PTC Thermistor: Ballast Resistor Purpose, Testing & Replacement

If you own a classic car, a vintage motorcycle, or deal with industrial lighting systems, you have likely encountered a white ceramic block bolted to the firewall or a mysterious wire that gets incredibly hot. This is the ballast resistor.

While modern electronics use sophisticated PWM (Pulse Width Modulation) to control current, older and simpler systems rely on this rugged passive component. To the uninitiated, it looks like a simple resistor. But to an engineer, a true ballast resistor is often a PTC (Positive Temperature Coefficient) Thermistor in disguise. It is a dynamic, self-regulating component that changes its behavior based on heat to protect your ignition coil from melting and your spark plugs from misfiring.

This guide, written from the perspective of an electrical engineer, will deconstruct the physics, the “Start vs. Run” logic, and the practical steps to test and replace a faulty unit.

What is a Ballast Resistor?

In electrical engineering, “ballast” refers to anything that adds stability to a circuit. Just as a ship uses ballast water to stay upright, an electrical circuit uses a ballast resistor to maintain a steady current despite fluctuations in voltage or load conditions.

Technically, a ballast resistor is a device placed in series with a load (like an ignition coil or an LED) to limit the current flowing through it.

The PTC Secret (The “Smart” Resistor)

While you can use a fixed resistor as a ballast, the best designs utilize the PTC effect.

PTC (Positive Temperature Coefficient): As the component heats up, its electrical resistance increases.

Why is this important?

Imagine an ignition coil. When the engine is running slowly (idling), the breaker points are closed for a longer time (high dwell). This allows current to build up to dangerous levels, potentially overheating the coil.

A PTC ballast resistor senses this heat (generated by the current passing through it) and increases its resistance. This automatically chokes back the current, protecting the coil. When the engine revs up and dwell time decreases, the resistor cools slightly, lowering resistance and allowing max power for the spark. It is an analog, thermal computer.

The Automotive Application: The Ignition Ballast

The most common search intent for “ballast resistor” involves vintage ignition systems (pre-1980s). Understanding this circuit is critical for troubleshooting “Start-then-Die” symptoms.

The Problem: 12V vs. 6V Coils

In the 12-volt era, car manufacturers had a dilemma.

Cold Cranking: When you turn the key to “Start,” the starter motor draws massive current, dropping the battery voltage from 12.6V down to ~9V. If you had a 12V ignition coil, it would produce a weak, yellow spark at 9V, making the car hard to start.

Solution: They used a coil designed to run on roughly 8V to 9V.

The New Problem: Once the engine starts and you release the key, the alternator kicks in, pushing system voltage to 14V. If you feed 14V into a 9V coil, it will overheat and burn out within minutes.

The Solution: The Ballast Resistor Circuit

Engineers inserted a ballast resistor (typically 0.8Ω to 1.6Ω) in series with the coil power feed.

Ignition State	Key Position	Circuit Path	Voltage at Coil	Result
Cranking	START	Bypass Wire (I-Terminal)	Full Battery (~10V)	Maximum Spark for cold starting. Resistor is bypassed.
Running	RUN	Through Ballast Resistor	Reduced (~7V – 9V)	Protected Coil. Resistor drops the excess 5V as heat.

This is why a bad ballast resistor results in the classic symptom: The engine fires while you hold the key, but dies the instant you let go.

Other Applications: Lighting and LEDs

While the ignition ballast is the most famous, the concept is used everywhere.

1. LED Turn Signals (Load Resistors)

When you switch from incandescent bulbs to LEDs, the power draw drops effectively to zero. The “flasher relay” thinks the bulb is burnt out and blinks rapidly (Hyper-flash).

You install a ballast resistor (often called a Load Resistor) in parallel to simulate the load of the old bulb. These are usually aluminum-housed power resistors.

2. Fluorescent Lamps

Old fluorescent tubes have a negative resistance characteristic. Once the gas ionizes, resistance drops. Without a ballast (either magnetic or electronic resistor), the tube would draw infinite current and explode.

Technical Specifications & Types

If you are looking for a replacement, you will encounter different physical forms.

Ceramic Block Style (Chrysler / Mopar)

The most iconic type. A white ceramic brick with two or four terminals.

2-Prong: Single resistor (Run circuit).

4-Prong: Dual resistor (Used in some ECUs to protect the transistor and the coil separately).

Inline Wire Style (Ford / GM)

Ford often didn’t use a ceramic block. Instead, they used a Resistance Wire (often pink) woven into the wiring harness under the dash. This is a hassle to replace, so many mechanics bypass it and mount a ceramic block on the firewall instead.

Material Composition

Wire Wound: Nickel-chromium wire wrapped around a ceramic core. High heat tolerance.

Iron-Wire: Used in very old systems (like the Ford Model A). Iron has a very high PTC value. As it glows red, resistance skyrockets, regulating charge rates.

Symptoms of a Bad Ballast Resistor

How do you know if this $10 component is the culprit?

Starts but Won’t Stay Running: The definitive symptom. The bypass circuit works (Start), but the run circuit (Ballast) is open.

Coil Overheating: If the resistor shorts internally (low resistance), the coil receives full 14V. The coil will get too hot to touch and eventually leak oil or fail.

Weak Spark / Misfire: If the resistance increases due to corrosion (high resistance), the coil gets starved of voltage, leading to misfires under load.

Burnt Wires: These resistors get HOT (often 200°F+). If mounting hardware fails and the resistor touches a wire loom, it can melt insulation.

Testing a Ballast Resistor: A Step-by-Step Guide

Testing is simple, but you must know whether you are testing it “Cold” or “Hot” due to the PTC effect.

Tools Needed:

Digital Multimeter (DMM)

Alligator clips (optional but helpful)

Step 1: Visual Inspection

Look for cracks in the ceramic housing. Look for the coil of wire inside—is it broken? Are the spade terminals corroded?

Note: A hairline crack in the ceramic usually means the unit has overheated and should be replaced.

Step 2: The Resistance Test (Cold)

Disconnect the negative battery terminal (Safety first).

Remove the wires from the ballast resistor. (Label them!).

Set your Multimeter to the Lowest Ohms setting (usually 200Ω).

Touch the probes to the two terminals.

Expected Values (Typical for V8 Engines):

Standard Ignition (Points): 1.2 Ω to 1.8 Ω.

Electronic Ignition (HEI/Duraspark): 0.4 Ω to 0.8 Ω.

MSD / High Performance: Often 0.8 Ω.

Engineer’s Insight: If you read “OL” (Open Loop) or Infinite Ohms, the internal wire is snapped. The part is dead. If you read 0.0 Ω, it is shorted (rare, but possible).

Step 3: The Voltage Drop Test (Live Circuit)

This is the most accurate test because it tests the resistor under load.

Reconnect all wires. Reconnect the battery.

Turn the Key to RUN (Engine Off).

Set Multimeter to DC Volts (20V scale).

Measure Input: Place Red probe on the “Battery side” of the resistor and Black probe on engine ground. You should see Battery Voltage (~12.5V).

Measure Output: Place Red probe on the “Coil side” of the resistor.

Points Closed: You should see 6V to 9V. This proves the resistor is dropping voltage correctly.

Points Open: You will see 12V. (Without current flow, there is no voltage drop. You may need to bump the engine to close the points).

Replacement Guide

Replacing a ceramic ballast resistor is one of the easiest jobs in the automotive world.

Time Required: 10 Minutes

Skill Level: Beginner

Removal

Identify: Locate the white ceramic block on the firewall or near the ignition coil.

Disconnect: Pull the spade connectors off. Inspect them for rust. If they are loose, squeeze them slightly with pliers for a tight fit later.

Unbolt: Usually held by one 8mm or 10mm bolt. Save the bolt.

Installation

Mounting: Bolt the new resistor to the firewall.

Critical: The back of the resistor gets hot. Ensure it is not touching plastic hoses, fuel lines, or wire looms. The firewall metal helps dissipate the heat.

Wiring: Connect the wires.

Polarity: A single ballast resistor is non-polarized. It does not matter which wire goes to which side.

Verify: Start the car. It should catch and stay running.

Bypassing (For High-Performance Upgrades)

If you are upgrading to a modern Pertronix Ignitor II/III or an MSD box, the instructions often say “Remove Ballast Resistor.”

Do not physically remove it; just bypass it.

Make a “Jumper Wire” (14 Gauge) with two male spade terminals.

Plug the jumper wire into the female connectors that used to plug into the resistor.

Now the coil gets full 12V ignition power.

Useful Resources

For those needing specific resistance values or wiring diagrams:

Mopar 4-Pin Ballast Guide: Detailed diagrams for the confusing Chrysler dual-ballast system.

Pertronix Installation Manuals: Excellent resource for understanding when to bypass the resistor.

DigiKey Power Resistors: If you are building a custom circuit, look for “Chassis Mount Resistors, Wirewound”.

Frequently Asked Questions (FAQ)

1. Does a ballast resistor have positive and negative sides?

No. It is a passive, non-polarized component. You can hook the ignition feed to the left and the coil to the right, or vice versa. The current flows through it just the same.

2. Why does my ballast resistor get so hot?

It is doing its job! It dissipates electrical energy as heat to drop the voltage. A normal operating temperature is often between 150°F and 250°F. This is why they are ceramic. Never touch one after the engine has been running.

3. Can I use a regular resistor from RadioShack?

Absolutely not. A typical electronics resistor is rated for 1/4 Watt. An ignition ballast resistor needs to handle 50 to 100 Watts of energy dissipation. A small resistor would flash-burn instantly. You need a “Power Resistor” or “Wirewound Resistor.”

4. What happens if I bypass the ballast resistor on a stock car?

The car will start faster and run great… for about an hour. Then the ignition coil will overheat, the oil inside it will boil, and it will fail. If you have mechanical points, they will pit and burn up rapidly due to the excessive arcing current.

5. How do I know if my car needs a ballast resistor?

Check the Coil.

If the coil says “Use with External Resistor,” you need one.

If the coil says “Internal Resistor” or “12V No Resistor Required,” you can wire it directly to 12V.

Most modern aftermarket coils (MSD Blaster 2, etc.) require a resistor if used with stock points, but no resistor if used with an electronic ignition box.

Conclusion

The ballast resistor is a relic of the analog age, but it is a brilliant one. By utilizing the properties of PTC thermistor physics—where heat creates resistance—it acts as a self-regulating valve for your electrical system.

For the classic car enthusiast, understanding this white ceramic brick is the key to solving the “start-stall” mystery. For the engineer, it is a reminder that sometimes the most robust solution isn’t a microprocessor, but a simple coil of wire that reacts to the world around it. Whether you are testing it with a multimeter or replacing it to save your spark plugs, respect the heat, and it will keep your engine firing smoothly.