However there are a few complexities that need to be settled before the design could be used for the required functions:. Since the base drive voltage to transistor is the major source for controlling the relay operations, it needs to be perfectly calculated for optimal results. The basic formula for calculating the base resistor of the transistor is given by the expression:.
The diode connected across the relay coil though is no way related with the above calculation, it still cannot be ignored. The diode makes sure that the reverse EMF generated from the relay coil is shorted through it, and not dumped into the transistor.
Without this diode, the back EMF would try to find a path through the collector emitter of the transistor and in the course damage the transistor permanently, within seconds. A transistor works best as a switch when it is connected with a common emitter configuration, meaning the emitter of the BJT must be always connected directly with "ground" line. This is already explained in the above discussions.
Here the relay may be connected across the negative line of the supply and the collector of the PNP. Please see the figure below for the exact configuration. However a PNP will need a negative trigger at its base for the triggering, so in case you wish to implement the system with a positive trigger then you may have to use a combination of both NPN and PNP BJTs as shown in the following figure:. If you have any specific query regarding the above concept, please feel free to express them through the comments for getting quick replies.
Normally, the supply voltage for a operating a relay is dimensioned to ensure that the relay is pulled-in optimally. However, the required retaining voltage is typically much lower. This is usually not even half the pull-in voltage. As a result the majority of relays can work without problems even at this reduced voltage, but only when it is ensured that at the initial activation voltage adequately high for the pull-in.
The circuit presented below may be ideal for relays specified to work with mA or lower, and at supply voltage below 25 V. Secondly, relays with higher voltage rating could be used with lower supply ranges.
The circuit can be seen wired to a supply voltage capable of holding the relay perfectly. During the time S1 is open, C1 gets charged via R2 upto the supply voltage. The moment S1 is presed, the T1 base gets connected to supply common through R1, so that it switches ON and drives the relay. The positive terminal of C1 connects to the common ground through the switch S1. Considering that this capacitor initially had been charged to the supply voltage its -terminal at this point becomes negative.
The voltage across the relay coil therefore reaches two times more than the supply voltage, and this pull in the relay. Switch S1 could be, certainly, be substituted with a any general purpose transistor which can be switched on or off as required. If you have any circuit related query, you may interact through comments, I'll be most happy to help! Your email:. Please T1 in the last circuit be any pnp transistor or Again what is the rational for the two 1k resistor in the npn and pnp transistor circuit above.
Yes it can be any pnp transistor for relay coils higher than ohms. The two 1K resistors ensure that the base of the transistor is never held floating, raher always connected to some potential, either positive or negative. I went through the post suggested by you so I got the formula to determine the value of a resistor to the base of a transistor thank you can we use two transistors parallel to handle higher current drawing load.?
Hai, what is the minimum voltage required to activate a normal 12volt relay?? I have some doubts. I connected two 12v relays in collector of one Will the 10k resistor fine as base resistor?
Dear sir, I need a dry run preventer cicuit for my open well, if the water goes down near to the submesible water bump the circuit will trip the motor if we want to run the motor we need to release the relay trip manually and the relay run through the no volt coil on the starter and also the circuit will not use of ic, please help me,. And which components should I adjust. If you give me the right values, it would be really appreciate it.
Hi, If I need to use two relays with single transistor, Can I change instead of in the above formula? So finally I suppose to use K resistor with BC?
Correct me if I am wrong. Kindly help. If you have calculated everything correctly and getting 95K then it may be the right value. Hi Hamid, you can connect by approximately judging the resistor values in circuits, it is not critical unless the value is referenced to some other dimension. In relay circuits I normally use a 10K resistor as the transistor base resistor for all relays whose coil resistance is between ohm and ohms. If you calculate the transistor base resistor for a ohm relay load, you will find it to be 56K, but I use 10k which does not make any difference expect a some mA more dissipation or wastage by the transistor and the relay.
Choosing transistor is also not so crucial, below 50mA load you can use BC, and at mA and above you can choose 2N Hello sir, I am ketan and my question is: I want to use B and 74f74 for my different projects,as a trigger of relay driver circuits so which parameters of d flip-flop ICs should I take into account?
Hello Ketan, the calculations are already explained in the above article, you can apply them in the following example applications. Hello sir, is it required to have a circuit for the operation of relay. I have a relay which has a triggering voltage of 12v, so please help help me with that. Hello Rajratan, if you want to operate the relay through some external triggering then you will require the transistor relay circuit explained in the above article.
If you want only to test the relay or want to operate it without an external trigger then you can join the relay directly with the available DC supply. Dear Sri. Swagatam Majumdar, After a very long period I am posting this query to you. My request to you is that How I can incorporate a single transistor and photo diode in this circuit so that I can operate as a remote relay circuit.
Thanks for your posting. It limits current to a safe level for the coil. Thus, when the current flows through the coil.
Then, a magnetic field occurs. It makes the contact of relay connect together as a switch is closed. To connect the voltage-V2 provides a high current I2 to Load as we need.
Sometimes you may use the relay with a digital circuit. Using the output pulse from a microcontroller or digital gate ICs. To control relay to works. But the most, its output is low current. So you need a helper, use a transistor to switch high current to drive the coil instead. In the circuit below is most transistor relay driver circuit.
The coil of relay needs a current about mA. And, the input current at output of a normal digital circuit is about 2 mA. You can calculate a limiting resistors-R from the input voltage and current. For example, input voltage of 5V, Current of 2 mA approximately. In the circuit above, Here is how you can choose a proper number. First, It is an NPN transistor type. Suppose the transistor has a currents Gain hFE about 50 times. Since the input current is about 2 mA.
It is enough to the needs of the relay coil. There are a lot of transistors that have the hFE gain is more than For example, 2N, 2N transistor, etc. Table 1 shows size of input voltage of various digital ICs and the relay coil need. By has various resistor-R limit proper current for circuit.
Figure 3 is the driver relay circuit by use input voltage to feed to the relay coil but has some voltage junction base and emitter lead of transistor. Which has value about 0. This circuit does not require a resistor-R. Because the circuit as emitter follower will have a high input impedance already.
On each relay feature will is determined the resistance of the coil is ohms unit. So if we know the voltage of the relay will also calculate the current of the coil. The resistance of relay coil ohms will calculate the current flowing through the relay coil is 12 divided by equals 0. Some have multiple stages, different driver devices such as ICs, and different methods for controlling those devices.
We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon. You must be logged in to post a comment. Skip to content What is a Relay Driver Circuit? This video breaks out a simplified transistor-based relay driver circuit.
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