What is the SPDT Switch and SPDT Solid State Relay?

If you need to use a solid state relay to control two devices with opposite operating states, such as device B turns off when device A turns on, and device A turns off when device B turns on, then you may need to select an SPDT Switch or SPDT Solid State Relay to meet this need. Through this article you will learn what is SPDT switch? How dose SPDT switch work? What is SPDT solid state relay? How does SPDT solid state relay work? What are SPDT solid state relays used for?

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CONTENTS

§1. What is SPDT Switch?

§2. What is SPDT Solid State Relay?

§3. How to select our SPDT Solid State Relay?

§1. What is SPDT Switch?

Single Pole Double Throw (SPDT) is a special switch structure. SPDT switch controls the operating status of two devices, such as Device A and Device B, which work in opposite status. If the SPDT switch is turned on, the Device A will be turned on, and the Device B will be turned off. Conversely, if the SPDT switch is turned off, the Device B will be turned on, and the Device A will be turned off. Therefore, it can be seen that whether the SPDT switch is turned on or off, there is always one device that is on. This is very different from the Single Pole Single Throw (SPST) switch, which can only control the turning on and off of one device.

1.1 What is the Structure of SPDT Switch?

In order to better understand the basic structure of SPDT switches, it is necessary to first understand SPST switches. Figure 02 shows the basic structure of an SPST switch, which consists of a pole and a throw, each of which is connected to a throwing circuit. When the pole is disconnected from the throw, it is open, and the throwing circuit will be turned off. Conversely, when the pole is connected to the throw, it is closed, and the throwing circuit will be turned on. If the pole and throw are open in the default state, and closed in the operating state, then it is called normally open (NO). Conversely, if the pole and throw are closed in the default state, and open in the operating state, then it is called normally closed (NC).

Compared with the SPST switch, the SPDT switch has two throwing circuits, namely throwing circuit 1 and throwing circuit 2, as shown in Figure 03. Then the function of pole is to switch between these two throwing circuits. For example, in the default state, the pole is closed to throwing circuit 1 and open to throwing circuit 2, then in the operating state, the pole will be open to throwing circuit 1 and closed to throwing circuit 2. In summary, we understand that the number of poles determines the number of actuators, while the number of throws determines the number of execution circuits. Therefore, if we come across DPDT switches, or switches with more complex structures, we can analyze them in a similar way.

1.2 How does SPDT Switch work?

SPDT switch has two input terminals, and three output terminals (containing two load terminals, and one common terminal). If only one load is connected to the output terminal, then the function of the SPDT switch is no different from that of the SPST switch. Of course, the advantage of the SPDT switch over the SPST switch is that it can control the operating state of two loads at the same time. Regarding how the SPDT switch controls two loads, we need to first understand how the pole on the common terminal works. According to how the pole works, SPDT switches can be categorized into BBM (break-before-make) SPDT switches and MBB (make-before-break) SPDT switches.

1.2.1 BBM (Break Before Make) SPDT Switch

Take the BBM SPDT NC switch as an example. In the default state, the pole of the BBM SPDT switch is connected to NC throwing circuit and disconnected from the NO throwing circuit. During the process of switching from the default state to the operating state, the BBM type SPDT switch will first disconnect from the NC throwing circuit, and then connect to the NO throwing circuit.

1.2.2 MBB (Make Before Break) SPDT Switch

Take the MBB SPDT NC switch as an example. In the default state, the pole of the MBB type SPDT switch is connected to NC throwing circuit and disconnected from the NO throwing circuit. During the process of switching from the default state to the operating state, the MBB type SPDT switch will first connect to the NO throwing circuit, and then disconnect from the NC throwing circuit.

§2. What is SPDT Solid State Relay?

SPDT solid state relays, also known as SPDT solid state relay switches, are a special type of solid state relay. This is because almost all power electronic devices, such as MOSFET, SCR, TRIAC, and IGBT, do not have the ability to output two signals at the same time through a single device, and therefore only act as SPST switches. Hence, it is necessary to design a circuit that can control two power electronic devices simultaneously with one control signal to output two distinct signals. Due to the use of power electronic devices, SPDT solid state relays are more efficient and have a longer life than SPDT switches. In addition, the optocoupler electrically insulates the input of the SPDT solid state relay from its output, which provides excellent protection for the control device.

2.1 How does SPDT Solid State Relay work?

Unlike SPDT switches, SPDT solid state relays do not have a mechanical structure, so their poles and throws are composed of electronic components that can perform similar functions. In general, a low-current control component is used as a pole, and a high-current output component is used as a throw. According to the combination of pole and throw, SPDT solid state relays can be divided into push-pull SPDT solid state relays and photodiode-array SPDT solid state Relays.

2.1.1 Push-Pull SPDT Solid State Relay

Figure 07 shows a common circuit diagram for a push-pull SPDT solid state relay, which use the General-purpose Push-pull Output (GPout) as a pole and the two Phototriacs (TRIAC1, TRIAC2) as throws. The GPout will output a high voltage level and a low voltage level through two LEDs (D1, D2) to control the operating state of two throwing circuit (CIRCUIT1, CIRCUIT2). And LOAD1 and LOAD2 are separately connected to the two output terminals of the push-pull SPDT solid state relay. It should be noted that there are some limitations and special requirements for GPout output and LED input, so push-pull SPDT solid state relays are not widely used. Based on the GPout signal, the push-pull SPDT solid state relay has three operating states:

● When a low voltage level is generated in the input terminal, D2 will be turned off, and D1 will be turned on, and TRIAC1 will be turned on, then the CIRCUIT1 will be closed, finally the LOAD1 will be turned on.

● When a high voltage level is generated in the input terminal, D1 will be turned off, and D2 will be turned on, and TRIAC2 will be turned on, then the CIRCUIT2 will be closed, finally the LOAD2 will be turned on.

● If there is no signal on the input terminal, D1 and D2 will be turned off, and CIRCUIT1 and CIRCUIT2 will not be turned on.

2.1.2 Photodiode-Array SPDT Solid State Relay

The photodiode-array SPDT solid state relay has five output terminals, which can operate in both DC and AC power supplies. Figure 08 shows a common circuit diagram for a photodiode-array SPDT solid state relay, which use the photodiode-array (D1) as a pole, and four N-MOSFETs as throws (MOS1, MOS2, MOS3 and MOS4). The photodiode-array D1 is a stack of photodiodes used to drive a pair of MOSFETs or one IGBT. And MOS1 and MOS2 are enhancement mode N-MOSFETs used to control the CIRCUIT1, and MOS3 and MOS4 are depletion mode N-MOSFETs used to control the CIRCUIT2. It should be noted that Vgs is the gate voltage of MOSFET, and Vgs(th) is the threshold voltage of Vgs. The enhancement mode N-channel MOSFET will be turned on when Vgs>Vgs(th), otherwise it will be turned off. The depletion mode N-channel MOSFET will be turned on when Vgs is zero, and be turned off when Vgs is negative.

1- Under DC power supply

If the photodiode-array SPDT solid state relay works under DC power supply, LOAD1 should be connected to PORT2, and LOAD2 should be connected to PORT4.

If there is no input control signal, D1 will not work. And MOS1 will be turned off since its gate voltage is below its threshold voltage, so that LOAD1 will be turned off. At the same time, MOS3 will be turned on since its gate voltage is zero, so that LOAD2 will be turned on.

If there is an input control signal, D1 will work. And MOS1 will be turned on since its gate voltage is above its threshold voltage, so that LOAD1 will be turned on. At the same time, MOS3 will be turned off since its gate voltage is a negative, so that LOAD2 will be turned off.

2- Under AC Power Supply

If the photodiode-array SPDT solid state relay works under AC power supply, LOAD1 should be connected to PORT3, and LOAD2 should be connected to PORT5.

If there is no input control signal, D1 will not work. And MOS1 and MOS2 will be turned off since their gate voltage is below their threshold voltage, so that LOAD1 will be turned off. At the same time, MOS3 and MOS4 will be turned on since their gate voltage is zero voltage level, so that LOAD2 will be turned on.

If there is an input control signal, D1 will work. And MOS1 and MOS2 will be turned on since their gate voltage is above their threshold voltage, so that LOAD1 will be turned on. At the same time, MOS3 and MOS4 will be turned off since their gate voltage is negative voltage level, so that LOAD2 will be turned off.

2.2 What are SPDT Solid State Relays used for?

SPDT solid state relays are widely used in many fields, such as:

Solar Battery Charger

SPDT solid state relays are typically used in solar power charger systems (such as portable solar charger) to control the solar cell charging device. The operating state (charging state and power state) of the solar cells is switched much frequently, so the SPDT mechanical relays cannot meet this requirement, but the SPDT solid state relay can.

Soft Starter System

When the electronic/electrical systems (and their subsystems) works normally, the switching frequency of its soft start state and stop state is very high. Therefore, the SPDT make-before-break switch need to be equipped to prevent overvoltage when switching.

Remote Control Device / Radio Transmitter

If the remote control device wants to control multiple objects, it needs to change the transmitted signal; if the communication tower wants to switch the recipient, it needs to change the transmission signal. This kind of equipment require strict operating frequency and operation accuracy, and SPDT solid state relays can meet these requirements in most cases.

Satellite Heater

In the space environment, there are high requirements (weight, anti-interference ability, adaptability, and so on) to every device. The SPDT solid-state relay (with lots of advantages, like small weight, high performance, long service life, low power consumption, low EMR and etc.) meet these requirements, and is much better than the SPDT electromechanical relay (simple structure, but high weight, poor anti-interference ability and cannot adapt to complicated situations due to overweight).

§3. How to select our SPDT Solid State Relay?

HUIMULTD is a company specializing in solid state relays, and supplies high-quality solid state relays and solutions to the world. Our Senior Management have collectively over 60 years` experience of production and development in Solid State Relay fields. And we are proud to offer high quality products and provide our customers with the services and support they deserve. We currently offer the following two SPDT solid state relays. Besides, we can design solid state relays in different forms and specifications according to customer needs, and also provide deep OEM customized services to meet different requirements.

MGR-1KB Series Single Phase AC Solid State Relay

MGR-1KB Series Dual Channel Panel Mount DC to AC Solid State Relay is a SPDT DC to AC solid state relay that uses one control signal to control two different loads. These two loads work under the same power supply, and only one load can be turned on at the same time.

MGR-1AKB Series Single Phase AC Solid State Relay

MGR-1AKB Series Dual Channel Panel Mount AC to AC Solid State Relay is a SPDT AC to AC solid state relay that uses one control signal to control two different loads. These two loads work under the same power supply, and only one load can be turned on at the same time.