- 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 series resistor that limits current so a load cannot draw an unbounded current. The broader term electric ballast covers any current-limiting device, including resistors, capacitors and inductors.
A fixed ballast resistor has a nearly constant resistance. A self-variable ballast, such as a barretter, raises resistance as it heats so current does not keep climbing.
The heated rise can hold current more nearly constant. A fixed ballast only drops voltage according to Ohm’s law at the current that happens to flow.
Unlike a load resistor, whose value stays nearly constant as voltage and current change, a self-variable ballast changes resistance as it heats. A fixed ballast is still an ordinary constant-value resistor used only to drop current.
Simple resistive lamp ballasts are uncommon now. Electronic and inductive ballasts, plus electronic ignition, have taken most of those jobs. Older vehicles still use a discrete ballast resistor or resistance wire in the ignition run circuit.
What does a Ballast Resistor do?
Ballast here means a current-limiting stabilizer, not a mechanical weight.
It sits in series so other parts see a limited current rather than the full source capability.
On a self-variable ballast, a rise in current heats the element. Resistance then rises because the material has a positive temperature coefficient.
The extra resistance then limits current through the rest of the circuit.
Older cars place a ballast resistor in the ignition run feed. It does not start the engine. While the starter cranks, a bypass feeds the coil from the battery around the ballast. After the ignition switch returns to run, current goes through the ballast.
Lighting uses include a resistive or inductive ballast on a fluorescent lamp, a series resistor on an LED and a transformer ballast on many neon signs.
Applications of a Ballast Resistor
A ballast resistor limits current. The voltage drop across it follows from that current. It does not by itself clamp overvoltage on a separate rail.
The usual jobs are automotive ignition and lighting.
Ballast Resistor for Automotive Applications
In a car, that part sits in the ignition system and is called an ignition ballast resistor.
It is wired in the run circuit to the coil positive terminal, often as a ceramic block or a resistance wire. During running it drops part of the battery voltage so the coil and, on points systems, the contacts do not overheat.
When the starter cranks the engine, the ballast is bypassed so the coil still sees a high voltage even though battery voltage sags. The ballast is back in circuit once the ignition switch returns to run.
The lower run current keeps coil temperature down and extends coil life.
Starting still needs a strong spark, so a jumper from the starter solenoid I-terminal or an extra ignition-switch start feed bypasses the ballast and applies near-battery voltage to the coil only while cranking.
Ballast Resistor in a LED Circuit
An LED is damaged by excess forward current. A supply that can force more current than the LED rating will overheat the die even when the open-circuit voltage looks only a little high.
A series ballast resistor drops the extra voltage so LED current stays near the rated forward current. The voltage across the LED then sits near the forward voltage.
Choose the resistance from the worked example below.
Consider one LED in series with the supply. The source voltage is higher than the LED forward voltage, so you cannot connect the LED directly.
![\[ R = \frac{E-V_F}{I_F} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-a364b7934a71e51272e91426c63c1512_l3.png "Rendered by QuickLaTeX.com")
Where:
VF = forward voltage of the LED
IF = forward current of the LED
R = resistance of the ballast resistor
E = supply voltage
For example, the DC source is 5 V. The LED forward voltage is 3.1 V and the forward current is 9 mA. From the equation above:
![\[ R = \frac{5-3.1}{0.009} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-f5783e4cbabac08380bd8e617c3ce5f8_l3.png "Rendered by QuickLaTeX.com")
![\[ R = \frac{1.9}{0.009} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-bcda1ecb473cb17eded8e88c5a042cc4_l3.png "Rendered by QuickLaTeX.com")
![\[ R \approx 211 \Omega \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-ee183dc266517f641bdf8d1d5e5406b6_l3.png "Rendered by QuickLaTeX.com")
For this example, use 211 Ω or the next standard value above that, such as 220 Ω, so current stays at or below 9 mA.
Types of Ballast Resistors
Ballast resistors fall into two groups:
- Fixed Ballast Resistor
- Self-variable Ballast Resistor
Fixed Ballast Resistor
A fixed ballast has a set ohm value. It is used where a constant drop is enough, often in simple DC circuits.
Fixed ballasts appear wherever a constant series drop will keep current inside a rating.
Examples include a neon indicator or LED series dropper. Some older multi-speed fans also use a series resistor pack.
On some older multi-speed fans, a tapped series resistor is switched in sections. The selector chooses how much resistance sits in series, and fan speed follows that drop.
Self-variable Ballast Resistor
This type of resistor raises resistance as current (and temperature) rise. When current through the element increases, the ballast heats and resistance increases.
A self-variable ballast, or the filament itself, is used with incandescent lamps. As current rises, resistance rises and more voltage appears across the ballast so lamp current does not run away.
When current falls, the element cools and resistance falls. The same thermal rise can limit overcurrent in other circuits that use a barretter or PTC ballast.
How to Test a Ballast Resistor
On older vehicles the ballast reduces coil voltage during run. Test it with an ohmmeter and a voltmeter (or a multimeter used in both modes).
If the ballast is shorted or omitted, full run-circuit voltage stays on the coil and the coil (and points, where fitted) run hot.
The ignition feed comes from the battery. Most cars are 12 V; some heavy vehicles are 24 V.
The ballast is there to cut run current after start, not to starve the coil during cranking.
A practical check is coil-positive voltage to ground with ignition in run and the engine not cranking.
On many 12 V systems that reading is about 6 to 9 V (shop notes often quote 7 to 8 V). Full battery voltage in run usually means a shorted ballast or a closed bypass. Open ballast gives little or no voltage in run, so the engine may start on the bypass then stall.
With the part isolated, an ohmmeter reading near the marked or workshop value is a healthy ballast. An open circuit or a dead short is a fail.
