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Do I Need a Diode in this Circuit for a Self-Powering Relay?

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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
Printed Date: July 28, 2021 at 12:37 PM

Topic: Do I Need a Diode in this Circuit for a Self-Powering Relay?

Posted By: mcbeginner
Subject: Do I Need a Diode in this Circuit for a Self-Powering Relay?
Date Posted: August 24, 2020 at 7:43 AM

I am redoing my custom wiring on my motorcycle, and the result will use two relays.

The first is simply to use a switched source so that the battery can only power items when the bike is powered, to prevent the battery draining when parked (and not being a free USB charger!)

The second is used to power auxiliary lighting and has two functions:

1/ Avoid the full current of the lights going through a simple handlebar mounted switch. So the coil is powered from the first relay, with the switch between pin 85 and ground.

2/ When the bike's power is turned off, I want the lights to remain lit if the handlebar switch is already closed. Obviously once the switch is opened the lights will turn off and closing the switch should have no effect until the bike is powered again. To do this I will connect pins 87 and 86 of the relay.

Is that alone enough, or do I need to use a diode when connecting those pins on the second relay? Is there not a conflict between current trying to flow between pins 86 and 87 (coming from the first relay to power the coil) and trying to flow from pin 87 to 86 (the intended direction so the battery positive connected to pin 30 can power the relay coil) when the bike is powered? Or does it not matter?

Here it is as a diagram, which I hope makes sense!


For reference, the switched source only provides 7.5A of which half is already used for other purposes. This is why I want to power everything directly from the battery as I can control the wiring and fuses to ensure there is no risk of overloading anything. I will have a 20A "master" fuse coming off the battery, and separate fuses for each application after the relay for added protection, but these are not on the diagram. I prefer to use the negative poles on the sources as ground. Also, the relays do have internal diodes as shown in the diagram.


Posted By: mcbeginner
Date Posted: September 02, 2020 at 7:08 AM
Just to answer my own question, there is an obvious flaw in my circuit so of course I need a diode when connecting the relay pins. I also got advice to add a second diode between pin 87 of the upper relay and 86 of the lower one. After working on some other areas of the bike, this will be my next project.

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