplier is hard to suggest but in the US I have used Mouser, Digi-Key and Allied electronics to name a few.
    When passing current through a wire the wire gauge is chosen based on Ampacity. There are ampacity charts out there but for 10 Amps I would look towards AWG 14 depending on service. Additionally if you expect to switch 10 Amps DC using a mechanical relay I would be looking for 15 Amp rated contacts.

    Ron
     
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  4. Smothers1690

    Thread Starter New Member

    Aug 11, 2017
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    Thanks for the replies. I've been doing more research and planning for this circuit I want to make and I've started to think about using a transistor instead of the relay. I want to use a 9v battery to power the transistor. I cannot figure out how to calculate which resistor I need to use to for the base. if I'm passing 12v 10a through the emitter to collector how would I figure it out? Also would it change if I were to pass say 5 amps through the circuit? Or 1 amp? Any help is appreciated. Thanks in advance
     
  5. Reloadron

    Distinguished Member

    Jan 15, 2015
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    There are formulas for calculating base resistor values. However, rather than think everyday transistor you may want to place some thought into a MOSFET and there are plenty to choose from. May also want to go with a Logic Level MOSFET requiring a low gate turn on voltage. There are dozens to choose from for example a FQP30N06L N Channel or a FQP27P06 P Channel. These will afford you the higher currents you seem to want to switch. They also on average run about $1.00 USD each making them inexpensive. A Google of either part number followed by circuits will get you example circuits.

    Ron
     
  6. Smothers1690

    Thread Starter New Member

    Aug 11, 2017
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    Awesome thanks for the help. Looking at the specs for mosfet you linked I found "gate threshold voltage" which is min of 1v and max of 1.5v. Is that the voltage level I need to turn it on? If so I don't need to worry about what voltage and current is going through the emitter and collector? Since I'm using a separate power source for the base I can just figure out what resistor I need to drop the 9v supply down to 1-2.5v and the mosfet should stay on correct?
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Not really. The gate threshold voltage is the voltage where the mosfet has a current of 250 μA going through it.
    You wil need a higher voltage to get real current.

    Bertus
     
  8. Reloadron

    Distinguished Member

    Jan 15, 2015
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    For the example FQP30N06L link take a look at Figure 2 VGS, Gate-Source Voltage [V]. The charts in the data sheets give you an overview of the gate voltage for current.
    "The threshold voltage, also called the gate voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage differential that is needed to create a conducting path between the source and drain terminals".
    So the Vgs is just a minimum to start conduction.

    Thank You Bertus. Saw your post as I typed. :)

    Ron
     
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