Alternator voltage regulator

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The regulator voltage of the alternator is a circuit responsible for maintaining the voltage level generated by the alternator within the specified limits that are between 13V and 14.5V.

Alternator voltage regulator

Looking for sometime on the internet, I did not get quality information about how to test the voltage regulators in alternators and how their pins or terminals work.

What is an alternator voltage regulator?

It is a circuit responsible for maintaining the voltage level generated by the alternator within the specified limits that are between 13V and 14.5V. This is done by exerting / turning off the rotor field. In addition to this, it performs the diagnostic function of the system in such a way as to inform the user about the bad operation of the system by turning on a lamp.

Terminals of an alternator voltage regulator

Here I bring you the case of Toyota.

regulador de voltaje toyota

Explaining the regulator pins:

E: Earth (negative)

Gives the negative power of the regulator circuit. Important to note that it is below ground input and has no communication with the heat sink.

P: Induction voltage also known as S-terminal "stator". (positive input signal)

This is the signal that comes from the alternator stator, once it starts generating the alternator, this induced voltage tells the regulator that the generator is already working and that it must start the monitoring process in addition to turning off the BAT light on the board.

F: Controlled field current output. (Negative output signal)

Controls the on and off of the rotor field. This exit is protected by C.C. and has an output modulator in ramp so that there are no voltage spikes induced product of connecting and disconnecting the field current.

B: Battery voltage. (Positive input signal)

Through this terminal the regulator measures the voltage of the system to exert control over the turning on and off of the rotor field.

L: Fault indicator lamp (Board battery light). (Negative output signal)

Indicates that there is a problem with the regulation system or if the alternator is not generating power.

IG: Power on the system. (Positive input signal)

Turn on the regulator and all its associated components. It is important to add that the (L) LAMP signal is always active so there is no IGN voltage, but in this case, it will be a poor ignition.

¿Cómo funciona?

Actually this module is not a regulator as such, that is, it does not work like a normal voltage regulator which keeps the output voltage stable under any regime of load consumption, rather it is like a brake or a generated voltage limiter.

To test the voltage regulator in this case of Toyota, just mount the following circuit:

diagrama para probar regulador de voltaje

By energizing the circuit, with the Ign suiches. And Generates. off we will have a dim light in the BAT lamp.


When we set ON to the suiche Ign. It will strongly light the BAT light and will light the Coal lamp very little. Turn it on a little because it is your put that we are going to start the engine and we need that all the power of the battery goes to the starter motor.

When starting the motor, with the weak field current the stator starts generating energy by polarizing the terminal P of the regulator (or S of stator for other marks), in this way the regulator turns off the battery lamp and puts at full current the rotor field. The Charcoal lamp will light intensely.

This remains stable only until the voltage measured by the B + terminal exceeds 14.5V, at which point the regulator opens the F terminal, thereby turning off the field. (Carbon lamp goes off).

When the voltage measured by the B + terminal drops below 14.5V, terminal F closes, connecting the alternator field.

Porque el Regulador de voltaje, no es un regulador de voltaje común.

As I commented a few paragraphs ago, this device does not work as a common regulator since the voltage is not stable in all the consumption regimes of the system, but it floats between 13V and 14.5V, even for discharged batteries it can arrive without problems up to 12.3V.

If we want an operational test to check the operation, we will mount the following one. (This is the one that I mounted in my YouTube video)

diagrama para probar regulador de voltaje operativo

In this case, to be able to notice if there is regulation of field current, a stable source is placed on the previous circuit that does not depend on the regulated voltage. This so that the luminosity is not affected when we lower or raise the tension in B +.


Below the disarming video of an alternator and the regulator test with this circuit:

Video of how I did it

Voltage regulator problems

These are quite reliable elements today, more than all because the protection against short-circuit currents and even the temperature for the most modern.

The problems are converted beyond the dirt of the heat sink that deteriorates the internal power transistor.

A regulator in semi-closed mode causes regulation problems (genera more than 14.5V) and flashing lights.

A regulator in open mode the alternator will not generate power.

It is all at the moment,

Do you have questions or doubts?

Questions or suggestions can use the form below and I will gladly answer them.


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Written by

Dennis García
Chica Mecánica.



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      Alternator voltage regulator

      Alternator voltage regulator

      The regulator voltage of the alternator is a circuit responsible for maintaining the voltage level generated by the alternator within the specified limits that are between 13V and 14.5V.

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