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What is the purpose of this zener?

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wisepie

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I found a circuit below can anybody please explain to me why purpose of this zener - zener connection?
 

Re: Question

from what is put up in the circuit diagram... i think zener is only to avoid the back emf that the relay coil generates... relay coil is an inductor... so when we apply a voltage to it... it could produce a large back emf which could damage the device attached to it... to avoid this we generally use a zener diode in series with the relay coil...
 

Re: Question

but i'm still confused why there are back to back zeners... one forward biased zener is enough to prevent the back emf... what is the other zener for....?
 

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I think maybe to reduce the voltage to the coil... but I not so sure....
 

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you mean to say that to reduce the voltage applied to the coil...?
 

    wisepie

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Re: Question

wisepie,
I believe that he diode on the right should be an ordinary diode. In this application, the lower terminal of the relay coil would be connected to the +16V supply. When the transistor turns off, the inductance of the coil will try to maintain current flow thrugh the coil. with no protection circuit, the voltage on the transistor collector would rise to a very high voltage. The Zener clamps this voltage to a value equal to the Zener voltage + 16V. The right hand diode isolates the 16V supply from the transistor collector when the transistor is turned on. Without this diode, there would be a current path from the 16V supply thru the forward biased (left) Zener thru the turned-on transistor.
.
A common method of suppressing the positive transient is to just use the forward diode (equivalent to shorting out the Zener). This limits the voltage on the collector to +16V plus the forward voltage drop. This technique causes the relay to go the the unacuated state more slowly, which is undesirable.
Regards,
Kral
 

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