Second Chance Garage
All About Car Restoration
Second Chance Garage
All About Car Restoration


Automobile Electrical Systems — Part 3: How Voltage Regulators Work

Voltage Regulators

As you may recall from last month's article on the function of generators in your classic car, there is no means of internally controlling the output of one. In other words, the faster it spins the more voltage goes into the car's electrical system. If this weren't controlled the generator would damage the battery and burn out the car's lights. Also, if the generator weren't cut out from the car's circuitry when not running, the battery would discharge through its case.

Voltage Regulator Diagram

That's where the REGULATOR (commonly called the Voltage Regulator, but that's only one component of the system) comes in. Regulators have seen many design improvements over the decades, but the most commonly used electro-mechanical regulator is the three-control units in one box type. Let's look at how these things work...

Cutout Relay

Sometimes called the circuit breaker, this device is a magnetic "make-and-break" switch. It connects the generator to the battery (and therefore the rest of the car) circuit when the generator's voltage builds up to the desired value. It disconnects the generator when it slows down or stops.

The relay has an iron core that is magnetized to pull down a hinged armature. When the armature is pulled down a set of contact points closes and the circuit is completed. When the magnetic field is broken (like when the generator slows down or stops) a spring pulls the armature up, breaking the contact points.

Voltage Regulator Diagram

An obvious failure mode is the contact points. As they open and close, a slight spark is generated, eventually eroding the material on the points until they either "weld" themselves together or become so high in resistance that they won't conduct current when closed. In the first case the battery would discharge through the generator overnight and in the second there would be no charging to the system.

Voltage Regulator

Another iron core-operated set of contact points is utilized to regulate maximum and minimum voltage at all times. This circuit also has a shunt circuit (a shunt re-directs electrical flow) going to ground through a resistor and placed just ahead (electrically) of the points. When the points are closed the field circuit takes the "easy" route to ground but when the points are open the field circuit must pass through the resistor to get to ground.

The field coil on the generator is connected to one of the voltage regulator contact points. The other point leads directly to ground.

When the generator is operating (battery low or a number of devices running) its voltage may stay below that for which the control is set. Since the flow of current will be too weak to pull the armature down the generator field will go to ground through the points. However, if the system is fully charged the generator voltage will increase until it reaches the maximum limit and current flow through the shunt coil will be high enough to pull the armature down and separate the points.

This cycle is repeated over and over in real time. The points open and close about 50 to 200 times per second, maintaining a constant voltage in the system.

Current Regulator

Even though the generator's voltage is controlled it is possible for its current to run too high. This would overheat the generator, so a current regulator is incorporated to prevent premature failure.

Similar in appearance to the voltage regulator's iron core, the current regulator's core is wound with a few turns of heavy wire and connected in series with the generator's armature.

Voltage Regulator Diagram

In operation, current flow increases to the predetermined setting of the unit. At this time, current flow through the heavy wire windings will cause the core to draw the armature down, opening the current regulator points. In order to complete the circuit the field circuit must pass through a resistor. This lowers current output, points close, output increases, points open, output down, points close, and so on. The points, therefore, vibrate open and closed much as the voltage regulator's points do, many times every second.

Good and Bad News

Because they are mechanical, voltage regulators are easy to troubleshoot. If you study the function of each of the three parts and how they interrelate, it becomes obvious which part is malfunctioning, depending upon symptoms. That means anyone who understands how everything works can easily troubleshoot problems. That's the good news.

The bad news is that the point gaps and spring pressures determine the voltage/current limits and they are exceedingly hard to adjust. Sometimes it can be done on the car using a voltmeter, but generally it is best to replace the entire regulator assembly when a certain part of it fails. Factory assembly of regulators required relatively sophisticated measurement instruments. Adjusting them by "feel" is a matter of luck, and frequently can result in damage.

Overall, the good news is that regulators are inexpensive and relatively easy to find. Replacement is always a good idea.

What about Alternator Regulators?

The same type of regulator was originally incorporated into alternator-fitted cars and they work in much the same way. However, since some cars used ammeters no current regulator was needed. Therefore, a "single unit" regulator was used to turn on the alternator's stator windings. It was just a regulator without a current regulator section.

It wasn't long thereafter that the automobile companies converted to transistor voltage regulators. Utilizing Zener diodes, transistors, resistors, a capacitor and a thermistor, these regulators maintain proper voltage and current flow throughout the system. Their circuitry operates as fast as 2,000 times per second and they are tremendously reliable. On the other hand, these regulators aren't easy to repair. They are designed to be thrown away and replaced.

Many "solid state" regulators are mounted inside the alternator and are not serviceable other than the ability to set the voltage limits. That's okay, because they work very well for long periods of time. To check their operation, just measure the battery voltage while the engine is off, then when it's running. You should see something between 13 and 15 volts when running. No change in voltage means either the regulator or alternator isn't working, while higher voltage means the regulator isn't properly "regulating."

What about Converting from Generators to Alternators?

Well, that's a double-sided question. We believe such conversions should be done if additional electrical devices have been installed during restoration or major updating of the car. Air conditioning, electric cooling fans, etc. eat up lots of current that can't be easily handled by old generators. Alternators provide three times the current and weigh much less than their old counterparts.

On the other hand, converting to an alternator will affect the car's "period" appearance. It's a personal choice, of course, but one worth considering. We'll be doing an article on a conversion very soon.

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