☑ Diode For Relay Back Emf

With a diode across the coil the back emf will be about 07 volt ie. 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.

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This entry was posted in newsworthy november 1 2013 by andi.

Diode for relay back emf. When the switch opens the diode conducts to let the inductor current flow in an alternate path and limit the back emf to the forward voltage drop of a silicon diode 07v. The diode does a very good job of suppressing the back emf and clamps the voltage to around a one volt or so. The forward voltage drop of a diode.

This is quite suitable for small solenoids like those found in strikes. A diode might be usually observed linked across such relay drivers stages which can be carried out for safeguarding the bjt from the lethal back emfs kicked from the relay coil whenever its turned off by the bjt. It is used in circuits in which inductive loads are controlled by switches and in switching power supplies and inverters.

However these are not suitable for door magnets. At the moment the coil circuit is opened the current that was flowing through the coil now goes through the diode. 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.

When the switch is closed the diode remains reverse biased and inactive. The most common technique with dc switching is to use a diode in parallel with the relay coil aka freewheeling diode as shown in figure 1a. So the diode needs to be able to withstand that current.

Since getting rid of all the inductors is not possible we have to suppress this back emf. The diode again becomes forward biased. When the load is energized the diode is back biased and has no effect.

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.