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This is the Circuit I Built

enter image description here

Circuit Source

I Assembled this circuit and once I connect the battery, it has to turn on the Relay and supply the Current to charge the battery.

but in my circuit nothing happens. So I checked with multi meter by putting one lead to Ground other lead to Coil terminals of relay and found that two coil leads of relay is receiving 12V.

as I Understand, Relay needs 0-12V to two ends of coil terminals to activate it.

I'm not sure whether i wired 50k preset correctly or not.

this is how I wired Preset

enter image description here

What could be the reason, My circuit not activating Relay?

Regards

lasita
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  • What is the part no. of relay you are using? – Aircraft Sep 29 '15 at 07:16
  • If you have a 12V relay, you put 12V over the coil, and it does not activate, the conclusion is that you have a faulty relay. (But do double/triple-check the premises before you accept the conclusion!) – Wouter van Ooijen Sep 29 '15 at 07:16
  • Relay seems to be working. i test it with a 9V battery and i can hear the relay activating noise and comm and N/O connection works. but with this circuit it seems both the coil terminals are receiving +12V. through the transistor it receives 12V and other terminal also receive 12V – lasita Sep 29 '15 at 07:48
  • The relay coil needs current flowing through it as well to energise. What is the voltage on the base of the transistor below it? Is it enough to turn the transistor on and supply enough current? Sorry sink enough current? – BenG Sep 29 '15 at 08:31
  • If both relay terminals have 12V, the left transistor doesn't switch. Does the LED turn on/off when you tune the trimmer? Pin 3 of the OpAmp should always have 5.1V. Does pin 2 cross this voltage while tuning? – sweber Sep 29 '15 at 08:41
  • @BenG base of the transistor seems to have 0.13V. – lasita Sep 29 '15 at 15:46
  • @sweber LED doesn't turn on or off when i tune the trimmer. pin 3 seems to have around 4.77V. while tuning pin 2 voltage is seem to change – lasita Sep 29 '15 at 15:48

1 Answers1

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You have very probably destroyed the drive transistor (the one with the collector to the bottom side of the relay coil).

As the is no Freewheeling diode, the first occurrence of the switch turning off will produce a high voltage spike at the collector (given by v = -Ldi/dt) which, under most conditions, will be a high enough voltage to cause catastrophic failure of the device.

I suspect your circuit switched on momentarily and the switch-off would destroy the transistor.

Place a diode (1N400x would be good choice) across the relay coil, anode to drive transistor collector, cathode to positive side of the coil.

See this question as well

Community
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Peter Smith
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  • Looks like transistor is faulty. Multi meter show some resistance between emitter and collector where it should be infinity for a good transistor. That info also i found it online though. – lasita Sep 29 '15 at 17:33
  • assembled the circuit with diode as you mentioned and now its working fine. :) thanks a lot for the help – lasita Sep 30 '15 at 15:54