What Is a Ballast Resistor? How to Test a Ballast Resistor with a Multimeter


What Is a Ballast Resistor? A ballast resistor is used in electrical devices to balance and maintain constant voltage flow through the circuits. A resistor does this by alternating its resistance to match the current from the power source.

This article explains how a ballast resistor balances current flow in an electrical circuit and its uses in various applications.

To learn more on ballast resistors, please take a read!

What is a Ballast Resistor Used for?

A ballast resistor is an electrical component integrated into an electrical gadget for balancing the amount of current flow and voltage in the device’s circuit. The ballast resistor in a circuit limits the current and hence prevents it from overcurrent faults. Hence, as the current in the device’s circuit (e.g., Neon lamps & light bulbs, LEDs, fans with alternating speeds, bright light bulbs, and lights) varies and increases above a particular threshold value, the resistance also starts to increase. This in turn, decreases the current and protects the circuit from overcurrent faults. Thus, maintaining the stability of the device’s circuit.



Currently, standard ballast resistors are rarely used because advanced electrical resistors in most circuits have replaced them.

How does a Ballast Resistor Work?

A ballast resistor works by alternating and matching its resistance with the current level through the circuit. For instance, if the current flowing through the resistor increases above the threshold value, the resistance increases. Similarly, the resistance can then correspondingly decrease as the current decreases.

Whenever this happens, the ballast resistor minimizes and balances the electrical current through the circuit in a system preventing any faults in the circuit. Thus, protecting the other parts of the electrical device from unbalanced voltage damages such as car-battery drainage cases.

Types of Ballast Resistors

There are two main types of ballast resistors which are;

  • Fixed Ballast Resistor

Fixed ballast resistors provide a fixed current resistance and are primarily in uncomplicated devices’ circuits.



Fixed ballast resistors are commonly used in uncomplicated electrical circuits that do not need heavy power threshold levels. For instance, ventilation speed fans, neon, and LED lamps/lights. 

  • Self-variable Ballast Resistors

Self-variable ballast resistors work by alternating their resistance threshold levels as the current changes. Whenever the current through the self-variable resistors rises, the temperatures rise too. The self-variable ballast resistor’s resistance level rises too.

Mostly used in the delicate incandescent lights for balancing the current levels in the circuits. Hence, safeguarding other components of the lights from excess current threshold levels.

Self-variable ballast resistors are more ideal for regulating voltage threshold voltage levels in a circuit than fixed resistors. Besides, they are perfect for minimizing the current passing through in lesser amounts per phase compared to the fixed resistors.

Ballast Resistors Applications

Electrical devices manufacturers use ballast resistors in a wide range of electrical devices to maintain the current levels through the circuits, such as;

  • Ballast Resistor in Fluorescent Lamps

The longer the lights are on, the more the temperature levels. Thus, the resistors are fixed with the fluorescent lamps to prevent the overheating damages caused by the excess current through the circuits.

A fluorescent lamp

A fluorescent lamp

A switching voltage equal to the power source is necessary for the lights to turn on. The ballast resistors integrated into the fluorescent lamps provide the ideal voltage through the arc created between the lamp’s electrodes.

Ballast Resistor for Automotive Applications

Vehicle manufacturers integrate ballast resistors into the engine ignition systems. The ballast resistors made for vehicles act as the link in-between the ignition coil and the coil stud. Placing the ignition ballast resistor between the two coils minimizes the malfunction risks of the ignition coil.

The ignition ballast resistor decreases excess voltage and current through the ignition coil once the starter motor ignites the car engine. Balanced current and voltage levels promote the durability of the car ignition coil.

LED Circuit using Ballast Resistor

A ballast resistor in a LED circuit works to minimize the voltage through the LED electric circuit to the constant current levels. Therefore, preventing potential damage to the Light-Emitting Diodes (LEDs) circuits.

Light-Emitting Diodes (LEDs)

Light-Emitting Diodes (LEDs)

For example;

For 1 LED in a series connection with a 5V direct current source, a 211Ω or more ballast resistor is ideal. Because the maximum voltage of the LED is 3.1V with a 9mA forward current calculated using the equation below.

Hence; R = (E-VF/IF), 

  • VF, as the ‘Forward voltage of LED.’
  • IF, as the ‘Forward current of LED.’
  • R, as the ‘Resistance of Ballast resistor.’
  • E, as the ‘Supply voltage.’
  • Ballast Resistor for an Ignition Coil
new automobile spark plug

new automobile spark plug

How to Test a Ballast Resistors with a Multimeter

On how to test ballast resistors with  a multimeter or an ohmmeter, you need to do the following;

  1. Connect the ballast resistor to the ignition coil to reduce the 12 or 24-voltage battery supply.
  2. Set the ohmmeter or a multimeter to an ideal range and connect the leads to measure the flow of voltage through the ballast resistors by connecting the leads. Ignore polarity levels.
  3. Check the readings. They should fall below the full battery voltage supply to show a good condition of the ballast resistors.

For a check using an ohmmeter, connect it to the resistor integrated with the circuit system and see if the readings are close to the specified resistance. Ohmmeter readings close to the resistance range show a healthy resistor [1.8-5Ω]. Watch this video on how to test ballast resistors with a multimeter.


To sum up the article, ballast resistors give stability to electrical circuits, and device manufacturers use them for balancing and regulating voltage threshold levels to prevent damage in various electrical circuits. These ballast resistors have major applications in areas of automotive and lamp circuits. For more information on ballast resistors, how they work, and their applications, contact us.

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