The transistor is the king of all electronic components because of its versatility.
It is a magnificent electronic component famous for its various uses, from a toy to a supercomputer. There is hardly an electronic device in the world which does not have a transistor in it. It is used as a switch, AC DC signal amplifier, modulator, etc., because you can easily control it. But the question is..
Can a Transistor be Used as a Relay?
Yes, it is possible. You only need to consider its connections in the circuit in which it will serve as a relay. Relay is an electromagnetic switch, while you can use a transistor as an electronic switch by controlling the power on its base. Transistors are way faster than relays.
You can use a transistor as a relay when you need to control more power or switch on and off a more powerful device. Otherwise, a relay is the best option as it provides a high level of isolation in the circuit. On the other hand, a transistor does not provide as much isolation as it has no mechanical parts.
Using Transistor as a Relay?
The transistor is an amazing solid-state component used for several purposes when designing an electronic or electrical circuit. It is because its uses are not limited to Electronics only. The only condition for accomplishing this is to use a good quality transistor with components of correct values.
And try to make the circuit as shown in the diagram providing enough voltage to make it functional. For using only a transistor as a relay, you will have to use it as an electronic switch. You can use any NPN transistor, like “2N2222A” or equivalent. You must use a “10K” or at least “1K” resistor at its base.
It will let you switch on and off your transistor as a relay without burning it out, as in the case of a resistor with a lower value. Next, connect the load in the series of the transistor. You can do this by providing “+VCC” to the load and connecting the negative terminal of the load to the “collector.”
Then, connect the “emitter” of your transistor to the ground so it can turn on when you provide the positive potential to the base resistor. Using a small switch, you can use the same positive voltage that you provided to the load connected to the transistor. Now when you do this, it will turn on as a relay.
When you give positive potential to the base resistor “R1”, your transistor will start conducting current. This current will flow from the “+VCC” power source through the transistor’s “collector” to the “emitter” and reach the negative ground voltage. As a result, your load will turn on at its maximum power.
For turning off the load, you can give negative potential to its base. It is the best method to do so. Leaving the base open is not recommended, as any stray electromagnetic signal can trigger your transistor-based relay circuit. You can try this by carefully touching your transistor’s base with your finger.
Use a transistor to pass enough current according to your load when forward bias. For powerful loads, you must need a powerful transistor. Otherwise, it will burn out within a second. It is bad for your circuit as a burnt transistor acts as a short or open circuit. Firstly, it will keep the load on for good.
But in the second case, it will keep the load off until you replace it with a new one. Hence, both kinds of faulty transistors will make your circuit dysfunctional. Each NPN transistor has its pinouts. So check the pinouts before making the connections, as wrong connections will not make the circuit useful.
Using Transistor as a Switch To Control a Relay?
Using a transistor as a switch to control a relay is the best option, providing two isolation levels and much more safety. When your transistor is faulty, your relay will be safe, and vice versa. First, connect one terminal of the relay to the transistor’s “collector” and another to “+VCC” for biasing.
After this, ground the “emitter” of your NPN transistor so the conduction can take place after switching it on. Next, connect a “10K” or at least a “1K” resistor to the base of your transistor, as it needs a very small amount of positive voltage to turn on. In the end, connect a diode and your load to the relay.
This diode is also known as Freewheeling or Flyback diode. It is a simple diode such as 1N 4007. This diode is used across the relay’s coil to protect the relay from sudden voltage spikes. It also conducts the unnecessary voltage that forms when the magnetic field across the relay’s coil collapses.
Providing positive voltage to the base resistor will turn on the transistor. As a result, it will conduct current. The coil in the relay is a conductor, so it will become energized. The mechanical switch in the relay will switch on, turning on the load. Removing this voltage from the base will shut it down.
Is a Transistor Better Than a Relay?
Transistors are way faster than relays when used as switches. The switching speed of a good quality relay is more or less 50 milliseconds, while the switching speed of a transistor is about 10-9 nanoseconds. Transistors are used in the real-time computer instead of slow mechanical, electromagnetic switches.
But of course, you can slow down the switching speed of a transistor according to your circuit requirements, as faster switching can burn out some electronic components, loads, or circuits. Relays need more power for switching on in terms of voltage, but transistors can operate on comparatively lower voltage.
Relays need a significant amount of space on an electronic circuit, but transistors can serve the same purpose efficiently using a very small space on the circuit board. The relays are mechanical switches inside that switch it on and make a noise. On the other hand, transistors do not make a sound during this.
Can You Use a MOSFET as a Relay?
MOSFETs can easily serve as a high-performance relay in many ways. Using an optocoupler increases the isolation and safety of the circuit when using them as a relay. MOSFET is also a transistor, pronounced as Metal-Oxide Semiconductor Field Effect Transistor. Hence, it can be used as a relay switch.
For using a MOSFET as a relay, you must make connections the same way as shown in the diagram. If you are using a “PC 817 optocoupler”, you must connect at least a “470-ohm” resistor to its “pin 1”. Otherwise, the infrared LED inside it will burn out when you give the input to switch it on.
“Pin 2” of the optocoupler must be grounded with the input. Next, connect a “4.7 K” resistor on “pin 4”, especially when the voltage is “12 volts”. The other side of this resistor must be connected to “+Vin” or “VCC.” The gate of the P-Channel MOSFET must be connected to pin 4 of the optocoupler.
“Pin 3” is the emitter of the transistor inside the optocoupler. Hence it must be grounded. The source of the PMOS will connect to the “+Vin” while you must connect the load on the remaining drain pin of the transistor. Connect the positive side of the load to this pin so your circuit can work properly.
The other negative side of the load you want to switch on and off must be grounded with the same source used to ground “pin 3” of the optocoupler. You can use a separate voltage source to switch on and off the optocoupler as its ground does not need to be common with the voltage source biasing the MOSFET.
You can use a small mechanical switch on the optocoupler, “pin 1” in series with the resistor or “pin 2”. It will allow you to switch on and off the circuit without repeatedly disconnecting and connecting the voltage source. The only thing to consider is the MOSFET and the optocoupler biasing.
Conclusion
A transistor can be used as a relay, and it is more energy efficient than a relay that needs more power to switch on. You can easily achieve this by using a transistor as a switch. Transistors are way faster switches than relays because they do not have mechanical parts in them at all.
Secondly, they operate on the speed of electrons as you provide voltage on their base to turn them on. But too fast switching can burn out the load if it is not meant to be turned on and off quickly. Instead of using a transistor as a relay, you can use it as a switch to control a relay as it is good at it.
Using freewheeling or flyback diodes across a relay in the right direction is necessary because it protects the circuit from power surges generated by the coil inside the relay. It conducts the stray charge that generates when the magnetic field across the coil collapses on it after switching off the relay.
Transistors are way faster than relays in switching on and off a load. They do not make noise during this, but the relays make noise as they have electromagnetic mechanical switches built inside them. Relays need more space on a circuit board, but you can adjust a transistor in a very small place.
MOSFETs can be used as relays, and they serve as a very high-performance relay in an electronic circuit for switching on and off a load. Whether you want to use a PMOS or an NMOS depends on you. An optocoupler will provide the same isolation and safety as a good-quality mechanical relay.
