Using diode circuit to realize the test of double-coil latching relay

Relays are common electromechanical devices in circuits, and there are two types: latching or non-latching. The latching relay will maintain its switch position even after it is completely de-energized, regardless of the single-coil or double-coil type. Single-coil latching relays use only one coil to set or reset the switch position, but require positive and negative voltages. When a positive voltage is applied, the current flows in one direction and allows the relay to enter the set state (ie, the relay switch is closed). If a negative voltage is applied, the direction of the current is reversed and the relay enters the reset state (that is, the switch is opened).

The dual-coil latching relay only uses positive voltage, but requires two power supplies or drivers. This relay has a setting coil and a reset coil. When the setting coil is energized, the relay enters the setting state. On the contrary, when the reset coil is energized, the relay enters the reset state. But the two coils are never energized at the same time.

If you want to use a dual-coil relay, but the available driver is for a single-coil relay, there is a way to easily convert a single-coil driver to a dual-coil relay, as shown in Figure 1. This conversion with only forward voltage drive is particularly useful for relay testing because it only requires one voltage polarity instead of two. This method can greatly simplify the test setup of the relay.

Figure 1: A diode can convert a single-coil relay driver into a dual-coil.

The principle of operation is very simple. When the coil driver output voltage is positive, current flows through the diode D1 to excite the setting coil, and the reset coil is not powered because D2 blocks the current. The relay enters the setting state. When the voltage is negative, the diode D1 blocks the current flowing through the setting coil, and the diode D2 starts to excite the reset coil.

The latching relay in Figure 1 has two independent coil connections and uses 4 pins. However, some dual-coil relays only use three pins and there is a common coil connection, as shown in Figure 2. The configuration is slightly more complicated at this time, involving four diodes.

"As mentioned earlier, when the driver voltage is positive, current flows through the diode D2, the setting coil, and D3. Diodes D1 and D4 are reverse biased to prevent current to the reset coil. Similarly, when the voltage is negative, current flows through the diode D4, the reset coil and D1, and the coil is set to be de-energized. Likewise, only one coil can be energized.

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Figure 2: When the coils share one connection, four diodes are needed to convert the single coil signal for dual coils.

Another advantage of the 　　 conversion circuit is that it is easier to test the AC performance of the dual-coil relay, such as operating time (opening time), bounce time, disconnecting time and frequency. Just 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 1000mA or more are required, signal generators alone may not be enough. In this case, a high-voltage function generator amplifier, such as Accel Instruments’ TS250, is needed to boost the voltage and current (see Figure 3).

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Figure 3: Only one function generator and one high voltage driver are used to test the dual-coil relay.

Diode circuit can provide a simple way to convert the single-coil relay drive signal into a double-coil. This method allows system designers to choose to use single-coil or dual-coil latching relays without having to replace the driver. In addition, it only needs one signal driver to test the double-coil latching relay.