zener diode

it looks like the diodes are being used to prevent a huge arc or surge when shutting the solenoids off again....
 
This is a zener diode symbol; I didn't see it in you pic.
http://en.wikipedia.org/wiki/Zener_diode
Zener_Symbol.jpg


The standard diode symbols across the coils are as DwSoft pointed out. Its not unusual to have them built into the solenoids but common to show them in the schematic.
http://en.wikipedia.org/wiki/Flyback_diode
 
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More usually a rectifier diode and reffered to as a flywheel diode. they are often built in to valve caps and their function is to suppress damaging back EMF voltages generated by the collapsing magnettic field around an inductive load e.g. solenoid.

Nick
 
I agree that the symbol on this drawing is not the correct symbol for a zener diode. If the part number specifies a zener, then it is most likely a zener sub-class more specifically known as an avalanche diode. The intention is obviously to act as a snubber circuit for the solenoid. Am I surprised that they were not actually installed?

No, I am not. ;)
 
As mentioned, the symbol in the drawing is for a standard diode. A zener diode would work, if the breakdown voltage was greater than the solenoid drive voltage. A zener would limit the supply voltage to the solenoid while also provide flyback voltage path when the solenoid shuts off.

If they were not installed, either they were not installed and should have been or the drawing shows more details than strictly necessary and the diodes are built into the solenoids (which is very possible, many solenoids and relays have the diodes built in).
 

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