This photo shows both built in efficiencies and inefficiencies. The pair of diodes are
used to rectify a small engines alternator, bypassing the engines built in regulator, and allowing higher output voltages.
A full wave rectifier or bridge rectifier (4 diodes) will further increase the output This is useful when adapting
a 24v alternator where a field voltage of 12v is inadequate. The Capacitor further increases the alternators output
by filtering the ripples to the field.
This diagram shows a slightly different way of exciting the field. A battery can be used to excite
the field for a few hours. The lamp is a 12v headlamp (if needed) wired in series to reduce the fields input.
The higher the wattage (usually stamped on the bulb) the more current is allowed to pass to the field, more
ouptut is being developed, and more load is applied to the engine. The lower the wattage the less current is allowed
to pass, less alternator output is developed, and the less an engine is loaded.
These principals are important to matching the alternators output to the engines highest power output at around
3600 rpm for most small engines. Too much excitement and the engine will be loaded excessively. Too little and
the welder will not develop its full output at all RPM's.
The output of the welding machine is regulated only by rpm's once it is all matched up. The faster the engine turns
the more amps it is capable of delivering,