Reversing Polarity in DC circuits

[Hydromechdude]

I've been trying to find why reversing the polarity in DC circuits is (except with 'special' devices) harmful. I haven't been able to find anything doing some Google searches, which leads me to believe the reason is so simple and straight forward that explanations are avoided and instead people are advised not to do it.

Thanks in advance. Electrical isn't my strong point.

 

[IRstuff]

?

Reversing the power connections to a DC circuit with transistors or ICs is like connecting you to the AC mains.

Too much current fries your heart, likewise a circuit.

TTFN

FAQ731-376

 

[Skogsgurra]

Reversing polarity is OK with lamps, heaters, magnet coils and some motors (like starting motors and drills with series excitation). They don't mind what polarity they get.

Batteries, computers and electronic devices with semiconductors in them are very sensitive to polarity. This is mostly because there are diodes and transistors that will conduct heavily (burn out) if reverse biased.

That is something most people should know about. You cannot test with one polarity and then, if it didn't work, test with the other polarity. Chances are that the other polarity won't work either - because you blew the device the first time.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

Since your field is "Aerospace", it's worth mentioning that most avionics has to meet standards that include the requirement for protection against reverse polarity DC power supply. It can be as simple as including a series diode (series! to block reverse current flow) at the appropriate point in the power supply circuit within the box.

 

[mcgyvr]

I'd think that a properly designed product (no matter if avionics or not) should include that series diode type of functionality. Unless there is no harm in reverse polarity hookup obviously.

 

[cranky108]

A series diode may in several low voltage applications require additional batteries. And thus could increase the size by as much as half.

But a special shape battery would not be out of the question. However people may become upset haveing to buy a special size battery, at a special price.

But in general it's not a bad idea.

 

[Hydromechdude]

Thanks guys, appreciate the responses.

 

[electricpete]

also some capacitors are sensitive to polarity

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[zzzjim]

It's electro chemistry that upsets electrolytic capacitors
these use an un-usual concept of electro-plating an insulating layer , that can repair / re-form its self .

Put the power the wrong way , and repair becomes self destruct !

 

[cranky108]

Still fun to watch newbes blow something up, because they don't follow instructions.

 

[Skogsgurra]

Maybe. But not if is your gear...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[xfmrman]

Always fun to accidentally hook up big electrolytic caps backward soon followed by a loud bang and confetti everywhere