Are there "inverted relays"?

My answer to SE EE question Are there "inverted relays"?

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This story is another very recent example of how the questions asked in SE EE awaken my creative activity. Thanks to the experience I have from electronic circuits, I was able to explain in a new (for me) way how the operation of the humble electromechanical relay can be reversed.

My answer

Short answer: A relay can be "inverted" by "inverting" either the coil (connecting to Vcc) or contacts (make them NC).

Structure. A relay consists of two cascaded parts (stages) - an input part (coil) and an output part (contacts). They can be considered as a kind of "converters" with input and output: the coil converts the input voltage into a magnetic field and armature movement; the contacts convert the armature movement into a resistance (on/off circuit). Each of them can be "noninverting" and "inverting" so that a total of four combinations are possible. To make an "inverted" relay, only one of "converters" should be inverting.

Coil inverting. We can assume that the relay coil is "noninverting" if its other (neutral) end is connected to ground... and the coil is "inverting" if the other end is connected to the positive rail:

• In the first case (grounded coil), there is a voltage across it and power is consumed when the input voltage is high. The input current enters the coil; so the driving stage must be able to source a current. When the input voltage is low, there is no voltage across and current through the coil; so power is not consumed.

• In the second case (connected to Vcc coil), there is no voltage across it when the input voltage is high since the two voltages are subtracted (mutually neutralized) and the resulting voltage across the coil is zero; this is a kind of voltage biasing. As a result, there is no current flowing and there is no power consumed. When the input voltage is low, there is a voltage across the coil and power is consumed. The input current exits the coil; so the driving stage must be able to sink a current.

Contact inverting. We can assume that the relay contact is noninverting (normal open) if it is on when there is a voltage across (current through) the coil... and the contact is inverting if it is off when there is a voltage across (current through) the coil.

An extravagant way to invert the relay operation is to connect its contacts in parallel instead of, as usual, in series to the load. However, in this case, the load should be powered by a current source.

Field inverting. An even more extravagant idea is to "invert" the magnetic field of a reed relay by an additional magnet (magnetic biasing)...

Note: I have used in my explanations above some non-standard (descriptive) names for the purposes of understanding. If necessary, I can give additional explanations.

Comments

Here is something surprising for me that is happening to me here for the first time - a manifestation of human attitude on the part of a moderator:

@Circuifantasist's answer contains some useful points. While the use of NC or NO contacts, as mentioned by others, will often do what you want. - 1. Changing the coil connection from ground to V+ is an obvious but easily overlooked method of "reversing" operation. 2. Using permanent magnet biasing so that the coil counteracts the pm field is also "obvious" but verges on magic in some applications. A relay is normally operated with NO voltage applies and releases when power is applied. Very seldom seen. Can be a life saver. – Russell McMahon♦ 21 hours ago

And here is my response:

@Russell McMahon♦, Thank you for your attention; it costs little but means a lot. As you can see, my goal is to make young people learn to think and understand, not just to know. That is why I focus on curious and little-known properties of elements and circuits... and not on well-known banal facts around them... I want to make them inventors, not just neat performers. Another interesting feature of relays is their hysteresis which can make them act as latches with "internal positive feedback"... and, of course, connecting the contacts back to the coil ("external positive feedback")... – Circuit fantasist 20 hours ago

This is what this platform lacks - normal human relationships in which participants encourage each other in the name of a common goal...