The diode is placed across a relay coil that is powered by DC, to absorb voltage spikes generated by the coil inductance when the current through the coil is shut off. Without this diode, the high voltage spike generated by the collapsing magnetic field of the coil ("inductive kickback") can damage solid state components, particularly a transistor or other semiconductor used to control the relay coil.
The diode is installed so it is normally reverse biased (cathode to the positive end of the coil). The voltage generated by the inductive kickback will be of the opposite polarity, and will cause the diode to conduct, absorbing the spike. I usually use a diode with a PIV rating at least 5X the applied coil voltage. Something in the 1N400x series is suitable for most situations.
I accidentally discovered inductive kickback when i was a teen. I was holding the wires on a relay, wired it up so that the coil was in series with the normally closed part of itself. (Tired to make it like a buzzer) Hooked it to the battery and got little shocks. Sometimes I've also seen a 0.1uF capacitor in parallel with the coil and diode. I often see the capacitor in parallel of small motors, such as in toy RC cars. The capacitor helps smooth out the inductive kickback because (i hope i got this right) a diode can take a small fraction of a second to work. I've even seen the caps on a bridge rectifier, although I'm not sure if it's for the same reason.
The capacitors perform a similar function, but not exactly the same.
The capacitor across a diode acts as a short circuit to any high frequency AC created when the diode turns on and off. The capacitor across a DC motor armature (and often from either end of the armature to ground, as well) helps filter out noise from the brushes, and prevents it from interfering with the control electronics.
I have gotta ask ... your User Name ... is it who I think it is modeled from (part of an "Older" Trio), or is it your real name.
I am thinking "Larry Fine" as in "Howard, Fine and Howard".
If needed, feel free to E-mail me at the Phoenetically listed address below:
setelectric at pacbell dot net (without the spaces or text for the symbols).
BTW, the "UnCap'ed" Brushes of DC Motors generate a lot of RFI - in the form of harsh pulses, which shows up big time on TV sets (mostly in the AM video part, but have seen in both AM video and FM audio parts), create havoc in other areas of Radio use (AM, HAM, etc.) , and may cause control signal errors being received on Carrier Current applications (or RF controls).
Damping the RFI is another reason to bypass the Brushes / Commutator with Non-Polarized Capacitors. Non-Polarized to allow the passage of "AC-Like Noise" ... I think that's correct (been some time!)
To add some more to the Clamped DC Coil thing, one very common application where a Capacitor was used in conjunction with a DC driven Coil, was the Automotive Ignition System.
The Coil was controlled by the "Breaker Points", which opened/closed the "Primary" circuit of the Coil, so a high voltage DC pulse was achieved at the "Secondary", and Distributed to the approporiate Spark Plug. The Points were clamped with a Capacitor (using the nomeclature "Condenser"), so the points would not become prematurely Barbequed from the surged DC - as result of opening the circuit.
It was a simple Parallel clamp method, using something like a 4.7 µF 125 VDC Cap., connected between Line side of points and Chassis "Ground" (the distributor)
Just some additional FYI.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!