This time – about a retro theme. It also has some interesting nuances.

In the picture: connection scheme of a “regular” alternator.


The scheme is simplified, a voltage regulator (called a “tablet”) is replaced with a rectangle and marked with 4TR.

Scheme of the voltage regulator is nothing complicated, but in this case – it has no principal meaning.

We can not replace or repair it – it makes to sense to bother.

In the image: 6 diodes – three-phase bridge, which turns alternating current to the direct current.


The alternator is connected to the car’s electrical system with 3 connections:

a) negative output of the bridge, in the scheme marked with (-);

b) positive output of the bridge, in the scheme marked with (+);

c) excitation and control output, in the scheme connected to “indicator”.


Regarding nuance – there are some tricks, which have to be taken into account, when connecting third – excitation and control output “wire”.


Attention: excitation connection and excitation winding are two completely different components! Excitation connection: “third” output of the alternator. Excitation winding: “coil” (Field coil) of copper wires, winded on the rotor.


1. Via this “third” alternator’s output, the voltage regulator receives an (initial) power supply, which is necessary for the alternator (its voltage regulator) to work at all. More precise – to start to work (because, as soon as the alternator has started to work, the voltage regulator is supplied by electricity via Trio diode block – additional power supply is not necessary). Accordingly – if this excitation output (important: connected in series with a control lamp or resistor with nominal of several tenths Ohm) is not connected to the power supply, by low/average RPM the alternator will not start to charge! You can increase RPM of the engine to 1200 ..1500 .. 2000 RPM, but the alternator will be “dead”!

2. If the excitation output (via a shunt, as mentioned before) is not connected to the power source, the voltage regulator has no initial voltage, to start to work. Accordingly, the current doesn’t flow via the Field coil, the alternator doesn’t produce electricity. But, there is a trick. Even if the current doesn’t flow thru Field coil, the rotor (its metal core) is slightly magnetized (as called persistent magnetism – physical peculiarity) and, significantly increasing RPM of the engine, alternator manufactures a small amount of current. This current is hundred times weaker than in a regular mode, but, usually reaching the redline RPM of the engine, is enough for diode Trio group to “feed” the voltage regulator, and it would be able to increase the current in Field coil. As soon the current in field coil increases at least a little, the avalanche-type process is initiated – the current, produced by the alternator, increases rapidly; the current, which via Trio can be directed to the voltage regulator, increases; the current in Field coil increases, etc. In a very short time, the alternator “turns on” and is able to supply itself till next time of starting. From this moment (when the alternator has “turned on”), the excitation connection is not necessary – it performs only indication function (regarding charging problems).

3. Excitation connection cannot be directly connected to +12 V, the bulb or resistor have to be included in series! If the excitation connection is connected directly to 12 V, the voltage regulator will get damaged!

4. Very often people misunderstand the “turning on” (if the excitation coil is not connected) mentioned above, with the ability of the alternator to produce necessary current. Let’s assume other situation – excitation coil via control bulb is correctly connected to +12 V, the alternator is in working order. In idle these “old” alternators  produce relatively small amount of current (several tenths of A), very often the current, generated by them, is not sufficient, to maintain the onboard voltage in the level, inquired by the alternator (14.0/14.4 V). Accordingly – part (of missing) current is taken from the battery. Increasing the RPM of the engine, onboard voltage increases to mentioned before – all current (which is consumed by consumers) is supplied by the alternator. But in this situation there is a fundamental difference from the situation, mentioned in point 1/2 – the alternator actually works also in idle, simply – its power is insufficient to ensure the car with 100% of necessary energy. So the “behavior” of the alternator is different – onboard voltage increases already by 1200 .. 1700 RPM, not as in point 1/2, when it’s necessary to turn (for one time) the engine close to the redline.

5. If the brushes are worn out, the Fiel coil is thorned, or the voltage regulator is damaged in a way, that the current doesn’t flow thru the Field coil, the alternator will not produce the current in any conditions. It will stop to produce the current also when the car will be driving, as soon as the current in the Field coil will dissapear. The current in the Field coil determines the torque of the alternator. No current in the excitation (Field) coil – no electricity.

6. If in series with the Field coil the control bulb is connected, it will signal regarding the situation when the alternator is not charging, or, more exactly – the voltage, generated by it, is significantly lower than car’s onboard voltage. As the alternator has only one regulator (there is no possibility of double control), charging lamp will in no way signal regarding, for example, increased voltage (regulator – damaged: its output transistor – short-circuited). The lamp will not signal also about the situation when any of the 6 main diodes will be damaged. But the lamp will light up in the situation, when, for example, the brushes will “die”.

7. Typical symptom – voltage “pulsates” with a frequency – one .. several “waves” in a second (by relatively small RPM/idle). In this case – usually, one of 6 main diodes is damaged (unconnected). This defect can be found using an oscilloscope, when checking the shape of the voltage (current). Measuring the voltage via multimeter, you will have to conclude the alternator is able to maintain necessary average voltage – the defect will not be identified!