Download Replace That Bad Diode

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 oneway 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.
12.7V is the terminal voltage of a fully charged 12V battery, but if that's what
you see when it's supposed to be charging it's no wonder the battery goes dead.
You can't recharge a battery with 12.7 volts unless you can wait about a hundred
years and you don't put any load on it in the meantime. :)
If you see 50VAC out of the alternator but always see 12.7V at the battery
terminals, you have an open circuit between them. Charging voltage should be at
least about 14V; the actual voltage will vary depending on RPM because the CB160
doesn't have a voltage regulator, but it will be substantially above the nominal
battery voltage (which is indeed about 12.7V, not 12.0V).
The brown and yellow wires from the alternator should go to the AC inputs of the
rectifier assembly (doesn't matter which wire goes to which input). Those
inputs should be marked with some sort of sine wave symbol. If the connections
are incorrect, the rectifier assembly will not work because it consists
internally of four diodes connected as a full-wave bridge.
Charging System
The charging system consists of the alternator (rotor/stator), rectifier, voltage
regulator and the battery. Problems with the charging system of these older bikes is
rare in bikes of 350cc and less, moderate in bikes from 500-650cc and very common
in bikes of 750cc and over. It will require a manual to do a good job in this area and if
you don't have one anyway, get one(Clymer is good). Here is a breakdown of what the
parts do:




alternator: produces an alternating current by passing a moving magnetic field
(rotor) through a nonmoving or static (stator) winding of wire. Gasoline is thus
converted from a chemical energy to a mechanical energy (within the engine)
to an electric energy (within the alternator). This energy is then sent to the
rectifier.
rectifier: converts alternating current (AC) to direct current (DC). Early
rectifiers simply took half of the energy (single wave), and sent it to the
battery. Since alternating current travels in both directions, the early rectifiers
simply grabbed the energy going in one direction and set it on. Later rectifiers
use a diode group to grab both directions (full wave) of current and combine
them into one direction and sent this on. Later rectifiers were much more
efficient but more prone to failure.
voltage regulator: governs the amount of current coming from the alternator
and limits it to about 14-15 volt. This is usually combined with the rectifier to
make one piece of hardware. If too much current is generated, the regulator
sends it to the ground or frame.
battery: stores energy and is the main place to dispense energy as needed, to
plugs and lights.
A sign of a bad charging system is when the headlight blinks or strobes at idle or
when there is a marked increase in headlight intensity when you gun the motor. All
bikes show some increase in headlight intensity with higher RPMs but if it goes from
dim to bright, it is probably the charging system.
I will tell you in advance that finding why your battery does not charge is a hard thing
to do! This is the procedure I follow to try and solve this problem.
1.
2.
3.
4.
5.
Make sure the battery is good! Use a volt/ohm meter to see if it is providing
near 12 volts. If it reads 10 volts or less, replace it. If it can hardly crank over
the engine, replace it. Motorcycle batteries are not real good providers. They
can go bad even if you take real good care of them. The small thin lead plates
within are not as vigorous as a car battery ones and can be damaged easily.
Using your manual, read on how to connect your volt/ohm meter up to your
bikes alternator. First check if there is an electrical leak in the wiring of your
stator. The insulation of the miles of coiled copper wire can deteriorate and
cause the wire to short and thus not provide the extended length of wire
needed to pick up the energy from the magnetic field. If a short is detected, it
must be replaced ($ ouch).
Start the bike and using your volt/ohm meter test the output of the alternator.
It must be within the specs of your manual. In the 750cc, the alternator
portion of the engine is low and sticks out, just waiting to be smacked when
the bike goes down. If your bike has been dumped or impact has occurred to
this portion of the engine, you may lose your charging power. The rotor is a
magnet and can lose power with an impact.
Using your manual, read on how to connect your volt/ohm meter to the
rectifier. With the bike running, check its voltage output. It should be within
specs (14-15 volts). Now here is a problem, the rectifier converts AC into DC
and this conversion requires that some of the electrical energy be converted
into heat energy. You can locate the rectifier because it has heat dissipating
fins located near it. Unfortunately this heat can cause a failure in the unit
which can not be detected except under load (blasting down the road)
conditions. So..... the rectifier may read good at idle or high RPMs in the
garage but it fails on the road! Many shops will swap rectifiers if they are
suspect and test ride to see the result. For us home mechanics this is not
possible!
I have found that the voltage regulator is rarely at fault. But you can still test it
by following the method outlined in the manual. The test consists of using two
batteries in series and a variable resistor. In effect you try to put over 14 volts
across it and see if it stops this too high voltage.
In my experiences with bad charging systems it is the rectifier. The exception is the
750 cc engines where the rotor can easily be bad.