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General Relay Principles


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pstroking99 
Member - Posts: 1
Member spacespace
Joined: August 24, 2015
Posted: August 25, 2015 at 9:35 PM / IP Logged Link to Post Post Reply Quote pstroking99
This time last week, I had very little experience with electrical circuits but I've spent a lot of time reading and researching and over the weekend I discovered this site. I used the Latched Output - Momentary to Constant Output - Positive Input/Positive Output to solve a problem.
The wiring diagram was easy to follow but I did stumble a bit on the diode shown on the relay on the left. Initially, I didn't understand why the diode was needed but a quick Googling answered that question. A snippet of one answer... "You need the flyback diodes because at the instant you open the switch, a relay's inductance will try to maintain the current. Adding a diode will create an easy path for this current to circulate until the relay's inductance has lost all its energy."
This leads me to two questions that Google has not helped with, likely because I've failed to come up with the proper search term.
First, why is a diode not needed across the coil of the relay on the right? That relay isn't directly connected to a switch in the same sense that the relay on the left is but it is ultimately switched (the ignition in my case).
Second, can someone explain what is going on that causes the coil to try maintain the current?
Thanks in advance!
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
Location: United Kingdom
Posted: August 26, 2015 at 1:23 AM / IP Logged Link to Post Post Reply Quote howie ll
How many months have you got?
Heard of a spark gap oscillator? Early form of radio transmitter?
What happens as the coil collapses is that a spike of about 200 volts forms across the relay coil. Also known as a quenching diode the diode stops that voltage racing back to the driver which circuit it will then fry.
The answer to your question about 1 diode in that diagram has just been answered.
Relay on left, its coil is driven directly via the blue wire hence the diode. Relay on right is connected to the relay on left which isolates it from the blue wire.
Frankly I've no idea on the theory as to why relay on right latches, never used that configuration in over 40 years (see Omron latching relays) PS Some relays come ready dioded, NB those diagrams also go against ISO convention, 85 is coil NEG, whilst 86 is coil POS. Some relays also use a resistor instead of the diode.
I don't do theory I install, caught that 200 volts down my hand in my 20's that's why I ALWAYS use the diodes (1N4004).

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