What is Ballast Resistor : Working & Its Applications

A resistor that has the property to increase the resistance when the flow of current through it increases & decreases resistance when current flow decreases are called a ballast resistor. This resistor maintains a stable current flow through it, even though differences within applied voltage or within the rest of the circuit. The ballast action can be obtained through resistive material that increases resistance when temperature increases. These resistors are used to compensate for variations within line voltage, because in some automotive ignition systems otherwise to compensate for negative volt-ampere characteristics of other devices like vapor lamps & fluorescent lamps. This article discusses an overview of a ballast resistor and its working with applications.

What is Ballast Resistor?

A ballast resistor is one type of electrical and electronic component, used to change the current flow within a circuit. This resistor has the property to increase the resistance when the flow of current through it enhances and decreases resistance when the flow of current decreases.

So the ballast resistor function is to maintain a stable current flow throughout the circuit even changes within applied voltage in the circuit. This resistor functions as a variable load on the system; thus it is different from load resistors, which have a stable resistance. The ballast resistor diagram is shown below.

Ballast Resistor
Ballast Resistor

Usually, these resistors are connected in series through the negative load, because the components connected in the series will get the same current. Some ballasts use series resistors, whereas others use capacitors & other complex components.

How does a Ballast Resistor Work?

The working of the ballast resistor is, whenever the current flow through the resistor increase then the temperature will be increased. So because of this temperature increase, the resistance will be increased. Thus, the increases within resistance will restrict the flow of current throughout the network.

These resistors are used to start the engine within automobiles. Once the motor starter starts the car engine, the resistor restricts voltage drain from the battery. It is also applicable in lighting-based applications like LED, neon lights, fluorescent lamps, etc.


Ballast Resistor Types

These resistors are available in two types fixed resistors & Self- variable resistors.

Ballast Resistor Types
Ballast Resistor Types

Fixed Resistors

As the name suggests, these resistors have fixed resistance. In most applications, high resistance value-based resistors are used but this type of resistor is applicable in simple circuits like LED or neon lamps because these circuits have low powered loads.

This resistor is also used to change the speed of the ventilation fan. The selector switch of fan speed bypasses the ballast portions such that for complete speed, a complete portion can be selected & for the less fan speed, no ballast resistor portion is chosen.

Self-Variable Resistors

Self-variable resistors have the property to change the resistance based on the current change like when the flow of current increases then the resistance will be increased similarly when the flow of current reduces then its resistance will be decreased.

These types of resistors are frequently used in incandescent lamps because the flow of current throughout this lamp increases, then this resistor will get heat. So the resistance increases when temperature increases & the voltage drop increases across the resistor. If the flow of current reduces, the temperature of this resistor will decrease, so its resistance & voltage drop will be decreased.

The benefit of using this type of resistor is, it provides precise current control as compared to a suitable fixed resistor, the power lost within the resistive ballast gets decreased because a smaller section of the overall power will be dropped within the ballast as compared to a fixed resistor.

LED Circuit using Ballast Resistor

We know that a light-emitting diode is a very sensitive device. So, if the voltage supply which is applied to this is above the rated voltage, then the diode may get damaged. To overcome this problem, a ballast resistor is employed within an LED circuit which is shown below.

LED Circuit with Ballast Resistor
LED Circuit with Ballast Resistor

The required components of this simple circuit are a LED, ballast resistor, and a battery. The connections of this circuit can be given as per the shown diagram. In this LED circuit, the ballast resistor is connected in series with a light-emitting diode. So it will decrease the voltage across the light-emitting diode as compared to its rated voltage.

The LED is connected in series with the voltage supply source like a battery. Here, the voltage supply value is higher as compared to the rated voltage, then, you cannot connect the circuit without a resistor directly. So, the resistance value of this resistor is to be used by using the following formula:

R = (E-VF/IF)

From the above equation,

‘VF’ is the forward voltage of the light-emitting diode

‘IF’is the forward current of light-emitting diode

‘R’ is the resistance of the resistor

‘E’ is voltage supply


For instance, the DC source value is 6V, the LEDs forward rated voltage is 4V & the forward current is 8mA, then calculate resistance value.

Example Problem Circuit
Example Problem Circuit

We know the formula for R = (E-VF/IF)

R = 6-4/0.008 = 250ohms

So, we need to connect 250 ohms or above the rating of the resistor in the circuit.

Ballast Resistor for an Ignition Coil

In an engine of an automobile, a ballast resistor can be used within the ignition system, so it is known as an ignition ballast resistor. Generally, this resistor is connected in between the main source of the ignition coil & coil stud. So, it decreases the malfunction risk of the coil. Once the starter motor starts the car engine, the ignition ballast resistor assists in reducing the voltage & current of the coil.

Thus, low current can cause a low rise in temperature and it results in longevity of the ignition coil. However, the ignition system requires maximum voltage which is equivalent to the power source voltage. Consequently, the jumper wire can be connected through an ignition ballast resistor. When starting the car engine, this wire provides the necessary voltage toward the ignition coil.


The advantages of a ballast resistor include the following.

  • These resistors help in regulating the voltage & current within an electrical system.
  • This resistor protects the equipment from over voltage & over current occurrence.
  • These resistors reduce differences in current, applied voltage otherwise in the rest of an electric circuit.


The applications of ballast resistors include the following.

  • These resistors are used mostly in lighting & automotive applications.
  • These resistors are used in LED circuits, fluorescent lamps, and automotive applications.
  • The ballast within the HeNe laser is used for stabilization & restricting the tube current. So the ballast resistance normally used for most HeNe lasers is approximately 75 KΩ.
  • These are used in low power-based devices like LEDs & neon lamps.
  • This resistor is mainly used in simple circuits because they have low-powered loads.
  • These resistors help in regulating the flow of voltage in an electrical system to stay away from battery drainage & overloads.
  • This resistor is used in a battery ignition system by connecting it in series through the primary winding to change the main current.
  • These are applicable in vapor lamps & laser circuits for compensating fluctuations within line voltage.

Thus, this is all about an overview of a ballast resistor and its working with applications. These types of resistors are mainly used for over-voltage or current or protection within different automotive & lamp circuits so that it gives stability to the circuit. Here is a question for you, what are the different types of resistors available?