npn or relay

Discussion in 'General Electronics Chat' started by duxbuz, May 4, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Hi

    I wonder what is needed to protect a 5v circuit whilst powering a motor.

    I initially thought that if something such as a motor was used, that needed some extra voltage from batteries, then a relay would be required.

    Since then I have seen a lot of circuits that just use transistors.

    In some examples transistors are used as relay switches with batteries and in other circuits transistors are used without batteries because of their amplification capabilities. (I don't know if this is correct I think I read this, but I may have read it wrong or misunderstood)

    I am wondering how big a voltage a transistor can be used to switch in stead of a relay whilst protecting the 5v circuit that switches it.

    Also how much power can it provide as an amplifier(if this is true). Could it power a small motor?

    Thanks
     
    Last edited: May 4, 2014
  2. Alec_t

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    Sep 17, 2013
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    Check the datasheet for a given transistor to see its maximum Vce rating. Some go to hundreds of Volts.
    A transistor doesn't provide power, it only controls the power (from batteries etc) used by a load such as a motor. In fact, the transistor actually wastes some power and gets warm/hot in the process.
     
  3. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Thanks

    Hundreds of volts.

    Why would a relay be used if higher voltages can be handled by transistors?
     
  4. R!f@@

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    Apr 2, 2009
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    What do you mean by protecting ?
     
  5. Alec_t

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    A relay can provide ciruit isolation and may have multiple sets of contacts, for greater versatility. Any relay must have a reverse-biased diode across its coil to protect the device/circuit which switches the coil voltage from the back-emf generated when the coil is switched off..
     
  6. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    That's where I am confused. It seems that circuit isolation can be performed by transistors.

    I presume it depends on the voltage?

    I want to use 9v and switch it with 3.3v but I don't want to damage the gpio.
     
  7. Alec_t

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    I don't see how. Perhaps we understand different things by 'isolation'. Can you give an example?
     
  8. KMoffett

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    Dec 19, 2007
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    If you experience a collector-base breakdown in the transistor, for whatever reason, you could be applying 9v to you GPIO terminal. There is no galvanic path from the relay terminals back to the GPIO terminal.

    Ken
     
  9. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Ok so relays are the only real solution here.

    Transistors offer no isolation and should be avoided at 9v?

    Thanks
     
  10. KMoffett

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    Dec 19, 2007
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    There are opto-isolators...Solid State Relays. Optocoupler driving a transistor that switches your motor. Good isolation.

    Ken
     
  11. duxbuz

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    Feb 23, 2014
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  12. Alec_t

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    The above-mentioned opto-isolator (aka coupler) would be my choice for optimum protection of the micro port. Cheaper than an SSR. The opto output would then drive a transistor (BJT or MOSFET) to switch the motor power.
     
  13. KMoffett

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    You never mentioned what your 9V(?) motor current is. Depending on the gain of the transistor, you can provide some protection to your controller's output with a transistor. Add a Schottky diode from the output pin to the +3.3V supply (cathode to +3.3V). The transistor's base resistor then becomes a voltage dropping and current limiting device, should 9V ever appear at the transistor's base.

    Ken
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A relay will only offer ON/OFF control, Transistor/Mosfet can offer RPM control.
    What do you need?
    Max.
     
  15. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Probably on off for now
     
  16. eetech00

    Active Member

    Jun 8, 2013
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    Just in case your not clear, a relay provides isolation between the control circuit and the "load". The isolation is, basically, an air gap between the control circuit (the relay coil) and the load (the circuit connected thru the relay contacts).

    A transistor doesn't, by itself, provide isolation. There is a silicon conductor between the control circuit (the transistor base) and the load (transistor collector-emitter junction).
     
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