The original Splitdorf generator design required removing the cap and adjusting the position of the third brush in order to change the charging rate. By moving the third brush in relation to the + or – brush (depending on rotation), the voltage applied to the field windings was changed. This in turn changed the strength of the magnetic field applied to the rotating armature and thus limited the maximum charging rate, or current output, of the generator. The generator would continue to charge at this rate all the time, regardless if the battery were boiling over or dead, lights on or off, it didn’t matter (see the DU-5 Three Brush curve in the graph at the end of this section). Many batteries and fine paint jobs have suffered from this poor regulating system!

The Splitdorf Solid State Voltage Regulator requires removal of the third brush and the often troublesome cutout relay. This will allow the solid state voltage regulator to accurately control the voltage to the + side of the field windings, which will in turn control the current output of the generator. Based on the battery voltage, the generator can now charge only as much as needed. When the lights go on, generator output increases in an effort to maintain the desired battery voltage. When the lights go off, the charge rate tapers down as the desired battery voltage is obtained, then a small trickle charge is applied to maintain the battery. If the battery is low, a high charge rate will be seen until the desired battery voltage or state of charge is obtained, at which point the charge rate tapers down to a trickle charge. Depending on the condition and size of the battery, this may take some time, as the maximum output of the Splitdorf generator is rather modest. See the DU-5 With Regulator curve in the graph below.