DC Generators and solid state controller therefore


I. Old DC-dynamos and how they work
Most common DC dynamos for use on motorcycles are either of

a) the three-brush type, or of
b) the electromechanically controlled type.
A common type of such dynamos, which from the outside all look essentially the same, is shown below in Fig 1



a) the three-brush type
The three-brush type dynamo, normally has an electromechanical cut-out switch, but lacks any separate external voltage regulation means. Voltage regulation is inherent to its construction and in fact depends on the amount of distortion of the magnetic field under different electric loads (see Figure 2). However, this manner of regulation is limited in its ability to keep the voltage and current in line with the actual requirements.





Examples of electrical circuits of different two-pole and four-pole dynamos of this type are shown below in Figure 3:

In these circuits Le is the field winding, Sg a fuse and Mg the cut-out solenoid switch. The brush arrangement shown in Figure 4 (two-pole arrangement) is suitable for easy change of direction of rotation of the dynamo.


b) the electromechanically controlled type
The electromechanically controlled type, or vibrating regulator type, permits the dynamo to maintain the same or constant voltage throughout the higher speeds and, if also equipped with current regulating contacts, limits voltage and current to a maximum allowable value. Such a regulator produces nearly perfect battery charging characteristics. However, because of the vibrating contacts these regulators are prone to wear and can be repaired only by a specialist having sufficient knowledge of the specific type of regulator. Furthermore, often the dynamo found at an autojumble is without its regulator or the wrong type is mounted and replacement proves very difficult to find nowadays even if one happens to know what type of regulator had been used originally.

An example of one of the great variety of such regulators and its electrical circuit is shown in Figure 5 



III. Additional considerations when restoring a DC-Dynamo for a motorbike
If the originally mounted generator is of the electromechanically controlled type and works well or is complete and still restorable, there is normally no need for a conversion of the regulator, especially the motorcycle's originality can thus be preserved.

However, for those who have no choice because essential parts are missing or are beyond repair or, if it is thought necessary to improve the reliability of the power output (in particular of a three-brush type of generator), it is suggested that use should be made of the solid state regulator presented in this article.

In some European countries electric lighting is mandatory even for very old motorcycles if they are used on the public roads and in such a case conversion to electric lighting and an electric power generator becomes unavoidable. The oldest generators, sold separately at the time as an accessory and therefore preferred for such a conversion, are nearly always of the 6-Volts three-brush type. They look nice, are small and installation is normally not too difficult, but the electric output is very low. However, when using the regulator presented in this article such a generator can be adapted to become a 12 Volts generator thereby effectively doubling its power output! It is also easy to change the direction of rotation of the adapted generator by just interchanging the connections of the filed windings.

Because the field windings are controlled directly by the regulator the third brush, the position of which with respect to the positive brush (see figures 3 and 4) normally determines the sense of rotation, is not needed anymore.


III. The "home made" regulator
The regulator circuit shown on the last page of this article, was developed from a state of the art regulator for modern alternators and gives a solution to most of the above stated problems. It can be used in combination with almost any D.C. dynamo and can easily be adjusted to particular needs without tampering with the dynamo itself. Total costs are only about 15 DM in Germany. The proposed regulator comprises a diode D2 which replaces the electro mechanical cut-out switch. The regulator can be build small enough to be inserted in place of the original cut-out switch or electromechanical regulator. When used in combination with a three-brush dynamo the dynamo can be controlled in the same manner as a normally regulated dynamo by controlling the field voltage directly. In that case the third brush should be dispensed with altogether and such conversion also has the advantage that power losses are to a great extent reduced.

IV. Functioning of the regulator
The circuit acts as a switch, supplying either full or no voltage to the field winding of the dynamo. When the dynamo voltage is below 13 volts, Zener diode Dz does not conduct, T1 is off and T2 is on, thus the dynamo voltage is fully applied to the field winding so it supplies increasing voltage to the regulator circuit and, through resistor R5 and diode D2, to the battery. When the circuit voltage exceeds 13 volts, Dz starts to conduct and the voltage across resistor P1 rises to a point, depending on the setting of P1, where T1 starts to conduct and T2 stops conducting, thereby switching off the current to the field winding with a subsequent decrease of the dynamo output voltage

During regulation these sequences follow in a fast mode such that the dynamo voltage is kept regulated at the desired voltage set by P1.

The components R4, R 5 and T3 form a current limiting circuit. The value of R5 is selected to limit the maximum output current in order to avoid overload of the dynamo. The value of resistor R5 is chosen such that, at the maximum allowable current load of the dynamo, the voltage across this resistor is so that T3 starts conducting whereby the voltage across P1 increases, even without diode Dz conducting, and the output voltage of the dynamo is consequently decreased.


V. How to convert a 6-Volts dynamo to 12 Volts
The maximum allowable power output of a d.c. dynamo is mainly dependent upon the maximum allowable temperature of the windings, which is essentially dependent upon the current (I) in accordance with the function Q=I².R (Q is the amount of heat generation in Watts and R is the resistance of the field coil).

Consequently, for a fixed maximum current rating and within the rated speed limit, the power output (P) of a dynamo is essentially directly proportional to the output voltage (V) in accordance with the function P=V.I.
Taking into account these principles and the fact that a 6-Volts dynamo easily gives 40 Volts when it is not regulated, the regulator described can also be used to convert a 6-volt dynamo to give 12 volts, and thus double its power output, without further modifications to the dynamo itself !

It is however advised to insert in series with the field coil a resistor R2 of the same resistance as the field coil resistance to limit the field current to the 6 volts value and so limit the load of transistor T2 and reduce the risk of overheating of the field windings at low dynamo speeds when, as follows from the above explanation, full voltage (thus up to 13 Volts) is supplied to the field windings.
With some dynamos difficulties have been encountered when used with a diode D2 instead of a cut-out. An electromagnetic cut-out allows the tension of the dynamo to rise before the connection to the battery is made and therefore it is easier for the dynamo to "start-up" in particular when the remaining magnetism in the field pieces is small. If  you have difficulties with a "black-box" for converting  6 Volts dynamo to 12 Volts or power is unsufficient, try out whether an electromagnetical cut-out switch works better and also check if the field windings get enough power. Normally a bad adaptation of the black-box controller to the dynamo concerned is the cause of the trouble.

If required, 6 volts regulation may be maintained by using a 6 volts Zener diode instead of a 13 volts Zener diode and by adjusting the trimpot P1 to give battery loading voltage of 7 volts.

It is always possible to maintain the original cut-out relay, which should then be installed instead of the diode D2.


VI. Details of the circuit and its adjustment
Diode D2 is a 50 volts, 6 amps, or dependent upon the dynamo output (for old dynamos generally in the range of 30-60 Watts), even higher rated type with low internal resistance to limit the voltage drop over the diode at the maximum current rating.

If maximum current should be limited to 3 amps (up to 5 amps), the resistor R5 should be chosen to be about 0.15 ohms (down to 0.08 ohms). If not available such a resistor can be made from a high load wire-wound resistor, cutting off a wire length in proportion to the known resistance of the wire wound resistor to obtain 0.15 ohms and wrapping this wire on a resistor of say, at least 100 ohms, as a support for the wire and for easy mounting in the circuit.

The value of resistor R2 is selected to be equal or slightly lower than the field coil resistance (to be measured with an Ohm-meter) for the reasons mentioned in point IV above.

Temperature compensation can be obtained by mounting one or two diodes in series with the zener diode Dz. Instead of the Transistor T2 also a darlington pair can be used, such as BD 645. However, in practise these further adaptations proved unnecessary and are mentioned only as alternatives for those who like to experiment.

Adjustment of the voltage should be carried out with a fully charged battery of the type to be used on the motorbike. The maximum voltage, normally between 14 and 14.5 Volts, should be adjusted so that there is still a small charging current, the intensity of which depends on the size of the battery (usually between 0.1 Amps for small and 0.5 Amps for bigger batteries).

Use a high quality trimpot (cermet or equivalent) for P1 and, after setting it to the required battery voltage, seal the setting screw with a blob of paint.

Adjusted in this manner the risk of overloading the battery is effectively prevented and it is ensured that the maximum capacity of the battery is always available.

T2 should be mounted on a small heat sink.

A great advantage of the regulator is that, in contrast to the three-brush type of regulation, its functioning is independent of the battery and can thus also be used in case the battery is disconnected without a risk of blowing the bulbs.


12 Volts regulator circuit



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