I think so. When the magnetic field collapses, the energy that was in the electro-magnet (coil), looks for someplace to go. Also, it increases the voltage to a very high number, while it arcs across the contacts. The diode gives the voltage somewhere to go, instead of back at the contacts, where it potentially erodes the contacts, much like an Arc-Welding machine does with consumable "sticks". It kicks that positive charge back at the positive source that feeds the relay, forming a dead-end. I'm not quite sure why that works, but I can take it on faith, rather than study the chemistry of semi-conductors.
That's a pretty good description.
The characteristic of inductance is that current through the inductor cannot change quickly. While the inductance is energised by current flowing from the power supply (via some contacts), current flows from the power source, in the top end, out the bottom end, and to ground. (This is "conventional" current, which flows from positive to negative, rather than the actual electrons, which flow the opposite way.)
When the circuit is interrupted, the inductor "tries" to keep the current flowing at the same magnitude and the same direction. This means that the grounded end now becomes positive and the top end swings negative, the opposite polarity from the voltage that was applied to it. The inductor can generate a large negative voltage in its attempt to keep the current flowing. This is the voltage that can arc across the contacts.
If you connect a diode in reverse across the inductor, it provides a path for the current to take, so the inductor doesn't need to generate a high voltage in order to try to keep the current flowing through the inductance. This obviously protects the contacts that are in series with the inductance from being damaged by high voltage arcing.
The diode provides a direct path to keep the current flowing in the inductor, and the current (and the magnetic field) collapse relatively slowly because little energy is lost in the diode (it has a forward voltage of about 1V). If the voltage wasn't clamped so hard, they will collapse more quickly. This is the reason for the zener; it clamps the voltage, but only to 100V or so, so it dissipates the energy from the inductor fairly quickly. In the case of the relay coil, this allows the relay to open quickly and cleanly.
I may have mis-stated it, but I understand to put the zener in series with the diode, pointing the (negative?) sides at each other. (The whole anode/cathode thing has me confused... cathode = negative?... or did I get that backwards?) Anyway, that's easy enough to look up, whenever I am ready.
The series combination of a diode and a zener doesn't conduct at all when the top end of the inductor is positive (i.e. while the inductor is energised by the current flow from the power supply), and when the top end of the inductor goes negative due to back EMF, the series combination conducts with a 100V drop, i.e. it clamps the voltage in that direction to 100V. (Or whatever voltage you choose.)
Understood. When this relay fails, as it most certainly will. I will take the time to add the zener to the diode, on the relay on the coil. (Did I get that right this time?
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Sure. Personally I would add it anyway, but that's up to you.
Is it just my imagimation, or is that zener large enough to plug my :ahem: porcelain throne?
The zener isn't particularly big. The 40A diode that goes across the pump is pretty meaty though. I wouldn't sit on it though; it has sharp edges
1n4001 diode, and 1N5378 zener reversed at each other across the coil of the relay
1N1190AR 40A, 600PIV Rectifier Diode, in parallel across the pump.
http://www.mpja.com/1N1190AR-40A-600PIV-Rectifier-Diode/productinfo/18393+DI/
Right.
I went to mouser, and searched for the parts discussed here. They all seem to come with letters and/or numbers on the end, and sometimes on the front, but contained in the middle were the "names" - so to speak.
I am assuming that the extra information is there for the purpose of cataloging, and warehousing the parts, rather than being a different spec?
It varies. With semiconductors in general, the prefix usually identifies the manufacturer, although 1Nxxxx diodes don't have a manufacturer prefix. Suffixes for semiconductors in general can indicate many things, including the package, voltage rating, gain range (for transistors), improved accuracy (op-amps), temperature grade, lead-free manufacture, packaging (tape and reel) for surface-mounted components, and other characteristics. In this case, the presence of an R in the suffix is important because it indicates "reverse" polarity - see CDRIVE's comments in an earlier post.