No diodes in the dynamo or its control box. One side of the the warning
light is connected to the output terminal of the dynamo and the other to the
ignition. When the dynamo generates sufficient voltage (just above battery
voltage) the cut-out relay operates and connects the dynamo output to the
battery. This means that both sides of the warning light carry much the
same voltage on both sides and is extinguished. I say 'much the same'
because in reality there are control-box relay windings in circuit which
necessarily have a resistance. The greater the current drawn from the
dynamo the greater the voltage dropped across these windings, the greater
the voltage differential between the dynamo output terminal and the battery
and hence across the warning light. Therefore at high currents the warning
light can have sufficient voltage across it to glow.
In an alternator the warning light is connected to the field winding and fed
via a different set of diodes to that which feed the output. With good
diodes the field voltage and waveform is always the same as at the output so
once the alternator has 'kicked in' there is never any voltage across the
warning light. When the current demand on an alt exceeds what it can supply
both the output voltage and the field voltage start to fall, but at the same
rate so the voltage across the warning light remains at zero. When one or
more diodes fails, depending on which ones and whether they are open or
short, the voltage and waveform at the field (warning light) and battery
(ignition) become different hence there will be voltage across the warning
light. As currents increase this disparity increases and hence the lamp
glows more.
Both types of warning light use the same bulb and hence both have the same
voltage and current requirements - pass a current through a bulb and you
will get a voltage dropped across it, apply a voltage to a bulb and you will
pass a current through it - Ohm's Law
Diodes and regulators are separate components in the alt - the former are
high-current items typically mounted on a multi-finned heatsink (in MGB-era
alts, dunno about modern ones). Any defect in either diodes or regulator
*always* causes problems. It could be low or high output voltage, excessive
heat which can damage windings and brushes, limited output current etc.
Both dynamo and alternator systems pass current through the warning light in
both directions at different times and so cause it to light. When you first
turn on the ignition there is 12v connected to the ignition side and a
virtual ground connected to the dynamo/alt side so it lights. When the
engine is running and charging there is circa 14v connected to both sides of
the lamp so it goes out. When you switch off a running engine the dynamo
and alt are still generating voltage but there is a virtual ground on the
ignition (from the coil and all the other ignition circuits) so the lamp
lights again while the engine is spinning down. It is this last mode that
causes the 'normal running-on' of later North American cars when there is a
problem in the emissions plumbing or anti-runon valve - because the only
other component on the ignition wire is the ignition relay (apart from the
SU pump which is effectively disconnected by its points 99% of the time) the
warning light gets its ground from the relay, and the current flowing
through the lamp is enough to keep the relay operated hence powering the
ignition. On these cars it is only the anti-runon system sucking the fuel
out of the jets which stops the engine.
PaulH.
> The dynamo used a mechanical (electrical?) system to operate the bulb. If
the
> current flowed from the battery it would light, if it flowed to the
battery it
> wouldn't. A simple diode did the trick.
>
> The alternator uses a tap off the regulator. As long as the voltage (not
> amperage) from the alternator is at or above the trigger point, it will
not
> trigger the light. The dim glow you see is because the voltage is near
the
> trigger point, most likely because one of the diodes is bad. My light has
> been dimly glowing for over a year, but with no ill effects. I will be
> replacing the regulator one of these days.
>
> To be concise. The dynamo light is triggered by amperage, the alternator
by
> voltage.
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