Semi-legal stuff first ----
This design may be distributed freely with my consent (I'd like to know
where it goes). It may not, under any conditions, be sold.
================================================== =================
As you have undoubtedly discovered, the price that Yamaha is asking
for it's OEM voltage regulator is outrageous.
The circuit described here works as a replacement at a fraction of
the cost. It also allows the voltage regulator to be relocated to a
cooler location on the bike, such as adjacent to the battery.
The circuit is based on a Texas Instruments TL594 pulse-width-modulation
control IC. The internal oscillator, in this case set at approximately
20 kHz, generates a sawtooth waveform sweeping between Vcc and ground.
That waveform is fed to some logic to create a square wave of variable
duty cycle. The duty cycle is controlled by the output of the error
amplifiers. In this circuit, the voltage sensed from the motorcycle
battery is divided down and compared against a precision 5 volt
reference. If the battery voltage drops, the output of the error
amp drops and the duty cycle decreases. Vice versa should the battery
voltage rise too far. The voltage divider is calculated to give a
divided output of 5v with a nominal battery voltage of approximately
13.8 volts. This corresponds to the battery charging at about 2/3
potential max charging rate.
The variable-duty-cycle square wave is fed through more logic and
then drives two independent output transistors. Only one is needed.
In this circuit, the output is used in an emitter-follower configuration
so that the waveform is inverted. That is to say that as the
battery voltage decreases, the waveform duty cycle is increased so that
is spends more time at the 'high' voltage.
This output drives the gate of a heavy-duty MOSFET which energizes the
alternator rotor field windings and in turn produces a charging current
from the stator windings. As the duty cycle of the MOSFET driving
waveform is increased, the average voltage across the rotor windings is
increased and therefore the average stator output current is increased.
(The catch diode between the drain on the MOSFET and the battery
voltage connection (i.e. across the stator brushes) is for protection.
Any good moderate current (the 1N4004 is rated for 1A continuous,
10A peak) rectifier diode will work here.)
*** Update: 8/23/95: Use a beefier diode that the above stated as the
catch diode. I think the 1A rating here is too light. ***
In slightly more lay terms, as the load on the electrical system
is increased (high beam turned on, grip heaters on, etc), more
energy is drained from the battery. The voltage regulator senses
the lower battery voltage caused by the drain and increases the
alternator output current to match the load and maintain battery
charge.
As I, and other FJ owners, have experienced, riding an FJ for a long
distance on a hot day can cause the battery to overcharge and boil
dry. Over time, this can become a permanent condition. What has
happened is that the IC in the stock regulator has succumed to the
heat (of both the big air-cooled engine and the air temperature) and
failed such that the driving transistor (in the OEM regulator, it's
a bipolar Darlington) is stuck on. This causes the average stator
voltage to rail high and the alternator just pumps out as much
current as possible continuously. Hence batteries overcharge and
boil dry in short order.
This replacement circuit can be constructed and wired to the
alternator using the stock IC regulator bracket. i.e. the
replacement regulator can be placed anywhere on the bike, within
reason, instead of in the stock position which is subject to
excessive heat.
When I called for a quote on a replacement voltage regulator for
my '89 FJ1200 in October '94, my local Yamaha shop told me it would be
$190. Mail order prices were barely less. The components in
the replacement circuit should be easily had for less than $10 or
$15. The TI TL594 IC is only about $1.
And OBTW, yes, it does work. ;-) It works very well indeed.
Jeff Earls, Portland, OR and by mail:
Old adress: Beaverton, OR
Old emailadress
Reply With Quote
Originally Posted by megarich
Bookmarks