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Why is a diode connected across a relay?
A flyback diode or freewheeling diode is placed with reverse polarity from the power supply and in parallel to the relay’s inductance coil. The use of a diode in a relay circuit prevents huge voltage spikes from arising when the power supply is disconnected.
What can I use for a flyback diode?
Schottky diodes are preferred in flyback diode applications for switching power converters, because they have the lowest forward drop (~0.2 V rather than >0.7 V for low currents) and are able to quickly respond to reverse bias (when the inductor is being re-energized).
Why do we use freewheeling diode?
Definition: Freewheeling diode is used to protect the circuit from unusual damage caused due to abrupt reduction in the current flowing through the circuit. It is also known as Flyback diode and forms connection across the inductor to remove Flyback voltage generated across it.
How to choose a fly back diode for a relay?
Here R is the resistance of the fly-back diode in conduction, I is the current flowing through the fly-back diode and L is the inductance supplying the current. If we solve for the power, something very interesting happens…
Can a Zener be used as a flyback diode?
For some high-end apps they use a zener across the relay coil, but in series with a diode and the zener is inversed to what is shown above. This provided the fastest relay release possible while still allowing the flyback voltage to be “clipped” by the zener at whatever voltage is needed to ensure the Vce safety fo the switching device.
What should the reverse voltage of a relay be?
In your relay, the coil current is shown as 79.4 mA. Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement. The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating.
What happens when the diode is turned off on a relay?
The diode current at turnoff is the same as the current when on, as Mike said above. This current through the diode has the effect of continuing the relay magnetic field until the current finally drops to zero. The effect is a slow release of the relay. In some circumstances this can cause arcing of the contacts as they are pulled apart too slowly.