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referenced by lowNslow
Yes, that schematic leaves a little to be desired. That diode should probably be put upon every airplane in the fleet regardless of what equipment is/is not aboard. There is another Cessna Service Letter which adddresses that matter, but basically.....
When the master switch is turned off, the electrical "ground" is removed from the battery solenoid/relay, which subsequently relaxes as it's magnetic field collapses. When that occurs it is common for an electrical "spike" to hit the electrical system. That "spike" can be as much as 600 volts!
If the pilot forgets to turn off all his avionics before turning off the master switch then all the avionics receive that "spike" which may cause damage to some solid-state components. Installing a diode across the master (battery solenoid, not the actual cockpit switch) will provide an "escape" for that "spike" back to the battery. The diode does not have to be large, but a 1watt diode will cost you almost a whole dollar at Radio Shack and will do the job just fine. Connect it across the two large terminals of the battery relay with the diode symbol "pointing" towards the battery's connection with the relay. (Installing it incorrectly will let the smoke out of it immediately the next time you turn on your cockpit master. If you let the smoke out of any electrical device they will not properly work ever again. Don't believe? Try it.)
To clarify, the diode shown on the schematic in the Alternator Conversion Kit addresses a somewhat different issue than the one installed on many master and starter contactors. The one referenced in SK-172-22 suppresses spikes generated by the voltage regulator switching the field circuit on and off (a normal part of regulator operation). The one on the contactor suppresses the spike generated when the contactor is de-energized to shut down the aircraft electrical system. Both involve the cockpit master switch, but on different poles (circuits) in that switch.
Concerning connection of a diode on the master contactor (and starter contactor if you have one), "Connect it across the two large terminals of the battery relay
" may or may not have been what George intended to say, but taken at face value, connecting the diode this way connects it across the CONTACTS of the relay. Connecting a diode in this manner will â€œprotectâ€ an already very robust set of contacts from the relatively small spike generated by the field collapsing in the aircraft wiring (essentially a 1-turn coil), and any small relays and motors in the system. However, the BIG spike is actually generated by the collapse of the magnetic field of the COIL in the contactor (many, many turns), so you would be better advised to connect the diode from the large terminal on the battery side of the contactor to the small terminal with the wire that goes to the master switch. (
The admonition to place the diode band toward the battery connection still holds
) This places the diode directly in parallel with coil, and limits the spike to usually 1 volt or less. Contactors that come with the diode already installed are wired this way.
Hereâ€™s the only illustration I could easily find. Contactors recommended for the 170 have the positive end of the coil connected to the battery terminal internally as opposed the external wire in the photo, but the idea is the same:
If the diode is not installed across the coil, the spike generated by the coil shows up as a high negative voltage across the contacts of the master switch in the cockpit, and can cause arcing in the switch, possibly reducing its life. For what itâ€™s worth, as far as I know my airplane has its original master contactor and cockpit switch, and has worked reliably for over 50 years, 2800+ hours, and Lord knows how many switch cycles, all without benefit of said diode. That is likely a testament to the quality of Cessnaâ€™s cockpit switches. That said, it is still a good idea to install the diode.
Also, only a tiny fraction, if any, of the voltage spike gets into the electrical system in the airplane, even if the diode isn't installed. (Explanation: voltage in a circuit distributes itself proportionally according to the impedance of the components in that circuit. The impedance of an open switch is MUCH higher than the impedance of other components in the circuit, like light bulbs, avionics, and even the battery. ) If you feel you need to â€œprotectâ€ avionics from this small spike, and the spike generated by the collapsing magnetic field in the aircraft wiring, place an identical diode from the power bus (or the bus side of the master contactor) to ground. Orient the diode with the band on the bus end.
If this isnâ€™t technical enough for you, take a look at this.