PA-28-140 Electrical system
Posted by hogwartsdropout__@reddit | flying | View on Reddit | 8 comments
Hi everyone, hoping someone might be knowledgeable on the electrical system for the Piper Cherokee 140. I’m trying to figure out how does the Battery not power the whole system. Is it because there’s an isolation diode? Is it because the alternator has a greater voltage than the battery? Or is it a different reason? Thanks for your help
xplanephil@reddit
The ELI5 is that the alternator pushes the electricity out at a higher pressure (voltage) than the battery.
Think of the consumers in the electrical system as your city water. Now imagine the city has two water towers, a short one and a tall one. If you open the valve on the short water tower, it will drain and supply the city with water. Now imagine opening the valve on the tall water tower with the higher water pressure. The higher pressure will force water into the smaller tower, refilling it, and also push into the city's water system, leading to more pressure on all taps in the city.
That's essentially what happens when you connect the alternator, which supplies 14V, to the battery, which supplies 12V when fully charged.
axnjackson11@reddit
great visualization
PM_ME_GOODDOGS@reddit
Speaking of. The one I fly has a fuse for "electric trim" that pops out sometimes, but there is no electric trim. I assume it's just mislabeled?
ComprehensiveEar7218@reddit
Are you asking why the battery does not power the electrical buses while the alternator is running?
hogwartsdropout__@reddit (OP)
Yes
ComprehensiveEar7218@reddit
When the alternator switch is on and the engine is running, the alternator will have a higher voltage than the battery and will run the electrical systems. The alternator voltage being higher than the battery will also stop the discharge of the battery and charge the battery.
bhalter80@reddit
I think of it as a spigot and a bucket. The alternator is the spigot dumping electrons into the battery which flow into the main electrical system. Now if the electrons flow out of the battery slower than the alternator dumps them in the battery charges and the alternator slows down the flow to keep up with the demand once it's full. If the electrons flow out of the battery faster than the alternator dumps them in then the battery discharges.
rFlyingTower@reddit
This is a copy of the original post body for posterity:
Hi everyone, hoping someone might be knowledgeable on the electrical system for the Piper Cherokee 140. I’m trying to figure out how does the Battery not power the whole system. Is it because there’s an isolation diode? Is it because the alternator has a greater voltage than the battery? Or is it a different reason? Thanks for your help
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