Relay Connections at the Collector and Emitter Sides of a Transistor: Which is Better?

Relays are really important in electrical circuits especially when it comes to switching things on and off.

These devices help control high power loads using low power signals, which makes them super useful in many electronic systems.

Often, transistors are the main parts that control how these relays work.

By using a transistor to manage the relay designers can make the switching process more efficient and reduce power loss, which improves how well the circuit works.

When you are designing a circuit to turn on a relay with a transistor, you can place the relay coil either on the collector side or the emitter side of the transistor.

This choice can really change how the circuit behaves and performs.

Each setup has its own pros and cons depending on what you need for your specific project.

In the collector side setup the relay coil connects to the collector terminal of the transistor.

This design is usually simpler because the transistor can easily turn the relay on and off by controlling the current through the coil.

A big plus of this arrangement is that it can handle higher voltages and currents, which is great for situations where the relay needs to control heavy loads.

Also, this design often leads to a stronger circuit since the transistor can be pushed into saturation making sure the relay is fully activated.

On the flip side the emitter side setup has the relay coil between the emitter of the transistor and ground.

This can be useful in some cases especially for low voltage applications.

One major benefit of this configuration is to take a close look at both setups to figure out which one works best in different situations.

Understanding Transistor Switching:

A transistor can work like a switch in two main setups:

• Common Emitter Setup (Relay connected to the Collector)
• Common Collector Setup (Relay connected to the Emitter)

1. Relay Connection at the Collector Side (Common Emitter Configuration):

In the circuit diagram above the 12V relay is placed between the +12V DC positive voltage supply and the Q1 transistors collector, while the emitter is connected directly to the ground.

This design is really important for the circuit to work properly because it lets the transistor control the relay, which works like a switch.

When a signal is sent to the base of the transistor, it makes the transistor turn on.

This happens because the signal gives enough current to the base allowing the transistor to let electricity flow.

As a result, current starts moving from the positive supply through the relay coil and into the transistors collector.

Now that the transistor is on, it completes the circuit by letting the current flow from the emitter to the ground.

As the current goes through the relay coil, it creates a magnetic field that powers the relay.

This magnetic field is really important because it makes the relays armature move, which can either open or close the relays contacts depending on how it is designed.

When the armature moves, it allows the relay to control another circuit turning the current on or off based on the signal sent to the transistor.

Also placing R1 resistor from the base of Q1 to the input for the relay control signal is an important and helpful design decision.

This change helps to control the current, keeps the system running smoothly and stops any weird switching behavior.

But, it is really important to choose the right resistor value so that there is enough base current to activate the relay properly.

In short, this setup shows a basic idea of how electronic control systems work where a small control signal can manage a larger power load using a transistor and a relay.

The transistor acts like a gatekeeper allowing the relay to turn on and do its job, while the relay keeps things safe and can handle bigger currents or voltages that the transistor might not be able to manage alone.

This design is commonly used in many areas, like automation and control systems where it is really important for things to work reliably and safely.

Note: In collector side relay configuration the base control signal has to be just 0.7V for the relay to operate correctly.

Advantages:

There are many great reasons to use this design and they are really important.

To start, one big plus is that it can handle high voltage and current levels very well.

When the transistor is in saturation mode it has a super low voltage drop, which means it can provide a lot of current to the relay.

This is really important for making sure everything works well.

Next, the way it switches is really efficient.

When the transistor acts as a common emitter switch, it can easily switch between being fully ON and fully OFF.

This helps to reduce wasted power and makes everything run more efficiently.

This design gives you better control over the load.

It allows the relay to be activated with a voltage that is higher than the base input voltage.

This means it can work with different logic levels like 3.3V, 5V or even 12V control signals, which is super useful in many different situations.

Finally, there is something called the Beta Effect that helps increase current gain.

This means that the transistors natural gain is used more effectively, so you need less base current to control larger loads.

This helps improve performance and makes better use of resources.

Disadvantages:

One big drawback of this design is that you have to have a good base drive circuit.

Since the emitter is linked to ground, it is really crucial to ensure that the voltage sent to the base is high enough for the transistor to function correctly and switch on and off as it should.

Additionally, you need to include reverse voltage protection, which means you should place a flyback diode D1 across the relay coil.

This D1 diode is really important because it protects the circuit from potential damage caused by voltage spikes when the transistor shuts off.

2. Relay Connection at the Emitter Side (Common Collector Configuration):

In the above circuit diagram the relay is placed between the transistors emitter and the ground, while the collector connects to the positive power supply.

This design is really important for the circuit to work properly because it lets the transistor act like a switch that controls the relay.

When the circuit gets power the transistor starts working when a base current is sent to its base terminal.

This base current is super important because it helps the transistor enter its active state allowing it to manage a larger current that flows from the collector to the emitter.

As the base current goes into the transistor, it turns on letting current flow from the collector to the emitter.

When the transistor is on, it creates a path for current to go through the relays coil.

The coil acts like an electromagnet and when current flows through it, it makes a magnetic field.

This magnetic field is strong enough to trigger the relays switch, which can either open or close another circuit based on how the relay is designed.

One of the best things about using a relay is that it can control a high power circuit with a low power signal from the transistor.

This means devices that need more current than the transistor can handle can still operate safely.

Also, the relay provides electrical isolation between the control circuit (the transistor and its base current) and the load circuit (the device being controlled), which makes the whole system safer and more reliable.

Lastly, this setup uses the transistor as a switch to control the relay, allowing larger loads to work while keeping control through a smaller easier to manage current.

The way the transistor and relay work together is really important in many areas, like automation systems, motor control and other electronic devices that need switching capabilities.

Note: In emitter side relay configuration the base control signal must be slightly higher than the relay coil voltage for the relay to operate correctly

Advantages:

This design lets the emitter closely follow the base voltage making it possible for logic circuits to control it directly without needing extra parts.

This is really helpful in situations where the relay needs to be connected to ground making it simpler to fit into different systems.

Disadvantages:

The emitter voltage is always about 0.7V lower than the base voltage, especially with a regular bipolar junction transistor BJT.

This voltage drop can reduce the amount of voltage available to power the relay coil, which might affect how well it works.

The transistor might not fully turn on leading to more power loss and lower efficiency when switching.

The voltage going to the relay depends on the base voltage, which can limit the ability to operate relays that need higher voltage levels.

Which Configuration is Better?

In most cases, it is usually better to place the relay on the collector side especially when using a common emitter configuration.

This design is popular because it helps the relay switch more efficiently gives better control over the voltage going to the relay and ensures the transistor works well, all of which lead to better performance.

When the relay is on the collector side in a common emitter setup the transistor can switch the relay on and off more effectively.

This arrangement allows for a higher voltage across the relay coil when the transistor is turned on making sure the relay gets enough power to work properly.

The improved switching efficiency happens because the transistor can fully saturate meaning it can carry a lot of current with little resistance, which cuts down on power loss and heat.

This is super important in situations where quick switching is needed, as it helps the relay respond faster and work more reliably.

Also, having the relay on the collector side gives better control over the voltage that goes to the relay.

In this configuration the transistor can be pushed into saturation more easily ensuring the relay gets the right voltage to turn on without the risk of not getting enough power, which could cause it to act strangely or not work at all.

This level of control is really important in situations where timing and reliable operation are key.

The common collector setup, also called an emitter follower is a type of transistor circuit where the input signal goes to the base and the output comes from the emitter.

While this setup has some advantages like high input impedance and low output impedance, it is not typically used for controlling relays.

One major reason for this is the noticeable voltage drop that occurs across the transistor when it is turned on.

In a common collector arrangement the output voltage at the emitter is roughly the same as the base voltage, minus a small drop of about 0.7 volts for silicon transistors.

This means that the maximum output voltage available to power a relay is limited, which can be an issue if the relay requires a higher voltage to function properly.

Additionally, the switching efficiency in a common collector setup is not the greatest.

When controlling a relay, it is crucial to switch quickly for it to work reliably.

However, this configuration might not switch fast enough due to its design.

The transistor can take longer to change between on and off states, which can slow down the relays performance.

Moreover, the current gain in a common collector setup is typically lower than in other configurations like the common emitter.

This means you need a larger input current to drive the relay, which can complicate the design and use more power.

At last, while the common collector configuration has some benefits in certain cases, its voltage drop, slow switching and lower current gain make it less suitable for driving relays.

Designers often prefer other designs like the common emitter or specialized relay driver circuits, for improved performance.

Formula:

Here we will calculate the base resistor value (Rb) for a current limiter circuit using a BJT transistor.

V_in = (V_s – 0.7) * H_Fe / Collector Current

where,

• V_in is the base voltage of the transistor usually in the range of 0.7V to 1.2V
• V_s is the supply voltage
• H_Fe is the DC current gain of the transistor (refer to the transistors datasheet)
• Collector Current is desired maximum current allowed through the collector (the current you want to limit)

Conclusion:

When creating a relay driver circuit using a transistor, it is usually best to connect the relay to the collector side, which is known as a common emitter setup.

This way you get better control over voltage and current making the switching more efficient and ensuring the relay works properly.

The emitter side setup might only be suitable for certain low power situations where it is more important to keep things simple for the base driving rather than focusing on efficiency.

By knowing about these different setups, designers can make smart decisions to improve how well the circuit works and how reliable it is.

References:

Why is a relay usually placed “after” a pnp transistor and “before” a npn transistor?