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Basic circuit with relay

Former Member

Hi.

First of all: I hope this is the place to post.

 

I am gonna make an easy circuit with a relay to control a pump.(see attachment)

I understand the principal, but I wonder if there is anything more I gotto think of, like resistors or something?

 

It is not to be used in a car.

 

I am thinking of using an car relay: 12V, Max 30 amp.

The pump use 12V and 4 amp.

The fuse need to be 4 amp?

 

Can the switch for example be wired directly to pluss and minus on the car battery or do I need a  resistor there?

Thanks!

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  • 0

    1n4002 is a PS diode, its okay but not really manufactured for this application.  Early ones were quite slow, now they are average.  The 1n914  is gold-doped for minority carrier lifetime suppression and is much faster.  Because you are using a relay, you will be fine with it, only better than nothing protecting a transistor.  Say, why are you using a relay rather than a transistor?

  • 0 in reply to D_Hersey

    Good point Don, picking the best diode depends on the specific circuit and objectives. Even the choice of using a flyback diode may not be the ideal way to control flyback pulses in some applications since using a flyback diode can delay the relay turn off until the coil has discharged.

    The specific relay may also impact diode choice - for example some 12V relay coils have an impedance of 28 ohms. This would result in a coil current of over .4 amps. Since the current through the coil cannot change instantly, the flyback diode would see the same current, which is above the rated current for a 1N914 - so its life would be shortened if it were used with this relay.

    Conversely the 1N4000 series diodes can take higher current, but may allow a short spike up to 500 ns as they turn on.

    You can of course get high speed, high current diodes - if the application justifies the price.

     

    For those who haven't seen these flyback voltages, here is a video...

    (Note he calls it back EMF which is not an accurate term for flyback voltage or inductive spike)

    https://www.youtube.com/watch?v=43HFng0CVKg

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  • 0 in reply to D_Hersey

    Good point Don, picking the best diode depends on the specific circuit and objectives. Even the choice of using a flyback diode may not be the ideal way to control flyback pulses in some applications since using a flyback diode can delay the relay turn off until the coil has discharged.

    The specific relay may also impact diode choice - for example some 12V relay coils have an impedance of 28 ohms. This would result in a coil current of over .4 amps. Since the current through the coil cannot change instantly, the flyback diode would see the same current, which is above the rated current for a 1N914 - so its life would be shortened if it were used with this relay.

    Conversely the 1N4000 series diodes can take higher current, but may allow a short spike up to 500 ns as they turn on.

    You can of course get high speed, high current diodes - if the application justifies the price.

     

    For those who haven't seen these flyback voltages, here is a video...

    (Note he calls it back EMF which is not an accurate term for flyback voltage or inductive spike)

    https://www.youtube.com/watch?v=43HFng0CVKg

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