Relays with diodes

[Gunner]

Your question is too vague to answer except in equally vague terms.

A relay should be used any time you want maximum current to a lamp, lamp string, heater motor, etc. without passing it through long wiring and switch.

Diodes are rarely needed except in those cases where you need to have multiple sources drive the same lamp (e.g., brakes, turn signal, emergency flashers). Diodes are kind of a kluge and can be avoided with correct wiring in most cases. They're used to patch in a new circuit without rewiring other parts of the system, such as when you add emergency flashers to an already-wired car.

If you have a more specific question I can try to give a more specific answer.

[kobrabytes]

http://www.kpsec.freeuk.com/components/relay.htm

[ccomt]

Here's a brief primer on relays that discusses the reason for diode protection:

http://www.rallylights.com/hella/Relays.aspx

Here's a link to specs and info on a Hella diode protected relay used in automotive apps:

http://www.rallylights.com/detail.aspx?ID=412

[IndyCobra]

Actually, voltage spikes (the perceived need for the diode) are mostly a figment of the imagination. The mostly likely scenario to produce a spike is when the alternator is removed and then added back into a charging circuit when the engine is running. In the aviation community, there's typically a switch that allows the pilot to remove the alternator from the circuit in order to run the electrics from the battery only. This is done for safety and redundancy reasons. Cycling this switch "may" cause a spike, but even that has been elusive to prove scientifically. In automobiles, the alternator is wired to always be in the circuit so there's no way to turn it off and then turn it back on again.

Bottom line is whether you believe in voltage spikes or not, I wouldn't worry much about diode protection in your relay. It won't hurt anything to have it, but it likely isn't doing anything to begin with.

[Gunner]

Originally Posted by IndyCobra

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

It might seem so in auto/aero electric circuits. They're certainly real when driving relays with electronic components - I've seen many a transistor or IC power output damaged by driving a relay without a protection diode. I suspect they're just as real in vehicles but there are no components sensitive enough to be affected by them. It becomes a "might as well" sort of protection under those conditions, probably never serving any purpose. As they don't cost anything (maybe a buck, for a new, high-power diode) and can't hurt... "might as well."

[IndyCobra]

I worked closely with two experts in the aviation electrical field a while back. The issue of voltage spikes is a big concern in the aviation community because you don't want a spike to destroy $50,000 worth of sensitive avionics if a simple diode can prevent it.

They tried repeatedly to create one in numerous aircraft under strict scientific testing and could not. That doesn't mean they don't occur, but they were sufficiently satisfied that current generations of personal and commercial aircraft are sans diode protection.

I admit that this doesn't necessarily transfer to automobiles where the comformance to strick electronic standards for component parts is probably less rigorous. So install a diode and drive happy.

[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!