Skip to main content

LED Relay Emergency Lamp

The post shows how to configure a simple LED relay changeover emergency light circuit using a battery back up which gets charged during mains presence and changes over to LED/battery mode as soon as mains fails. The idea was requested by one of the members of this blog.

Discussing a Circuit Idea

The following discussion explains the application details for the proposed LED relay changeover emergency lamp circuit

I am trying to make very simple changeover circuit.. where I am using a 12-0-12 Transformer to charge a 12v Motorcycle battery via mains.. when the mains go off the battery will power a 10w LED. But, the problem is the relay is not switching off, when the mains goes down.. Any ideas. Want to keep it really simple.. 12VDC Relay / 2200uf-50v cap on Transformer.

Hi, make sure that the relay coil is connected with the rectified DC from the 12-0-12 transformer. The relay contacts should be only wired with the battery and the LED

Hi Swag,

Firstly Thanks for the Reply.

1. Yes the Relay Coil is connected with the Rectified DC.

2. If I connect the relay contacts to Battery / LED only, then how will the Battery get charged when Mains is ON?
If i am not missing anything..

The Design

The above circuit is self explanatory and shows the configuration for implementing a simple LED relay changeover emergency lamp circuit.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. I think your connection is right in theoretical method, So relay should not remain in on state After mains off, because reverse voltage supply of Diode 5408 is negligible and not enough for Drive Relay.
    1. Please check the reverse bias Leakage current and voltage of IN5408. Read Again Electrical Properties in Datasheet of Diode 5408
    It may possible to reverse leakege current/voltage of diode IN 5408 is High and enough to drive this relay
    2. A Reason may cause to take the relay on, The Capacitor which is used in Filtering to Rectifying voltage on the AC to DC conversion. So A discharge component at capacitor may resolve this problem.

    Or Both Diodes are connecting in circuit make a connection series with each other (one diode is reversed with other) . And one terminal is common of both connect with ground in this case this makes like transistor .

  2. that's simply impossible unless your 1N5408 is FAULTY.

    You mean to say the voltage from the battery will hold the relay ON even after the mains is off, that can never happen...

  3. reverse bias Leakage current can be a problem if the diode gets hot, or the ambient temperature is significantly high


Post a Comment