Relays are common electromechanical devices in circuits, and there are two types: latching and non-latching. Latching relays retain their last switching position even after complete de-energization, regardless of whether they are single-coil or double-coil types. Single-coil latching relays use only one coil to set or reset the switching position, but require both positive and negative voltages. When a positive voltage is applied, current flows in one direction, causing the relay to enter the set state (i.e., the relay switch is closed). If a negative voltage is applied, the current direction is reversed, causing the relay to enter the reset state (i.e., the switch is open).
Relays are common electromechanical devices in circuits, and there are two types: latching and non-latching. Latching relays retain their last switching position even after complete de-energization, regardless of whether they are single-coil or double-coil types. Single-coil latching relays use only one coil to set or reset the switching position, but require both positive and negative voltages. When a positive voltage is applied, current flows in one direction, causing the relay to enter the set state (i.e., the relay switch is closed). If a negative voltage is applied, the current direction is reversed, causing the relay to enter the reset state (i.e., the switch is open).
Dual-coil latching relays use only positive voltage but require two power supplies or drivers. This type of relay has 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. However, the two coils are never energized simultaneously.
If you want to use a dual-coil relay, but the only available driver is designed for a single-coil relay, there is a way to easily convert a single-coil driver to one that can drive a dual-coil relay, as shown in Figure 1. This conversion, which only requires a positive voltage drive, is particularly useful for relay testing because it only requires one voltage polarity instead of two. This method can greatly simplify the relay test setup.
Figure 1: A diode can convert a single-coil relay driver into a dual-coil one.
The operating principle is simple. When the coil driver output voltage is positive, current flows through diode D1 to energize the setting coil, while the reset coil is not powered because D2 blocks the current. The relay enters the setting state. When the voltage is negative, diode D1 blocks the current flowing through the setting coil, and diode D2 begins to energize the reset coil.
The latching relay in Figure 1 has two independent coil connections, using four pins. However, some dual-coil relays use only three pins, with a common coil connection, as shown in Figure 2. The configuration is slightly more complex in this case, involving four diodes.
As mentioned earlier, when the driver voltage is positive, current flows through diode D2, the setting coil, and D3. Diodes D1 and D4 are reverse biased, preventing current from flowing to the reset coil. Similarly, when the voltage is negative, current flows through diode D4, the reset coil, and D1, while the setting coil is de-energized. Likewise, only one coil can be energized at a time.
Figure 2: When coils share a single connection, four diodes are needed to convert the single-coil signal for use with dual coils.
Another advantage of the conversion circuit is that it makes it relatively easy to test the AC performance of dual-coil relays, such as operating time (open time), bounce time, disconnect time, and maximum frequency. Simply replace the relay driver with a square wave voltage signal generator. Since many relay coils require high voltage (up to 48V), and in some cases, large currents from 20mA to over 1000mA, a signal generator alone may not be sufficient. In such cases, a high-voltage function generator amplifier, such as the Accel Instruments TS250, is needed to boost both voltage and current (see Figure 3).
Figure 3: Testing a dual-coil relay using only one function generator and one high-voltage driver.
Diode circuits offer a simple method to convert single-coil relay drive signals for dual-coil use. This method allows system designers to choose between single-coil or dual-coil latching relays without changing the driver. Furthermore, it requires only a single signal driver for testing dual-coil latching relays.
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