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Did you pay attention to details of using relay?


Application of a relay, I think everyone knows that it can work as long as it is turned on and off in circuit.

However, I don't know if you paid attention to the details of its application. Let me share my point of view.


Current popular connection method as shown

Did you pay attention to details of using relay?

In figure, relay coil passes through Q1 like a switch that turns it on and off. D1, as a free current, consumes energy in coil.


Relay characteristics

1. The pull-in current is greater than release current

Did you pay attention to details of using relay?

2. The holding current is less than pull-in current and more than release current.

The above two points are "common faults" of relay, you can experiment or read manual.


Advantages and disadvantages of popular schemes

As we all know, a relay coil is equivalent to an inductor and its current cannot suddenly change. At moment Q1 is released, coil will still maintain original current level, if diode D1 is not connected, generated voltage is theoretically infinite (when external circuit load is infinite), in popular circuits, connecting D1 provides a channel for releasing energy in coil.

However, if (theoretically) diode is ideal, i.e. conducts only in one direction without any power consumption, then when relay is released, current in coil will always maintain maximum current when it is pulled in (and if coil is ideal), this situation will not allow relay to release.

The real diode and coil are not perfect, so they can be removed. The pull-in to release of relay is determined by current in coil. If equivalent resistance (DC) of diode and coil is small, its release time will be very long, otherwise it will be short.

From this point of view, advantage of popular circuit is that it provides a channel for releasing power when Q1 is turned off, but disadvantage is that release time can be further reduced.


Other links

I have seen circuit in figure below, and also connection without diodes in figure below. These connections took into account reverse voltage when Q1 muting switch is open, but did not take into account release. time.

Did you pay attention to details of using relay?


Suggested Connections

1. Add resistor R1 for faster energy release.

Did you pay attention to details of using relay?

In figure above, when coil is turned off by Q1, energy is mainly consumed by R1, so that relay can quickly drop to release current.

The choice of resistor R1 is determined by largest backpressure Q1 and operating current of coil. The greater resistance, shorter release time. ---- Let's not talk about calculations.

2. Reduce power consumption while holding relay.

As we all know, a relay requires a lot of current when it's closed, and current doesn't have to be same as current when it's closed to keep it closed.

Connecting resistors R1 and C1 in figure below will greatly reduce relay's holding power consumption. Before retracting relay, C1 was charged to supply voltage, and at moment of retraction, C1 will be charged

The relay is energized to provide high current needed to operate. In off state, current supplied to coil comes from R1, which limits current to a smaller state.

Did you pay attention to details of using relay?