Replace That Bad Diode

A common problem in any restoration is fixing and replacing inoperative or missing electrical components. If the exact part cannot be located then the restorer is forced to improvise. Most amateur mechanics do not have too much trouble figuring out switches and lamps but usually are stopped cold when they encounter a diode rectifier lurking under the seat. In this article I will tell you what a rectifier is and how a simple replacement from Radio Shack can be used to not only replace a bad unit but to actually improve it.

From the mid-fifties on, most of the british bikes abandoned the old generator in favor of an alternator. In the typical alternator arrangement a permanent magnet on the end of the crankshaft rotates inside a fixed set of stator coils. The movement of the magnets causes an alternating voltage to be induced in the stator coils. While the alternating current supplies a lot of energy, it can not charge a battery which needs direct current. A generator with its complicated set of brushes and commutator puts out direct current but the brushes wear and have to constantly be replaced. This complexity of course equates with unreliability. The alternator with only one moving part, the magnet, is the epitome of simplicity and hence relibility.

The alternator became practical in the fifties with the advent of the solid-state rectifier, often called a diode. Think of a diode as a one-way valve for electricity. Electrons will pass one way (the direction indicted by the arrows in the figure) but not the other. The two leads from the stator coils go to a bridge rectifier which actually contains 4 diodes. During one phase of the cycle the diodes pass current from the positively charged stator lead to the positive pole of the bridge. When the current reverses the diodes direct the current from the other lead to the bridge's positive pole. Thus, the alternating current is rectified to a direct current by the diodes. Most Japanese systems used two sets of stator coils. With the lights off one set of coils whose output closely matched the ignition current were connected to the diode. Turning on the light connected a second set of stator coils which would supply additional current for the headlights. In the Lucas system shown in the figure, the stator put out enough current to run the lights all the time and soaked up the additional current with a zenner diode when the lights were off. Note that the Lucas system used a positive ground.

Assuming that the wires are intact, failure to charge the battery will invariably be either a bad stator coil or a blown diode. The stator coil can be tested by connecting the two stator wires to an old automotive head light bulb and starting the engine. Only run the engine at idle speed so you do not blow out the bulb. If the bulb lights then the stator is OK and the problem is probably in the diode. Old diodes also suffer from high back current. On many bikes the positive pole of the bridge rectifier is connected directly to the battery all of the time. A leaky diode will insidiously draw current from the battery and after a week or so without riding the leaky diode will discharge the battery.

Lets assume that you have determined that your patient needs a new rectifier. You will probably be shocked at the price as NOS venders typically charge between $20 and $40 for one and that is assuming you can even find one. Not to worry! Diodes are rated in only two critical parameters the maximum current and the peak backwards voltage. A typical motorcycle alternator puts out about 8-10 amps. Radio Shack offers a full wave bridge rectifier* (part number 276-1185) that is rated at 25 amps and it will withstand up to 50 volts, much more than any bike will put out. The price? A paltry $2.60 at the time of this writing. Furthermore, the RS diode is state of the art and will be far more efficient and reliable the original item. The RS diode has 4 spade lugs on it so it will plug right into your triumph or BSA. On a Honda Dream I simply replaced the screw lugs on the wires with female spade ends.

The diode that was on the bike probably had only 3 connections but the RS diode has 4. Where does the extra wire go? The bike's original unit had a 4th connection but you didn't notice it because it was on the mounting lug, the ground. As you look at the diode, 2 of the wires are marked AC. Connect one of the 2 stator wires to each of the AC posts. That will leave a "plus" and a "minus" lug. If the bike is a positive ground like a triumph, connect the "minus" lug to the battery wire (it originally went to the center lug of the Lucas diode). Make a jumper lead and connect the "plus" lug to the motorcycle frame for a ground. If the bike is negative ground like a Honda Dream, Hawk, CB 160 or 90), then reverse the process. Put the positive battery wire on the "plus" lug and ground the "minus" lead. William Silver sent me the diagram below that covers most older Hondas and other Japanese bikes will be similar.

The diode gets warm when it is operating so it needs to be mounted to something that will dissipate the heat. If you just wrap the thing with tape and stuff it under the seat you may generate some smoke. The RS diode has a hole through it and usually it can be mounted right where the original diode was. The unit is electrically isolated so don't worry about the metal backing touching the frame. Unfortunately, the mounting hole is only about 3/16" so a 1/4" or a 6 mm screw will not go through it. Use a smaller screw and do not try to drill the hole in the diode bigger.

* Radio Shack has recently reduced the number of components that it sells in many of its stores. If you have trouble finding it you can order it on line from www.radioshack.com/ click on components/semiconductors/diodes.