RL Circuits (resistor – inductor circuit), also called RL network or RL filter, is a type of circuit having a combination of inductors and resistors and is usually driven by some power source. As such, an RL circuit has the inductor and a resistor connected in either parallel or series combination with each other. They are either driven by the current (parallel) or a voltage (series) source. Besides, the resistor (R), inductor (L), and capacitor (C) form the basic passive linear circuit elements. They can form an electrical circuit in four different ways like the RC circuit, the LC circuit and the RLC circuit.
Alternatively, an RL circuit is also described as an electric circuit with resistance and self-inductance. We already know that the process of induction occurs when an emf source is applied by a continuous change in the magnetic flux. The mutual inductance is an effect of the laws of induction presented by Faraday, while self-inductance is an effect of the laws of induction of Faraday of a device coming on itself. The inductor is a circuit or a device component that exhibits self-inductance. However, since there is a presence of a resistor in the ideal form of the circuit, an RL circuit will consume energy, akin to an RC circuit or RLC circuit.
Consider the above circuit having a battery, a resistor, and a switch. The switch can either have the battery in the series connection to the circuit or the battery can be removed from the circuit. When the switch is closed, the current jumps to the maximum value and when the switch is opened, the value of current decreases immediately.
When an inductor is added in series with the resistor of the circuit, we come to observe changes in the current. The role of an inductor in the circuit is to oppose the change in the magnetic flux, i.e., the inductor does not allow the spontaneous changes in the current. When we close the switch of the circuit, there is a gradual increase in the value of current to a maximum value. When we open the switch and remove the battery, the inductor voltage causes the current to reduce gradually to the value of zero again.
A first-order RL circuit mainly comprises one resistor and one inductor to form an RL circuit. The power factor of this circuit is low because of the inductive load like a 3-phase induction motor. Even the lamps, transformers, welding devices operate at low lagging power factors.
In the RL series circuit, the flow of current is lagging behind the voltage through an angle ‘ϕ’ due to the inductor effect. So here, the power factor (PF) can be given like the cosine of lagging angle ‘ϕ’
The power factor = Cos ϕ = Resistance/Impedance = R/Z
RL Series Circuit
A circuit that contains a resistance R connected in series with the coil having an inductance L is known as an RL Series Circuit. When a supply voltage (V) is applied across the current element I flowing in the circuit. IL and IR are the currents flowing in the inductor and resistor, but the current flowing across both the elements are the same as they are said to be connected in the series connection with each other. The diagram of the RL Series Circuit is as shown below-
where VR is the voltage across resistor R
VL is the voltage across inductor L
V(t) is the total voltage across the circuit
In the above simple RL Series circuit where the resistor, R and the inductor, L are combined in series combination with the voltage source having V volts. The current flowing in the whole circuit is I amps and the current through the resistor and the inductor is IR and IL. Since both the resistance as well as the inductor are connected in the series combination, the current in both of the elements of the circuit remains the same.
Here, IL = IR = I. Consider VL and VR to be the voltage drops across the inductor and resistor. By the application of Kirchhoff voltage law (sum of the voltage drops must be the same across the circuit to apply the voltage) to this circuit, we get,
V(t) = VR + VL
Thus, this is the equation for the voltage across the RL series circuit.
RL Parallel Circuit
The parallel RL circuit is generally of less interest than the series circuit. It can be interesting until it is fed by a current source. This is the case with a parallel RL circuit mainly because the output voltage Vout is equal to the input voltage Vin. As a result, this circuit does not act as a filter for a voltage input signal.
With complex impedances:
The inductor lags the resistor (and source) current by 90°.
The parallel circuit can be found on the output of many amplifier circuits. It is basically used to isolate the amplifier from capacitive loading effects at high frequencies.
Use of RL Circuit
- In chokes of luminescent tubes.
- For supplying DC power to radio-frequency amplifiers where the inductor is used to pass DC bias current, and block the RF returning into the power supply.
- RL circuits can form a single-pole filter. The filter is low-pass or high-pass is dictated by whether the reactive element (C or L) is in series with the load or parallel with the load.
- Radio Wave Transmitters.
- Resonant LC Circuit/RLC Circuit.
- Communication Systems.
- Processing of Signal.
- Oscillator Circuits.
- Magnification of Current or Voltage.
- Variable Tunes Circuits.
- Filtering Circuits.
Also Read About:
| Types of Circuits |
| Circuit Components |
| Circuit Diagram |
| Integrated Circuit |
| Difference between Series and Parallel Circuits |
Frequently Asked Questions on RL Circuit
What is the power factor of an RL circuit?
The power factor of an RL circuit is the ratio of the actual power dissipated to the apparent power.
What are the applications of RL circuits?
RL circuits are used in communication systems, radio wave transmitters, oscillator circuits, RF amplifiers, filtering circuits, variable tuned circuits, magnification of current and voltage, etc.
What is an RL circuit?
An RL circuit is a circuit consisting of the passive components like the resistor and the inductor connected together, driven by a current source or a voltage source.
What is the time constant of an RL series circuit?
Time Constant, τ = L/R
L = inductance
R = resistance
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