Diode Across Ac Relay Coil

On the attached schematic theres a diode on the coil of the relay to protect the system from the coil spike when the relay opens. Kind of like a zener diode for ac.

Relays Rig Nitc

X2 Pc792h 1a C2 12c Dn X 2 4 Pin Spst 12 Vdc Coil 60 40

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Why do we put a diode across a relays coil.

Diode across ac relay coil. Ie is a clamping diode always stopping positive from reaching negative or vice versa or. To prevent damage you must connect a protection diode across the relay coil. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes.

You have probably seen it many times. For example use an mov across coils and contacts that operate on 120vac etc. This explains why theyre rated at off voltages.

This allows the coil field to collapse without the voltage spike that would otherwise be generated. Normally a designer puts in plenty of reserve in the reverse rating. The diode provides a path for current when the current path to the relay is interrupted ie.

It is used in circuits in which inductive loads are controlled by switches and in switching power supplies and inverters. Todays back to basics tutorial topic why flyback or snubber diodes are used around relay coils when switched or controlled by low power electronics. Many articles will say something like to protect the transistor from damage due to the back emf.

The diode reverse voltage rating should be at least the voltage applied to the relay coil. Why is it done. Well talk about how and why dangerously.

Relay coils produce brief high voltage spikes when they are switched off and this can destroy transistors and ics in the circuit. The diode again becomes forward biased. Again 1n4001 will do the job.

A diode in your application having 50 volts would be more than adequate. In the industrial world they get used lot with ac relay coils in place of flyback diodes as well as snubbers across the contacts of relays to control arcing. A flyback diode is a diode connected across an inductor used to eliminate flyback which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted.

Does it matter which way the 1n4004 diode is pointing. Generated in the relay coils inductance when the transistor turns off. Placing a diode across a relay coil passes the back emf and its current through the diode when the relay is energized as the back emf drives the flyback protection diode in forward bias.

Diode across the coil of a relay. When the power supply is removed the voltage polarity on the coil is inverted and a current loop forms between the relay coil and protection diode.

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