[Sharing] Summary of 7 relay switch circuits (bipolar transistors can be replaced by enhancement MOSFETs)

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[Sharing] Summary of 7 relay switch circuits (bipolar transistors can be replaced by enhancement MOSFETs) [Copy link]

 

A relay is an electromechanical device that uses an electromagnet to operate a pair of movable contacts from an open position to a closed position.

The advantage of a relay is that relatively little power is required to operate the relay coil. However, relay switching circuits can be used to control motors, heaters, lights, or AC circuits, which themselves consume much more voltage, current, and power.

An electromechanical relay is an output device (actuator) that comes in many shapes, sizes and designs, with a variety of uses and applications in electronic circuits. However, while relays can be used to allow low-power electronic or computer-type circuits to turn relatively high currents or voltages "on" or "off", some form of relay switching circuit is required to control it.

The designs and types of relay switching circuits are vast, but many small electronic projects use transistors and MOSFETs as their main switching devices. This is a small subset of some of the more common relay switching methods because transistors can provide fast DC switching (ON-OFF) control of the relay coil from a variety of input sources.

NPN relay switch circuit

A typical relay switch circuit has a coil driven by an NPN transistor switch TR1 as shown in the figure, depending on the input voltage level. When the base voltage of the transistor is zero (or negative), the transistor is cut off and acts as an open switch. In this case, no collector current flows and the relay coil is de-energized because, as a current device, if there is no current flowing into the base, no current flows through the relay coil.

If a sufficiently large positive current is now driven into the base to saturate the NPN transistor, the current flowing from base to emitter (B to E) controls the larger relay coil current flowing through the transistor from collector to emitter.

For most bipolar switching transistors, the amount of relay coil current flowing into the collector will be between 50 and 800 times the base current required to drive the transistor into saturation. The current gain or beta value (β) of a common BC109 is shown to be typically around 290 at 2mA (datasheet).

NPN relay switch circuit

Note that the relay coil is not only an electromagnet, but also an inductor. When power is applied to the coil due to the switching action of the transistor, the maximum current will flow according to the DC resistance of the coil defined by Ohm's law (I = V/R). Part of the electrical energy is stored in the magnetic field of the relay coil.

When the transistor is turned "off", the current flowing through the relay coil is reduced and the magnetic field collapses. However, the energy stored in the magnetic field has to go somewhere and this creates a reverse voltage across the coil as it attempts to maintain the current in the relay coil. This action creates a high voltage spike across the relay coil which, if allowed to build up, can damage the switching NPN transistor.

So to prevent the semiconductor transistor from being damaged, a "flying diode", also called a freewheeling diode, is connected across the relay coil. This freewheeling diode clamps the reverse voltage across the coil to about 0.7V, dissipating the stored energy and protecting the switching transistor. The freewheeling diode is only applicable when the power supply is a polarized DC voltage. AC coils require a different protection method, and an RC snubber circuit is used for this purpose.

NPN Darlington relay switch circuit

The previous NPN transistor relay switch circuits are great for switching small loads like LEDs and miniature relays. But sometimes you need to switch larger relay coils or currents that are beyond the range of the BC109 general purpose transistor, and this can be achieved using a Darlington transistor.

The sensitivity and current gain of a relay switching circuit can be greatly improved by using a pair of Darlington transistors instead of a single switching transistor. A Darlington transistor pair can be made from two individually connected bipolar transistors as shown, or it can be provided as a single device with the standard: base, emitter and collector connection leads.

The two NPN transistors are connected as shown so that the collector current of the first transistor TR1 becomes the base current of the second transistor TR2. Applying a positive base current to TR1 automatically "turns on" the switching transistor TR2.

NPN Darlington Configuration

If the two individual transistors are configured as a Darlington switch pair, a small value resistor (100 to 1,000Ω) is usually placed between the base and emitter of the main switching transistor TR2 to ensure that it is fully turned off. Likewise, a freewheeling diode is used to protect TR2 from the back EMF generated when the relay coil is de-energized.

Emitter follower relay switch circuit

In addition to the standard common emitter configuration of the relay switching circuit, the relay coil can also be connected to the emitter terminal of the transistor to form an emitter follower circuit. The input signal is connected directly to the base, while the output is taken from the emitter load as shown.

Emitter follower relay switch circuit

The common collector or emitter follower configuration is very useful for impedance matching applications because the input impedance is very high, in the hundreds of kiloohms range, while having a relatively low output impedance to switch the relay coil. As with the previous NPN relay switch circuit, switching is achieved by applying a positive current to the base of the transistor.

Emitter Darlington relay switching circuit

This is a Darlington transistor version of the previous emitter follower circuit. A very small positive base current applied to TR1 causes a much larger collector current to flow through TR2 due to the multiplication of the two Beta values.

Transmitter Darlington Configuration

The common emitter Darlington relay switching circuit can be used to provide current gain and power gain, while the voltage gain is approximately equal to 1. Another important characteristic of this type of emitter follower circuit is that it has a high input impedance and a low output impedance, which makes it ideal for impedance matching with large relay coils.

PNP relay switch circuit

In addition to using NPN bipolar transistors to switch relay coils and other such loads, we can also switch them using PNP bipolar transistors. The PNP relay switching circuit is no different from the NPN relay switching circuit in its ability to control a relay coil. However, it does require operating voltages of different polarities. For example, with the PNP type, the collector-emitter voltage, Vce, must be negative in order for current to flow from the emitter to the collector.

PNP transistor configuration

The PNP transistor circuit works the opposite of the NPN relay switch circuit. When the base is forward biased and the voltage is more negative than the emitter, the load current flows from the emitter to the collector. In order for the relay load current to flow through the emitter to the collector, both the base and collector must be negative relative to the emitter.

In other words, when Vin is high, the PNP transistor is switched "OFF", as is the relay coil. When Vin is low, the base voltage is less than the emitter voltage (more negative), and the PNP transistor turns "ON". The base resistor value sets the base current, which sets the collector current that drives the relay coil.

A PNP transistor switch can be used when the switching signal is opposite to that of an NPN transistor, such as the output of a CMOS NAND gate or other such logic device. The CMOS logic output has a drive strength of logic 0 to sink enough current to turn the PNP transistor "on". The current sink can then be turned into a current source by using a PNP transistor and a power supply of opposite polarity.

PNP collector relay switch circuit

The working principle of this circuit is the same as the previous relay switch circuit. In this relay switch circuit, the relay load has been connected to the collector of the PNP transistor. When Vin is low, the transistor is "on", and when Vin is high, the transistor is "off", and the switching action of the transistor and the coil occurs.

PNP Collector Configuration

We have seen that both NPN and PNP bipolar transistors can act as switches for relay switching, or any other load for that matter. But when current flows in two different directions, there are two different conditions that need to be understood.

Thus, in an NPN transistor, a high voltage relative to the emitter is applied to the base, current flows from the collector to the emitter, and the NPN transistor switches “on.” For a PNP transistor, a low voltage relative to the emitter is applied to the base, current flows from the emitter to the collector, and the PNP transistor switches “on.”

N-channel MOSFET relay switch circuit

The MOSFET relay switch operation is very similar to the bipolar junction transistor (BJT) switch operation seen above and any of the previous circuits can be implemented using MOSFETs. However, there are some key differences in the operation of the MOSFET circuit, the main difference being that MOSFETs are voltage operated devices and because the gate is electrically isolated from the drain-source channel they have a very high input impedance, so the gate current is zero for a MOSFET and therefore no base resistor is required.

The MOSFET conducts through a conductive channel that is initially closed and the transistor is "off". As the voltage applied to the gate terminal is slowly increased, the conductive width of this channel gradually increases. In other words, the transistor operates by enhancing the channel as the gate voltage increases, and this type of MOSFET is therefore called an enhancement MOSFET or E-MOSFET.

The N-channel enhancement MOSFET (NMOS) is the most commonly used type of MOSFET because a positive voltage on the gate terminal switches the MOSFET "ON" and zero or negative voltage on the gate switches it "OFF", making it ideal for use as a MOSFET relay switch. Complementary P-channel enhancement MOSFETs are also available, and they operate at opposite voltages like the PNP BJT.

N-Channel MOSFET Configuration

The above MOSFET relay switch circuit is connected in a common source configuration. Under zero voltage input, low level conditions, the value of V GS, the gate drive is not enough to open the channel, and the transistor is in the "off" state. However, when V GS increases above the MOSFET lower threshold voltage VT, the channel opens, current flows, and the relay coil starts to operate.

The enhancement mode MOSFET then operates as a normally open switch, making it ideal for switching small loads such as relays. The E-type MOSFET has a high "off" resistance and a moderate "on" resistance (suitable for most applications), so its R DS value needs to be considered when selecting one for a specific switching application.

P-channel MOSFET relay switch circuit

The P-channel Enhancement MOSFET (PMOS) is constructed the same as the N-channel Enhancement MOSFET, except that it only operates with a negative gate voltage. In other words, the P-channel MOSFET operates in the same manner, but with the opposite polarity, as the gate must be more negative than the source to "turn on" the transistor by forward biasing it as shown.

P-channel MOSFET relay switch circuit

In this configuration, the P-channel source terminal is connected to +Vdd and the drain terminal is connected to ground through the relay coil. When a high level is applied to the gate, the P-channel MOSFET will be "off". An "off" E-MOSFET will have a very high channel resistance, almost like an open circuit.

When a low level is applied to the gate, the P-channel MOSFET will turn "on". This causes current to flow through a low resistance path through the e-MOSFET channel operating the relay coil. Both N-channel and P-channel e-MOSFETs make excellent low voltage relay switching circuits and can be easily connected to a variety of digital logic gates and microprocessor applications.

Logic control relay switch circuit

The N-channel enhancement-type MOSFET is very useful as a transistor switch because in its "off" state (gate biased at zero), its channel has a very high resistance that blocks current flow. However, a relatively small positive voltage greater than the threshold voltage VT on its high-impedance gate causes it to begin conducting current from its drain terminal to its source terminal. ---Focus on Circuits

Unlike a bipolar junction transistor which requires a base current to turn it "on", the e-MOSFET only requires a voltage on the gate, and because of its insulated gate structure, zero current flows into the gate. This then makes the e-MOSFET, whether N-channel or P-channel, very suitable for being driven directly by a typical TTL or CMOS logic gate as shown in the figure.

Logic control relay switch circuit

Here the N-channel E-MOSFET is driven by a digital logic gate. The output pins of most logic gates can only provide a limited amount of current, usually no more than about 20 mA. Since the e-MOSFET is a voltage operated device and does not consume gate current, we can use the MOSFET relay switch circuit to control high power loads.

Microcontroller relay switch circuit

In addition to digital logic gates, we can also use the output pins and channels of microcontrollers, PICs, and processors to control the outside world. The circuit below shows how to connect a relay using a MOSFET switch.

Microcontroller relay switch circuit

Tutorial Summary

In this tutorial, we saw how to use bipolar junction transistors (NPN or PNP) and enhancement mode MOSFETs (N-channel or P-channel) as transistor switch circuits.

Sometimes when building electronic or microcontroller circuits, we want to use transistor switches to control high-power devices such as motors, lights, heating elements, or AC circuits. Often these devices require a larger current or higher voltage than a single power transistor can handle, and then we can use a relay switch circuit to do this. ---Follow the official account: Circuit One-stop

Bipolar transistors (BJTs) are very good and cheap relay switching circuits, but BJTs are current operated devices because they convert a small base current into a larger load current to power the relay coil.

However, the MOSFET switch is an ideal electrical switch because it requires almost no gate current to turn “on”, converting the gate voltage into load current. Therefore, the MOSFET can be used as a voltage-controlled switch.

In many applications, bipolar transistors can be replaced with enhancement-mode MOSFETs, which provide faster switching action, higher input impedance, and potentially lower power dissipation. The combination of very high gate impedance, very low power dissipation in the "off" state, and very fast switching capability make MOSFETs suitable for many digital switching applications. Also, with zero gate current, their switching action will not overload the output circuitry of a digital gate or microcontroller.

However, because the gate of the E-MOSFET is insulated from the rest of the component, it is particularly sensitive to static electricity, which can damage the thin oxide layer on the gate. Therefore, special care should be taken when handling or using the component, and any circuit using the e-MOSFET should include appropriate static and voltage spike protection.

Additionally, for additional protection of the BJT or MOSFET, always use a freewheeling diode across the relay coil to safely dissipate the back EMF generated by the switching action of the transistor.

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Thanks for sharing~