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I have been away from the forum for a while but i am back to share information. I saw someone hook up a battery backwards the other day and it got me wondering why that fries stuff. I know it normally takes out at least the alternator, radio, some wires, and some fusible links but it doesn’t really make sense why. Doing some research on the internet i saw some people saying that the alternator was destroyed because the diodes got fried. This makes sense until I realize that the positive of the battery would be going to the n region of the diode and thus the diode should not allowed any current to flow. Older cars used to be positively grounded, what makes them different? What in a radio makes it so susceptible to reverse polarity destruction? Even simple things like relays are commonly fried by reverse polarity but all the control side of a relay is is a coil, swapping coil negative should have no negative consequences except the relay not working because the contacts would be repelled not attracted on the load side. My question is not what happens but rather why is happens? Any information would be much appreciated, there is not a lot of info on the internet regarding the why’s?
Josh
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I had a nephew who did that. I kind of wonder how it is even possible. The cables normally barely reach where they are suppose to connect let alone reversing them.
For the radio – those things are full of transistors and diodes and other polarity-sensitive components (like some capacitors, IC’s (or microchips). Reversing current, well it would be a long technical explanation but yeah it fries stuff.
Plus say for sake of argument that current made it to the starter. It would try to start the engine in the reverse direction which just does not work.
Another thing – the alternator would try to charge the battery in reverse polarity.
It doesn’t always destroy the items you mentioned. In fact, more often than not, it just blows the main fuse and once it’s replaced, things run fine again.
I don’t want to get into a long explanation here, but the short of it is, electricity is designed to flow in one direction in automotive systems and that’s positive to negative. Hooking things up the other way is like petting a cat backwards, they don’t tend to like it very much. I actually cover a lot about how electricity works in this article I wrote.
http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems
I hope that answers your question.
While it’s uncommon to connect a battery backwards, it’s quite common to connect jumper cables backwards, with the same results. Just ask my wife. Fortunately, the fix was just a $6 fuse.
Most semiconductors are built into p-type substrate including all the sensors, drivers, and computer in your car. This p-type substrate is grounded and that’s part of how all those devices on the chip are isolated from each other.
When you hook up the battery backwards, you are forward biasing this p-type substrate allowing current flow through tp any n-type material on these chips that can find its way out.
Some circuits have reverse diode protection to prevent damage but this cannot be easily done on any output transistors, those can fry.
The opposite would happen in a houses. They use alternating current AC; it can go both ways. Anything that uses a battery (Car radio ect) is DC it can only go one way.
[quote=”EricTheCarGuy” post=116483]It doesn’t always destroy the items you mentioned. In fact, more often than not, it just blows the main fuse and once it’s replaced, things run fine again.
I don’t want to get into a long explanation here, but the short of it is, electricity is designed to flow in one direction in automotive systems and that’s positive to negative. Hooking things up the other way is like petting a cat backwards, they don’t tend to like it very much. I actually cover a lot about how electricity works in this article I wrote.
http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems
I hope that answers your question.[/quote]
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