This is just a little information on current rectifiers in general.  Thought it would be appropriate to explain a little about the types of rectifiers in use, since the Selenium Rectifier is used on the Honda Benly 150.

A rectifier is a device which transforms alternating current into direct current by limiting or regulating the direction of current flow. The principal types of rectifiers are dry disk, solid state, and vacuum tube rectifiers. Solid state, or semiconductor, rectifiers have effectively replaced all other types.  This is necessary since the generator ("Dynamo" in the manual) generates alternating current (AC).  Since the ignition circuit and battery are direct current (DC) devices, it becomes necessary to convert the AC to DC through the use of Rectifiers.  Below is a discussion of how a Selenium Rectifier, or Dry Disk Rectifier, works.

Dry Disk Rectifiers

Dry disk rectifiers operate on the principle that electric current flows through a junction of two dissimilar conducting materials more readily in one direction than it does in the opposite direction. This is true because the resistance to current flow in one direction is low, while in the other direction it is high. Depending on the materials used, several amperes may flow in the direction of low resistance but only a few milliamperes in the direction of high resistance.

Three types of dry disk rectifiers may be found in various applications: the copper oxide rectifier, the selenium rectifier, and the magnesium copper-sulfide rectifier. The copper oxide rectifier (figure 8-234) consists of a copper disk upon which a layer of copper oxide has been formed by heating. It may also consist of a chemical copper oxide preparation spread evenly over the copper surface. Metal plates, usually lead plates, are pressed against the two opposite faces of the disk to form a good contact. Current flow is from the copper to the copper oxide.

The selenium rectifier consists of an iron disk, similar to a washer, with one side coated with selenium. Its operation is similar to that of the copper oxide rectifier. Current flows from the selenium to the iron. This is the type of rectifier used on my Honda Benly 150 motorcycle.

The magnesium copper sulfide rectifier is made of washer shaped magnesium disks coated with a layer of copper sulfide. The disks are arranged similarly to the other types. Current flows from the magnesium to the copper sulfide.

Solid State Rectifiers

Diode Bridge Rectifier Circuit

To illustrate how a bridge rectifier performs, consider a sine wave input which is on its positive alternation as denoted on the schematic of figure 8-243. With the secondary of T1 functioning as the bridge rectifier's power supply, point A is the most positive point of the bridge, while B is the most negative. Current flow will be from B to A through the forward biased diodes. As an aid in finding the path of electron flow, consider the redrawn bridge circuit in figure 8-244. The forward biased diodes, CR3 and CR4 are easily recognized. Voltage is dropped across each voltage loop as indicated. Thus, on the positive half cycle input CR3 and CR4 are both forward biased and CR1 and CR2 are reverse biased.

As long as diode breakdown voltage is not exceeded, current flow will be from point B up and across CR4 down and to the left across RL. After current crosses RL, it will flow to point A through CR3. Notice that current flow across RL is from right to left, or in respect to polarity, a negative half cycle output for positive half cycle input.

Remember that, when tracing current flow for the negative half cycle, electron flow through a diode is against the symbolic arrow and from negative to a less negative or positive point. Therefore, no confusion should arise when tracing electron flow up to and away from the common point between CR3 and CR1. Although it may appear that CR1 as well as CR4 is forward biased, such is not the case. The collector of CR1 is more negative than its emitter; therefore, it is reverse biased.

Since, on the negative half cycle, CR1 and CR2 are forward biased, the output signal on the negative half cycle is negative.