Relays are common electromechanical devices in electrical circuits that come in two types: either latched or non-latched. Latched relays retain their last switch position even after complete power loss and are available in either single coil or dual coil versions. The single-coil latched relay uses only one coil to set or reset the switch position, but requires positive and negative voltage. When positive voltage is applied, current flows in one direction and enters the relay into the set-state (i.e. relay switch closed). A negative voltage reverses the current direction, placing the relay into the reset-state (i.e. switch opened).

Dual-coil latched relays, on the other hand, use only positive voltage but require two power sources or drivers. Such relays have a set-coil and a reset-coil. When the set-coil is energized, the relay enters the set-state. Conversely, when the reset coil is energized, the relay enters the reset-state. The two coils are never energized at the same time.

If you wish to use a dual-coil relay, but the only driver you have available is for a single-coil relay, there is an easy way to convert that single-coil driver handle the two-coil relay, shown in Figure 1. This conversion to positive-only drive is particularly useful for relay testing, because you need only one voltage polarity instead of two. This reduction can significantly simplify the relay testing setup.

Figure 1 Diodes convert a single-coil relay driver to dual-coil use.

The operation is simple. When the coil driver output voltage is positive, current flow through diode D1 to energize the set-coil, while the reset-coil is unpowered because D2 blocks the current. The relay enters the set-state. When the voltage is negative, diode D1 blocks the current flow through the set-coil and diode D2 energizes the reset-coil.

The latched relay in Figure 1 has two independent coil connections using 4 pins. Some dual-coil relays, however, use only three pins with a common coil connection as shown in Figure 2 . This configuration needs a slightly more complicated configuration, involving four diodes.

As before, when the driver voltage is positive, current is flowing through diode D2, the set-coil, and D3. Diodes D1 and D4 are reverse biased, blocking the current to the reset-coil. Similarly, when the voltage is negative current flows through the diode D4, the reset-coil, and D1, while the set-coil is de-energized. Again only one coil is on at a time.

Figure 2 Four diodes are needed to convert the single-coil signal for dual coil use when the coils share a common connection.

The conversion circuit has the additional benefit that it makes things easier when testing dual-coil relays for their AC performance characteristics, such as operate-time (turn-on time), bounce-time, break-time, and maximum frequency. Just replace the relay driver with a square-wave voltage signal generator. Because many relay coils require high voltage, up to 48V, and high current anywhere from 20mA to over 1000mA in some cases, a signal generator alone may not be enough. In such cases a high-voltage function generator amplifier, such as the TS250 from Accel Instruments, is needed to boost the voltage and current (Figure 3).

Figure 3 Testing the dual-coil relay using only one function generator and a high-voltage driver

The diode circuits provide an easy way to convert single-coil relay drive signals to dual-coil use. This approach gives system designers the option of using either singe-coil or dual-coil latched relays without needing to change the driver. Furthermore it enables dual-coil latch relay testing with just one signal driver.

“Why not use this topology for the common-connection case, and save a couple diodes? https://airgunwarriors.com/wp-content/uploads/wpforo/attachments/177/5536-RelayDriver.JPG“

“PS: In other words, the return path for the “set” coil current will (predominately) be through D2, not through the “reset” coil. You neglected to show this path in your diagram.”

“Hi Stephen, Thank you for the comment. I don’t think your idea will work. See the below link for diagram. The relay driver current will energize both coils. In order for the relay to work properly, only one coil is energized at a time. I will have to test it in the lab to confirm it. https://ibb.co/jKBgje“

“Please do so. I think you’ll find it will work because, for example, when the driver output is positive, the reset coil will see only shunt diode D2’s forward-conduction voltage (i.e., ~ 0.7V) and similarly for the set coil when the driver output is negative and D1 conducts. If Shottky diodes are used, the shunt voltage would of course be even less.”

“I think you are right Stephen. The coil operate (activation) voltage is much higher than the 0.7V diode drop. So it should work. “

“Comment from EDN's Twitter (@EDNcom): If that's a single supply H-bridge driver, those grounded nodes are fatal to operation. There needs to be a wire between those two nodes and no grounds anywhere.”

“Just ground the coil common and lose the diodes. You have 2 totem poles in there.”

“Figure 1 is correct if the driver is a single totem pole, like an NE555. Once you drop in an H-bridge, now you have two separate drivers and you don’t need any diodes at all. Unless for some reason you want to drive 4 coils.\n\nOld school would use a capacitor to store up charge to operate the second coil, but that was so long ago I don’t recall the circuit off hand. Would conjure it up if needed.”

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