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How to drive relays?

Discussion in 'General Electronics Discussion' started by s57aw, Dec 18, 2013.

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  1. s57aw

    s57aw

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    Dec 18, 2013
    Hello guys,

    I am quite newbie in electronic and more or less self-taught in this area.

    I would be happy if you take a look into these schematics with Arduino:
    http://remoteqth.com/hw/band_decoder_sch.png

    I have project completed and digital outputs from Arduino send signal through resistor and led diode, optocoupler and NPN transistor.

    There are 12 open collector outputs and I want to drive 12v relays (about 30ma, 360 ohm coil resistance) on the SV2 outputs.

    I have applied +12v to the relay, put diode over coil and connected specific output to other side of the coil but relay doesn't switch, I got only appropriate led diode on but it looks no output for the relay.

    What I am missing - appreciate your suggestions. What about pin 13 on the SV2 output, should it be connected somewhere?

    Regards,

    Robert
     
    Last edited: Dec 18, 2013
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,646
    2,016
    Nov 17, 2011
    PIn 13 is Ground. You connect it to the - of the 12V source.
     
  3. s57aw

    s57aw

    9
    0
    Dec 18, 2013
    Thanks Harald,

    I was not sure as schematics tells notning about it, it is floating now and probably that's the reason why I get no current throught. Will try that

    Robert

     
  4. s57aw

    s57aw

    9
    0
    Dec 18, 2013
    FYI..

    This is "Band decoder" for radio transmitter (TRX). TRX is connected to Arduino with serial cable (only TX, RX and GND is needed). Arduino reads frequency information from the TRX and based on it puts signal to one of the 12 outputs. I need a relay at the output to drive an antenna switch (switching different antennas for different frequencies).

    Arduino needs +5v and I would like to use 12v relays at the output. This is not an issue itself, I supply 12v to the board and output relays and using a simple DC voltage regulator from 12 to 5V for Arduino. I don't have two separate PSU, ground is the same and sources are not isolated in current configuration.

    Anyway, I didn't change anything, all soldered on PCB as per schematics and I have connected PIN 13 to the same ground for test and relays are working now, but... I would do it again I would use a another simpler approach.

    I still don't understand kit schematics / author completely - why the combination of optocoupler, switching transistor, and relay, it looks redundant. I know I need a transistor to effectively and safely drive the relay, but the relay (plus the diode across the coil) would be enough for isolation I think, why optocupler? And why there is a diode across NPN transistor (I am using another diode over relay coil).

    Robert
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,646
    2,016
    Nov 17, 2011
    You are right, a transistor alone would suit the task here. I can only guess that the kit ma be used for other purposes, too. Then the potential isolation offered by the optocoupler may be required if there are no relays. A potential application is the control of mains voltage via a triac.

    You can do either. A diode across the transistor has the same effect as it will conduct the reverse current from the relay coil back to the power supply. The author of the circuit could not fresee all possible uses and may have provided the diode across the transistor just in case. Having both diodes in place wil not harm the circuit.
     
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