BLEVEN’S TECH TIPS

Tech Tips March 2004

An Electrical System and its Circuits

Not all airplanes have electrical systems. Those that do have them, divide the system into circuits. Each of these circuits has components connected in series with the device which consumes or uses the energy as the final component. The circuits in airplanes and autos are somewhat similar. When breaking down and electrical system into circuits, a simple understanding of the entire system is revealed.

Where does this whole subject of electrical systems in airplanes best begin? It most logically begins with the first circuit, which is the storage circuit. This circuit contains the battery. One of the battery's terminals is connected to a common ground. All of the circuits in the system will lead eventually to this ground. The other battery terminal is connected to a solenoid switch and also directly to the starter solenoid switch. Sol switches are magnetically controlled high amperage switches, controlled by activating the magnet with a small amount of power. The ground for the magnet is connected directly to the power switch and then to the common ground. The other large terminal of the solenoid switch is connected to one side of all of the fuses or breakers, which are to be powered by the battery. Each of these fuses or breakers is the first component in a secondary circuit. Once the power switch is activated, the solenoid is also activated and all of the secondary circuits are turned on.

Each of these secondary circuits is developed from its respective fuse or breaker, and they all follow the same sequence. As an example, the landing light circuit will go as follows: from the fuse or breaker to the switch, from the switch to the light, from the light to the common ground.

Now about the generator/alternator? What does it do? It is the “pump” that fills the storage battery. The generator/alternator is powered by the battery. Therefore it also will require a fuse or breaker and a switch. The power that comes out of the generator/alternator is connected directly to the same source that feeds all of the fuses/breakers. If the power switch is turned off while the engine is running, and if the generator/alternator is producing current output, it will no longer have the battery as a cushion, and it will longer have the voltage regulator to control the voltage output. If this set of circumstances happens, you need to protect your radios and lights from a possible over voltage condition (pilot induced by the way). The surest way to avoid this and to protect the radio and lights from possible damages is to leave the master switch on until the prop stops turning END OF DISCUSSION!. Some people insists on turning only the radio master off. THIS IS WRONG! Landing lights and strobe lights can be very costly. Wouldn't turning off the generator/alternator circuit eliminate the possibility of over voltage? Sorry. The reality is that the wire feeding the charging system and the long winding of the magnetic field it is feeding, along with the commonly used noise suppressor on this line, will store enough power to cause and over voltage condition. This power has no place to go other than through any circuit that will allow it to pass through on the way to the ground. (At this point the soldiers are in double file).

A voltage gauge indicates the electrical pressure that currently exists is the system. It can be connected to almost any power source in the line to ground and it will be accurate. This circuit does not require a fuse/breaker.

A voltage regulator is used to control the voltage produced by the generator/alternator. In order to charge the battery, the voltage in the system must be greater than the voltage in the battery. This voltage, or pressure, is normally between 13.8 and 14.2 volts (in a 12 volt system), and it allows the amperage to flow into the battery without bringing the liquid in the battery to a boil, and without burning out the electrical compos along the way.

An amperage gauge is used to indicate the flow of current to or from the battery. The important point here is to be able to tell if your system is filling or draining the battery. The amp gauge should be connected between the feed line from the battery to the fuses/breakers, and it needs to be before the feed from the generator/alternator (battery side). Amp gauges that can not handle this current are generally fed information than an inline shunt mounted on this wire.

Dick, lets keep it simple, it really is.

Next month, radio circuits, still bored ? i.d.k.s about this either