Relays with diodes

[PDUB]

Originally Posted by IndyCobra

Actually, voltage spikes (the perceived need for the diode) are mostly a figment of the imagination.

If this in fact were true, then your ignition system would not create a sprark! Engine no runny...

Quick primer:

Anytime the magnetic field surrounding a coil collapses, i.e. when you shut it off, you get an electric field, and thus a voltage, induced that is associated with said collapse. This is associated with inductance. The induced voltage attempts to try and keep the same amount of current flowing in the circuit as just before the switch was opened to turn off the relay. These induced voltage spikes can reach extremely high values, depending on conditions. This is basically how an ignition system generates a spark. And, it is this induced voltage that can damage componants.

The diode, often called a fly-back diode, is mostly important when you are using an IC to turn the relay on and off, as in the diagram for which Kobrabytes provided the link. In this case the transistor is in an open collector configuration as a low-side driver, and operates as the switch control for the relay coil circuit. Keep in mind that we are talking about the relay coil and not the relay load being switched, which could also have a separate inductive spike issue. The diode provides a discharge path for the inductive spike when the relay coil circuit opens, thus protecting the transitior switch. Again, this has nothing to do with the load switching.

If you want to see just how big inductive spikes can get, look here:

http://www.youtube.com/watch?v=0pRZ-...eature=related

or here:

http://www.youtube.com/watch?v=pqMYYlYq4Ho

and don't forget to duck!