Why does reverse polarity cause damage

[caphair]

Is it because it can exceed the reverse breakdown voltage of semiconductors?

For instance in low voltage devices say 1-5v is the reverse voltage rated a lot lower than that for damage to occur?

Also I watched a video where a car battery was accidentally connected backwards and the main 80 amp fuse blew yet no other damage occurred. Makes me wonder what failed that drew so much current?

[stj]

Re: Why does reverse polarity cause damage

Also I watched a video where a car battery was accidentally connected backwards and the main 80 amp fuse blew yet no other damage occurred. Makes me wonder what failed that drew so much current?

nothing failed, the current flowed across the diodes in the alternator.

[eccerr0r]

Re: Why does reverse polarity cause damage

Usually it is due to some parasitic (or protective) diode that is normally reverse biased but when you connect the battery backwards, they become forward biased and conduct massive current, causing thermal breakdown.

Indeed the alternator rectifier is one example. If you want an easier example take a look at a 4-diode bridge rectifier: compare it when you have the battery connected to charge it. Then compare what happens if you connect it wrong...

[caphair]

Re: Why does reverse polarity cause damage

If the current flowed through the diodes that caused the fuse to blow, why doesn't that happen with correct polarity? Or are they orientated in one direction normally?

[stj]

Re: Why does reverse polarity cause damage

take a look at the direction of the diodes in a bridge rectifier.
if you put power across the + and - outputs with the polarity reversed the current will flow through the 2 series diodes.

[R_J]

Re: Why does reverse polarity cause damage

We had a customer with some dc equipment they would hook up via clips to their car batteries to operate, someone always connected them backwards at least once. we installed bridge recitfiers with the + and - going to the equipment and the wires going to the clips went to the ~ ac in on the bridge. that way it did'nt matter which way the clips were connected to the battery the equipment always got the correct polarity.

[tom66]

Re: Why does reverse polarity cause damage

There are a few reasons. The main reason is in modern CMOS devices, there is a body diode from Vdd to GND, which is reversed biased normally. (The diode is actually formed by millions of parallel transistor body diodes.) Reversing power and ground causes current to flow through these diodes, causing a high amount of power dissipation and eventually destruction of the device.

Additionally, you have electrolytic capacitors and conventional semiconductors that may not play well with reverse biased sources.

[Longbow]

Re: Why does reverse polarity cause damage

It should be noted that installing a rectifier bridge on the output of a battery introduces a significant voltage drop to the downstream equipment. The diodes may also have to be mounted on large heat sinks if big current is being drawn by the load. At least the bridge prevents unnecessary blown fuses. Tradeoff.

[caphair]

Re: Why does reverse polarity cause damage

There are a few reasons. The main reason is in modern CMOS devices, there is a body diode from Vdd to GND, which is reversed biased normally. (The diode is actually formed by millions of parallel transistor body diodes.) Reversing power and ground causes current to flow through these diodes, causing a high amount of power dissipation and eventually destruction of the device.

Additionally, you have electrolytic capacitors and conventional semiconductors that may not play well with reverse biased sources.

What are those diodes in CMOS devices there for in normal operation?

[budm]

Re: Why does reverse polarity cause damage

May be you should read some of these about MOSFETs.
https://cdn.badcaps-static.com/pdfs/...3b1facbbe8.pdf
http://www.microsemi.com/document-po...osfet-tutorial