Up to the 1950's most bikes used a generator with direct lighting and a bulb
horn. The reason being that batteries were cumbersome made from either a glass
jar or vulcanised rubber with liquid acid splashing around. This resulted in dimming lights when the revs
dropped and no lights when the engine stops, and so the British manufacturers turned
to the alternator, which generates an alternating current
induced in the stator coils. But AC is no good to the motorcycle which runs on
Direct Current (DC) and battery.
To convert AC to DC a Bridge rectifier is used, which consists of four diodes, each of which acts as a one-way
street for electricity. The positive part of the AC current is fed to earth, whereas
the negative part is fed to the electrical circuit, and in this way the alternating
current is rectified to a direct current by the diodes.
next problem is what to do with the excess DC current that is generated. The
generator produces more than enough to run the engine and the lights as well as
charge the battery. As the battery is charged another diode, which is called the Zenner diode,
progressively leaks the excess
current to ground, converting it to heat in the process. Without this arrangement
the battery would boil.
When it doesn't work
Lack of current is usually due to either a bad stator coil or a blown diode. The stator coil can be
tested by connecting the two stator wires to a 12V head light bulb
and starting the engine at idle speed only. If the bulb lights then the stator is OK and the problem is
probably in the diode.
A replacement rectifier from a motorcycle shop is not a pleasant experience,
which will generally set you back by £30 or more. Alternatively you can get a
cheaper version from RS components. Diodes are rated by two parameters the maximum current and
the peak backwards voltage. A typical motorcycle alternator puts out about 8-10
have a full wave bridge
rectifier, part number AR84F,
this is a std KBPC2501 25A Single Phase Bridge Rectifier, 4 terminal type rated at 70v RMS and
25a. This is a large square metal type with a bolt through the centre
and no heatsink is required. Cost £1.29
RS have a full wave bridge
rectifier, part number 227-8621, a 25A Single Phase Bridge Rectifier 100V 26MB10A, which is rated at 25 amps and it will withstand up to
volts RMS. Cost £4.57
Both diodes have much more capacity than any bike will put out as the peak voltage is about double the
RMS value. They show a good saving on price and the components will be more efficient and reliable the original item. The
4 spade lugs on them fit directly onto the BSA wiring as detailed below.
Wiring in the Rectifier
The diode that was on the bike probably had only 3 connections but the
4th connection was the mounting bolt onto the frame. Connect the 2 stator wires to each of the AC
terminals on the rectifier. That will leave a "positive" and a "negative"
terminal. The BSA is a positive earth, so connect the "negative"
terminal to the battery wire NW (Brown-White) and take a wire (Red) to earth on
the frame. Now mount the rectifier onto the frame with a suitable diameter bolt through
the hole in the middle. The frame will dissipate the heat,
or you can sandwich a piece of 3mm aluminium between the two, which will help in
heat flow. Do not drill out the hole to a larger size. The unit is electrically isolated so don't worry about the metal
backing touching the frame.
Check out the RS Components
& Maplin web-pages or
(Note: Use Radio Shack in the USA)