What is SPDT Switch and SPDT SSR? banner

WHAT IS SPDT SWITCH AND SPDT SOLID STATE RELAY

INTRODUCTION:

If you need to use a solid state relay to control two devices with opposite working status (when A is on, B will be off; when B is on, A will be off), you may need to select an 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? Where SPDT Solid State Relays are used?

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CONTENTS



§1. What is SPDT Switch?

1.1 What is the Structure of SPDT Switch?

1.2 How does SPDT Switch work?

§2. What is SPDT Solid State Relay?

2.1 How does SPDT Solid State Relay work?

2.2 Where SPDT Solid State Relays are used?

§3. How to select MGR (HUIMULTD) SPDT Solid State Relay?

§1. What is SPDT Switch?

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SPDT (Single Pole Double Throw) is a special switch structure. Different with the common SPST (Single Pole Single Throw) switches, SPDT switches control the working status of two devices, and these two devices work in opposite status. For example, if a SPDT switch is connected to Device A and Device B, when turning off the SPDT switch, the Device A will be on status and the Device B will be off status; conversely, when turning on the SPDT switch, the Device A will be off status and the Device B will be on status. Therefore, it can be seen that no matter what state the SPDT switch is operating in, only one device will be turned on.

1.1 What is the Structure of SPDT Switch?

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The basics of the SPDT switch structure:
Pole - We usually use single pole, double pole, or etc. to show how many separate circuits that controlled by a switch. We usually use single pole, double pole, or etc. to show how many separate circuits that controlled by a switch.
Throw - The throw means the circuit that will be controlled by the pole. We usually use single throw, double throw, or etc. to show how many circuits will be controlled by one Pole.
Open - If the pole is disconnected to one throw, the state of this throw is called open state or on state.
Close - If the pole is connected to one throw, the state of this throw is called close state or off state.
NO (Normally Open) - If the throwing circuit is disconnected from the pole by default (under the premise that the pole is not affected by any external factors), this throwing circuit is called a normally open circuit (NO circuit, N/O circuit), and this switch is called a normally open switch (NO switch, N/O switch).
NC (Normally Closed) - If the throwing circuit is connected to the pole by default (under the premise that the pole is not affected by any external factors), this throwing circuit is called a normally closed circuit (NC circuit, N/C circuit), and this switch is called a normally closed switch (NC switch, N/C switch).

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SPDT switch is a five terminal switch device — two input terminals, and three output terminals (two load terminals, and one common terminal). And if one of load terminals does not connect to any load, which means only one load terminal is working, then the SPDT switch can only work as an SPST switch.

Generally, the SPDT switch will be more suitable and efficient for some special applications than two SPST switches, like switching two different power supplies in opposite status, or making two devices with different functions work in opposite states.

1.2 How does SPDT Switch work?

According to the way of operating, the single pole double throw switch can be divided into BBM (break-before-make) type SPDT switch and MBB (make-before-break) type SPDT switch.

1) BBM (Break Before Make) type SPDT Switch

In the Nature State, the pole of the BBM type SPDT switch is connected to NC throwing circuit and disconnected from the NO throwing circuit. During the process of switching from the Nature State to the Working State, the BBM type SPDT switch will first disconnect from the NC throwing circuit, and then connect to the NO throwing circuit.

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2) MBB (Make Before Break) type SPDT Switch

In the Nature 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 Nature State to the Working State, the MBB type SPDT switch will first connect to the NO throwing circuit, and then disconnect from the NC throwing circuit.

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§2. What is SPDT Solid State Relay?

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SPDT Solid State Relay (also known as SPDT SSR Switch) is a special solid state relay, because all the switch component (MOSFET, SCR, TRIAC, IGBT, etc.) in the solid state relay only acts as the SPST switch, and furthermore, there is also no discrete component in the solid state relay has SPDT function. Besides, the opto-coupler (with optoelectronic isolation feature) (with optoelectronic isolation feature) is used in the solid state relay as the signal transmission component, so the input terminal of the solid state relay is electrical insulated to its output terminal. Consequently, SPDT solid state relays can only be made by modifying SPST solid state relays by customizing the circuit. And because of the electronic switching components, SPDT solid state relays are more efficient and have a longer life than SPDT electromagnetic relays.

The Structure of SPDT Solid State Relay

The basics of the SPDT SSR switch structure is a little different to the SPDT switch structure:
"Pole" – "Throw A and Throw B" - By combining the components acting as a pole and the components acting as a throw, we can get combinations such as "General-Purpose Push-Pull Output (GPout) " – "Diodes and Photodiodes", and "Photodiode array " – "N-channel enhancement mode MOSFET and N-channel depletion mode MOSFET".
Open - If the pole is disconnected to one throw, the state of this throw is called open state or on state.
Close - If the pole is connected to one throw, the state of this throw is called close state or off state.
NO (Normally Open)- If the throw circuit is disconnected to the pole by default (when the pole is not energized or charged or the voltage level on the pole is zero), this throw circuit will be called the normally open circuit (NO circuit, N/O circuit), and the solid state relay switch will be called normally open SSR switch (NO SSR switch, N/O SSR switch).
NC (Normally Close) - If the throwing circuit is connected to the pole by default (when the pole is unenergized or uncharged or the voltage level on the pole is zero), this throwing circuit is called a normally closed circuit (NC circuit, N/C circuit), and this solid state relay will be called normally closed solid state relay (NC SSR, N/C SSR).
① Push-Pull Output: Normally, there are two transistors in the push-pull output cirdcuit which are controlled by two complementary signals, so the push-pull output circuit can output the high voltage level and low voltage level at the same time, and both of which have the capability to drive the switch components, that is, if one transistor is turned on, the other transistor must be turned off.
② Photodiode array: the stack of photodiodes is used to drive a pair of MOSFETs or one IGBT.

2.1 How does SPDT Solid State Relay work?

According to "Pole" – "Throw", SPDT solid state relays can be divided into two types: Push-Pull type, and Photodiode-Array type.

1) Push-Pull Type SPDT Solid State Relay

A common circuit diagram for a push-pull type SPDT solid state relay is shown in the figure below (see Figure 7). TThe push-pull type SPDT solid state relay consists of two LEDs (D1, D2), and two Photo-TRIACs (TRIAC1, TRIAC2). D1 and TRIAC1 form the CIRCUIT1, and D2 and TRIAC2 form the CIRCUIT2. The push-pull output signal (GPout) is generated in the input control terminal of the push-pull type SPDT solid state relay, and LOAD1 and LOAD2 is separately connected to the two output terminals of the push-pull type SPDT solid state relay.

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Based on the GPout signal, there will be three working statuses of the push-pull type SPDT solid state relay:

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

Note: There are some limitations and special requirements for GPout output and LED operating voltage, so push-pull type SPDT solid state relays are not widely used.

2) Photodiode-Array Type SPDT Solid State Relay

The following shows a common circuit diagram for a photodiode array type SPDT solid-state relay (see Figure 8), which can operate in both DC and AC power supplies. The photodiode-array type SPDT solid state relay consists of one photodiode-array (D1), and four N-MOSFETs (MOS1 and MOS2 are enhancement mode N-MOSFETs; MOS3 and MOS4 are depletion mode N-MOSFETs). MOS1 and MOS2 form the CIRCUIT1, and MOS3 and MOS4 form the CIRCUIT2. The photodiode-array type SPDT solid state relay has 5 output terminals: 5 output terminals, Port1, Port2, Port3, Port4, Port5. And the Port1 is the common terminal. LOAD1 and LOAD2 are connected to output terminals of the photodiode-array type SPDT solid state relay.

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③ The enhancement mode N-channel MOSFET will be turned on when Vgs>Vgs(th), otherwise it will be turned off.
④ The depletion mode N-channel MOSFET will be turned on at zero input, and be turned off when its Vgs is negative input.
⑤ Vgs is the voltage from gate to source; and Vgs(th) is the threshold voltage from gate to source.


1. DC Power Supply
If the photodiode-array type SPDT solid state relay works in a DC power supply, LOAD1 should be connected to PORT2, and LOAD2 should be connected to PORT4, so that MOS2 and MOS4 are disabled.

When there is no control signal, D1 will not conduct, and MOS1 will be turned off because its gate voltage is below its threshold voltage, therefore, LOAD1 will be switched off. At the same time, MOS3 will be turned on because its gate voltage is zero input, so LOAD2 will be switched on (see Figure 9).

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When there is a control signal, D1 will conduct, and MOS1 will be turned on because its gate voltage is above its threshold voltage, therefore, LOAD1 will be switched on. At the same time, MOS3 will be turned off because its gate voltage is negative input, so LOAD2 will be switched off (see Figure 10).

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2. AC Power Supply
If the photodiode-array type SPDT solid state relay works in an AC power supply, LOAD1 should be connected to PORT3, and LOAD2 should be connected to PORT5.

When there is no control signal, D1 will not conduct, and MOS1 and MOS2 will be turned off because their gate voltage is below their threshold voltage, therefore, LOAD1 will be switched off. At the same time, MOS3 and MOS4 will be turned on because their gate voltage is zero input, so LOAD2 will be switched on (see Figure 11).

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When there is a control signal, D1 will conduct, and MOS1 and MOS2 will be turned on because their gate voltage is above their threshold voltage, therefore, LOAD1 will be switched on. At the same time, MOS3 and MOS4 will be turned off because their gate voltage is negative input, so LOAD2 will be switched off (see Figure 12).

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Note: Due to the advantages of N-channel enhanced MOSFET and N-channel depletion MOSFET, as well as compatibility with DC and AC power supplies, Photodiode array SPDT solid-state relays are widely used.

2.2 Where SPDT Solid State Relays are used?

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 working 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 MGR (HUIMULTD) SPDT Solid State Relay?

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1) MGR-1KB series Single Phase AC Solid State Relay

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Type meaning:
MGR - 1:[Single phase] - KB:[SPDT (Single Pole, Double Throw)] - 48:[Load voltage 480VAC] - 40:[40A]

Control Voltage:
3-32VDC (such as 3VDC, 5VDC, 9VDC, 12VDC, 18VDC, 24VDC, 32VDC or customized control voltage)
Control Current:
Ultra low input current, ≤20mA
Load Voltage:
Single phase 24-480VAC (such as 24VAC, 48VAC, 110VAC, 230VAC, 380VAC, 480VAC or customized load voltage)
Load Current:
10A, 15A, 20A, 25A, 30A, 40A
Dimensions & Mounting:
57.4mm*44.8mm*28mm; Panel mount solid state relay with transparent cover,metal base and screw terminals

2) MGR-1AKB series Single Phase AC Solid State Relay

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Type meaning:
MGR - 1:[Single phase] - A:[Control voltage 85-265VAC] - KB:[SPDT (Single Pole, Double Throw)] - 48:[Load voltage 480VAC] - 40:[40A]

Control Voltage:
85-265VAC (such as 100VAC, 110VAC/120VAC, 200VAC, 220VAC/230VAC, 240VAC or customized control voltage)
Control Current:
Ultra low input current, ≤10mA
Load Voltage:
Single phase 24-480VAC (such as 24VAC, 48VAC, 110VAC, 230VAC, 380VAC, 480VAC or customized load voltage)
Load Current:
10A, 15A, 20A, 25A, 30A, 40A
Dimensions & Mounting:
57.4mm*44.8mm*28mm; Panel mount solid state relay with transparent cover,metal base and screw terminals



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