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Feeding a DC relay with a 20 mV ac signal using two cascaded op amp and a NPN transistor, is it doable?

hooman KAKAVANDI
Prodigy 10 points

I want to use two cascaded op amps with a NPN transistor to feed the coil of a relay. Does anyone have any design suggestions? The input voltage is in range of 0 to 20mV and it is not DC. it is harmonic. I mean by harmonic is that it goes to a speaker and create a sound. 

Your help is much appreciated.

  • 0
    TI__Mastermind 23200 points

    Hi Hooman,

    We do have some content that goes into design considerations for this design. Please take a look at the Voltage-to-current (V-I) converter circuit with BJT to understand the design considerations needed to driving a BJT with an op amp.  That would be a good start.  

    Do you know how much current you would need to drive into the speaker? If it's not a lot of current, then we do have some op amps that might be able to drive the speaker directly. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 10 points

    Thanks Robert, yes, the link was really helpful. 

    I think I might clarify my meaning in this design a bit more.

    I have a device which has a 2 watts speaker with 9-ohm resistance. I am trying to use the output to the speaker for triggering an alarm for our DCS system which has 24 V base voltage.

    The level of the voltage to the relay which i found needs to be over 3.75 volts to be able to energies the coil. The output to speaker is in range of 20 mVs and current can be calculated by the wattage of the speaker I think, and I think it is not very much. I was hoping to use two op amps one in non-inverting amplifying and other one as the buffer. Then the NPN transistor to drive that coil.

    I hope my intend is clearer. 

    Thanks again for your time on this.

  • 0 in reply to hooman KAKAVANDI
    TI__Mastermind 23200 points

    Hi Hooman,

    Glad that link helped!

    Is the content I sent over sufficient or do you need further assistance? 

    Best Regards, 

    Robert Clifton