Relay or other components for three input and one output?

Discussion in 'General Electronics Chat' started by FroceMaster, Oct 15, 2015.

  1. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Hi
    Need this setup up as simpel as possible.,

    Have a 12v input at max 10 mA,

    With this input i should be able to supply the outgoing 12v 5A wire.

    When Theres comes 12v 5A input, also set the outgoing 12v 5A wire. without sending power back in the 12v 10 mA wire.

    Have 12v 10A wire ingoing also and ofcourse ground / 0V,

    How should i do it ?

    Rgds
     
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  2. wayneh

    Expert

    Sep 9, 2010
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    I don't fully understand your two sources: Do you have two independent 12V supplies, one rated to 5A and one to 10A?

    Based on what you've described so far, I'd use a MOSFET switch. It would be positioned on the ground return line of your 5A load, which could be supplied power from either 12V supply. The MOSFETs drain pin goes to the load, source pin to ground, and your 12V signal on the gate pin. The other side of the load is connected directly to the power supply. The MOSFET will function much like a relay, but needs almost no current on the gate. A relay is likely to need more than 10mA although you might find a suitable one.

    You didn't say how quickly you expect to switch the load or what the load is. A relay could be fine for slow switching (less than once per second, for example), but the MOSFET can handle much faster switching.
     
  3. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Have 10A supply and a 5A and a 10mA to set the output. But i will have to be possivtive controled. Can't use ground

    Not fast switching.
    Reason to use other thing than relay/transistor is due to space
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    It is called a "high-side switch".


    283.gif
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    might really help to explain exactly what you are doing and maybe why a typical diode or'ing scheme wouldn't suffice.
     
  6. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Should maybee do the job. But can it handle up to 14-15 V not precise 12v
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you are replying to my post #4, then the circuit will work up to 20V provided that the Vds rating for the PFet is >20V, the Id rating is > 5A, and the Ron is low so that the PFET does not have to be on a heatsink.

    Also check the Vgs rating. If it is less than ~15V, then add a 2K to 4.7K resistor between the collector of Q1 and the gate of M1 to create a voltage divider with R1 to reduce the Vgs to ~10V.
     
  8. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    Is the 12V, 10mA signal a switch or is it a low-current supply that should drive the output when none of the high-current supplies are available?

    If all three 12V inputs should be presented to the output, and have their own current limiting so that the load on the "5A output" won't damage the input that only supplies 10mA, you can use FET-based "Ideal Diodes". The small circuits act as de-facto diodes but have a tiny fraction of the voltage drop of actually using diodes to OR the three supplies together.

    The "Ideal Diode" circuit can be as simple as putting in a P-FET in series with each input. The trick is to wire the P-FET "backwards" with its drain toward the supply input (anode of the body diode). The body diode will block the back-voltage when the FET is off. The only trick is how to control the gate. I have a couple of ideas about that... Like diode-or the two high-current supplies together and feed them to the gate of the low-current supply P-FET with a resistor to ground, so that it switches off if either of the high-current supplies is on.

    You could create a hierarchy this way. The 10A supply would tie back to the gate of the P-FETs on both the 5A supply and the 10mA supply. The 5A supply would tie back to the gate of the P-FET on just the 10mA supply.

    Forgive the crude drawing.

    upload_2015-10-15_13-21-19.png
     
  9. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    As i can understand your drawing,
    If the 12v 10A supply wire is with "power" the output is also "high" 12V
    if this i correct the schematics is wrong.

    The 12v 10A supply is allway there.
    When i "turn on" 12v 10 mA the output should be "high" 12v. and could be drawing up to 10A.
    If then 12V 5A is "turn on" the output should reamain high , or be high if 12v 10 mA is off.

    Could you also set some info on what value you would use to it, MOSFET and diode.
     
  10. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    A Little idea, but dont know how or what value the componets should have.
     
  11. ScottWang

    Moderator

    Aug 23, 2012
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    1. D1~D6 are the dummy avoid, if you don't want them then you can take them away.

    2. D7~D9 are used for when the inputs voltages are not equal, if the inputs are complete equal then you don't need them, the labeled 20/10A/1A are for the rating values of diodes, the labeled also the same in mosfet.

    for three input and one output_FroceMaster_ScottWang.gif
     
  12. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
  13. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Have now cooked this together.
    Will i Work or do i need other components ?
    Reason for D1-D2 : must NOT send any power back in either "on" wires.
    Output should only be turned on if 1 or 2 of the 2 inputs is "on"
    The output should at least be able to handle 10A 12-14V.

    Please check that the MOSFET is ok.
    Data is.
    Continuous Drain Current Id: 180A
    Drain Source Voltage Vds: 20V
    On Resistance Rds(on): 0.004ohm
    Operating Temperature Max: 175°C
    Power Dissipation Pd: 210W
    Rds(on) Test Voltage Vgs: 10V
    Transistor Case Style: TO-220AB
    Transistor Polarity: N Channel

    Do i need heatsink for my Circuit ?
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Nope. The MOSFET can handle 10A with ease. That design ensures it is either fully on - with its low Rds - or fully off, so it should never need to dissipate much heat.

    Here's an exception: If the voltage applied to the 3904 transistor is NOT a full 12V or open, but is instead is just a couple volts, you could create a scenario where the MOSFET is only partially conducting. This would cause it to heat up.
     
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