Ballast Resistor: What is it? (And What Does It Do?)

Contents

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Key learnings:

Ballast Resistor Definition: A ballast resistor is a component that controls the amount of current in a circuit by changing its resistance based on the current flow.
Types of Ballast Resistors: There are two main types of ballast resistors: fixed, which have a static resistance, and self-variable, which adjust their resistance with changes in current.
Testing Ballast Resistors: To ensure a ballast resistor is functioning properly, measure its resistance and the voltage across it using an ohmmeter and multimeter.
Application in LED Circuits: Ballast resistors are used in LED circuits to prevent damage by ensuring the voltage across the LED does not exceed its rated level.
Significance in Automotive: In automotive systems, ballast resistors protect the ignition coil by reducing the voltage supplied during engine start-up, thereby extending the coil’s lifespan.

What is a Ballast Resistor?

A ballast resistor is a resistor used to limit current in a circuit and prevent over-current faults. The term “electric ballast” refers more broadly to any device, such as resistors, capacitors, or inductors, that helps stabilize a circuit by controlling current and voltage levels.

Ballast resistors adjust their resistance based on the current. As the current exceeds a threshold value, the resistance increases, and it decreases when the current falls.

In this way, the ballast resistor tries to maintain a constant current flowing through a circuit.

Unlike a load resistor, which has a constant resistance under varying currents and voltage, a ballast resistor acts as a variable load within the system, adjusting its resistance as needed.

Ballast resistors have largely been phased out, replaced by modern electronic circuits that serve the same purpose.

What does a Ballast Resistor do?

The term “ballast” implies stability, indicating that a ballast resistor helps maintain the stability of an electrical circuit.

A ballast resistor is used in a device to compensate for changes and secure the other components of a network.

When the current flowing through the resistor increase, the temperature also increases. And due to the temperature rise, the resistance also increases.

Therefore, an increase in resistance limits the current flowing through the network.

Ballast resistors are widely used in automotive applications to start the engine. When the starter motor starts the engine, the ballast register limits voltage drain from the battery.

It is also used in the lighting applications such as a fluorescent lamp, LED, and neon lights.

Applications of a Ballast Resistor

A ballast resistor helps to regulate the current and voltage in an electrical system. It helps equipment from experiencing overcurrent and overvoltage events.

Ballast resistors are mostly used in automotive and lighting applications.

Ballast Resistor for Automotive Applications

In a car engine, a ballast resistor is used in the ignition system. And it is known as an ignition ballast resistor.

Generally, an ignition ballast resistor is placed between the primary source of the ignition coil and coil stud. It reduces the failure risk of the ignition coil.

When the starter motor cranks the engine, the ignition ballast resistor helps to reduce the coil voltage and coil current.

Therefore, low current causes a low-temperature rise. And it results in a long life of the ignition coil.

But the ignition system needs a higher voltage which is equal to the voltage of the power source. So, the jumper wire is connected with an ignition ballast resistor. And while starting the engine, the jumper wire provides the required voltage to the ignition coil.

Ballast Resistor in a LED Circuit

The LED (Light Emitting Diode) is a very sensitive device. It may damage if the supply voltage is more than the rated voltage.

To avoid such conditions, a ballast resistor is used to connect in series with the LED. It will reduce the voltage across the LED to its rated value.

We need to connect the appropriate resistance value of ballast resistance. For that consider the below example.

Consider a single LED is connected in series with the supply source. Here, the value of the supply source is higher than the rated voltage. Therefore, you cannot connect directly without a resistor.

$R = \frac{E-V_F}{I_F}$

Where;

V_F = Forward voltage of LED

I_F = Forward current of LED

R = Resistance of Ballast resistor

E = Supply voltage

For example, the value of DC source is 5V. The forward rated voltage of LED is 3.1V and forward current is 9mA. From the above equation;

$R = \frac{5-3.1}{0.009}$

$R = \frac{1.9}{0.009}$

$R \approx 211 \Omega$

Hence, for this example, we need to connect a 211Ω or more than this rating of the resistor.

Types of Ballast Resistors

The ballast resistors are classified into two types;

Fixed Ballast Resistor
Self-variable Ballast Resistor

Fixed Ballast Resistor

As the name suggests, the resistance of this type of resistor is fixed. This type of ballast resistor is generally only used in simple circuits.

Fixed ballast resistors used in various applications where a high resistance value is preferred.

For example, it is used in a circuit with a neon lamp or LED. It is also used in the variable speed fan.

In a variable-speed fan, a fixed ballast resistor with two center taps is used. The fan selector switch sets the value of resistance with the help of the center tap. And according to the value of resistance, the speed of a fan varies.

Self-variable Ballast Resistor

This type of resistor has a property to change its resistance with changes in current. When the current flowing through the resistor increase, it increases the temperature of the ballast resistor. As a result, the resistance also increases.

This type of ballast resistor is used in an incandescent lamp. When the current passing through the lamp increase, the resistance of the ballast resistor increases. And it also increases the voltage drop across the resistor.

When the current decreases, the temperature goes down and resistance decreases. This type of resistance is also used to protect circuit equipment against overcurrent.

How to Test a Ballast Resistor

The ballast resistor is used in automotive applications. It is used to reduce the voltage level of the ignition coil. To test the ballast resistor, we need an ohmmeter and multimeter.

If the ballast resistor is not connected with the ignition coil, the full supply voltage is applied to the ignition coil.

Generally, the battery is used to supply the ignition system. And in most cases, the battery voltage will be 12V or 24V.

To increase the life of the ignition coil, we need to reduce voltage. and to fulfill this purpose, we connect a ballast resistor with the ignition coil.

So, to test the ballast resistor, we can measure the voltage across the ignition coil.

If the ballast resistor is good, it reduced the voltage level to 7-8V. If the ballast is damaged, the voltage level of the ignition coil will be high.

We can check the resistance of the ballast resistor with the help of an ohmmeter. If it is near the rated resistance, this is a good sign that the ballast resistor is in a healthy condition.