Voltage Monitoring Circuit Question

Discussion in 'General Electronics Chat' started by rainyday101, Apr 27, 2010.

  1. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    I have a voltage monitoring circuit built as shown below. Everything works fine. When the reference voltage of the TL431 is equal to or greater than the 2.5 volts, the LED will light. This tells me that my power supply circuit is above 22.5 volts. I would also like a to ad a red led that will come on if the voltage drops below 22.5 volts. I have tried several things with no luck. Any ideas on a simple way?

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  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You can have both if you include comparators into ur circuit.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
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    Try this. You can use any small power NPN transistor. Keep your original circuit 5K VR connection and ignore R4 which is there for simulation purpose only.

    [​IMG]
     
  4. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    I was gonna head the comparator direction as my next step, but eblc1388 has done what I was unable to do. Looking at his add-on I was close and had the right idea- but not close enough. Thanks much, I am going to add the transistor and give that a try.
     
  5. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    My circuit I tried was almost like the one you posted except I was using a 6.8K resistor on the base of the transistor and did not have diode d3 on the emitter. Of course with this the transistor was always on. I increased the value of the base resistor, but not high enough. For couriosity's sake, what is the purpose of d3?
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
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    It's one of the safety measures to ensure that the NPN will never turn ON when the TL431 is ON.

    When the TL431 is ON, it would still has about 1.1V across its terminals. A NPN transistor biased with some value of base resistor(even with 47K) will still easily get turns ON, as you have found out in your design.

    I have done two things to prevent this from happening. First I placed the LED on the emitter and that adds about 1.8V to the turn ON voltage. Then I added the diode to get another 0.6V margin. Now it would require about 3V at the base of the NPN to turn it ON.

    You can remove the diode and the circuit will still work, but I always want to be absolutely sure.
     
  7. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Thanks alot for all the help!!! That makes sense because the way I did it I had 1.8v on the base with my lower value of base resistor.
     
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