FLYBACK DIODE USED ON RELAY CIRCUITS

Typical Electromagnetic Relay

Typical Electromagnetic Relay

The chatter of a relay is not often heard in the modern ham radio, but when I first got my amateur radio license there would most likely be more then one relay circuit somewhere in the shack. Today, most of the time, solid state circuitry is used rather then the bulky electromechanical relay or electromagnetic relay for switching circuits off and on.
The relay is a rather simple device. It has an electromagnet winding that attracts an iron armature when it is energized. The armature is connected to an electrical switch contact that will move when the armature moves. The movement will either bring the contact in contact with another switch contact thus completing the circuit or it will move the contact away from a contact it is touching while the coil is not energized thus opening a normally closed circuit; it is also possible that the contact can be moved off one contact (normally closed) and connected to another (normally open). The armature might move one contact or more.

Relays are not completely relegated to history though most of the time when modern hams come in contact with them it is when they are using older equipment. One place you might want to use a relay is if you want your mobile radio to automatically shut off when you turn off the ignition. This can easily be done by using a power relay that is energized by taping its coil into a circuit that is turned off and on by the vehicle’s ignition and switch the 12 VDC through a normally open contact on the relay. Power for a mobile radio should always come directly from the vehicle’s battery and not be connected a wire inside the cockpit.

Vacuum tubes are high voltage devices thus they can be very forgiving to voltages that will destroy solid state equipment. When we hooked relays up in or associated with tube radios we normally did not have to be concerned with voltage spikes caused by the collapsing magnetic fields into the coil but with solid state equipment that is a whole different story.

When the electromagnet is energized it builds up a magnetic field around it. If the coil is energized by Direct Current (DC) then when the circuit is opened that magnetic field will collapse back into the coil thus creating a very high reverse voltage. A 12 volt relay coil can generate over 200 volts reverse voltage. This voltage can destroy solid state devices.

This effect is known as “flyback”. Flyback is used to generate the voltage needed to operate the spark plugs in a gasoline engine. It is also used in the flyback transformer in a TV set to generate the voltage needed to operate the CRT.

To prevent flyback voltage from causing damage a diode called a “diode snubber,” “flyback diode,” or “free wheel diode” is placed SCHMETIC DIODE ON RELAYacross the coil in reverse polarity to energizing voltage.

When voltage is applied to the relay the diode snubber serves no purpose.  It is reverse biased so an insignificant amount of current passes through it. But when the relay is de-energized the resulting flyback voltage will be a forward biasing voltage which the diode will short out thus protecting any components that might be damaged by the high voltage.

A sine wave alternating current (AC) has constant smooth rise and fall of voltage so a strong magnetic field does not form thus a flyback diode is not only unable to be used but is not needed.

Square wave AC voltage should not be used on relays because of the sudden rise and sudden reverse in voltage will cause voltage spikes.

 

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