I am wondering if there is a certain label or naming convention for a relay that can mechanically hold its contact state after the coil is de-energized. I thought this is what a "latching" relay does, but after some googling, it looks like latching relays hold their state by opening/closing a current path for a secondary coil. I would like for there to be no power consumed by the relay once the switch has changed state. Does such a device exist?
As others say, a latching relay will stay in its last set poition without power.
The latching relays you mention, that need power to hold in one state, are better described as relays with a self-latching contact or circuit. One application for this sort of latching system is as a relay controlling a motor - when you push the "start" button, the relay pulls in, and one contact closes to maintain power on the coil when you release the button. If power fails, the relay releases, and the motor will not start unexpectedly when power is restored.
Latching Relay coils tend to be high power and demand short periods of power. Whereas Non-latching relays can have a high effective current gain of I_contact(max)/I_coil = 1000 to 5000. Faster power Relays tend to be lower ratios and Latching Relays even lower ratio.
They come in 2 versions;
- single coil with bipolar voltage +Set , -Reset, 0V idle these are used all types of "relays" aka ...solenoid, relay , Contactor or transfer switch body.
- dual coil with +Set and +Reset being exclusive inputs that are permitted to operate one at a time. Here the common coil end is often metal case grounded.
There are other ways to make a latching relay too.
I thought this is what a "latching" relay does
It is called a latching relay and all the ones that I have seen retain their state of latchedness through any power cycle.
Wiki says this: -
A latching relay (also called "impulse", "keep", or "stay" relays) maintains either contact position indefinitely without power applied to the coil.
Such relays are known as "latching" or "bi-stable". They mechanically stay in their current state when power is removed. You seem to have gotten the wrong idea what "latching" means for a relay. These do exactly what you are asking for.
Such relays either have two coils, one to drive the relay to each state, or a single coil and the direction of the current selects the state that the relay is driven in.
There are also relays called "magnetic latching relay" that use a coil to pull the relay contact arm into one position where a magnet then can hold the arm in position even though power is removed from the coil. This type of relay has three typical methods that are used to release the magnetically held contact arm.
- A second relay coil is energized and can pull the contact arm back away from the magnetically held position. This second coil exerts a stronger pull on the arm to overcome the magnet.
- The primary pull in coil is used with a reversed current flow direction. This produces a repulsive field to cancel out the magnet field and allows the arm to return to its non-latched position (usually by a spring).
- The relay may have a mechanical button or lever which can be physically activated to release the magnetically held contact arm position. Again a spring is used to return the arm back to the unlatched position.