Easiest way for reverse polarity protection.

[65535]

I was thinking about this, and how do you make sure that you get the current flowing in the proper direction regardless of how you power a DC device.

Simple, put a full wave bridge rectifier (or 4 diodes) on the input leads. no matter how you power it AC/DC+/- it will get the proper polarity DC current.
Obviously you wouldn't want to run something off AC if you could help it.

Anyways, to any circuit builders, you could use this to protect your drivers from reverse polarity.

 

[lolzertank]

Now just don't solder the bridge rectifier in backwards...

 

[Bullzeyebill]

How much room does this rectifier take up?

Bill

 

[rantanplan]

More important question is ... how big are the losses due to the voltage drop at the diode(s)?

Guess there is a reason that the bridge rectifiers in a power supply are mounted to a heatsink. In a low voltage application (like flashlight drivers) the voltage drop should have a significant influence on the efficiency ...

 

[Illum]

I think the easiest way is to use a recessed positive polarity contact that can only be made to contact the battery's nipple and should the battery be placed in backwards there would be no way for the contact to force shut...you could use the same concept for battery holders as well..

 

[65535]

Physical protectors are good and all, but a proper sized rectifier would work perfectly.

And not necessarily just on a light, on any DC application that could be reversed, like battery chargers and such.

 

[greg_in_canada]

A bridge rectifier puts two diode drops in series with the battery so you are losing 1.2V or so with silicon diodes and 0.6 or 0.7V with Schottky or Germanium diodes.

A single diode can also provide protection (with half the voltage drop) but won't give you light when the battery is backwards.

Greg

 

[Illum]

yeah...but diodes have its own voltage drop to worry about as well...

 

[Mugrunty]

There is also a way to set up a reverse polarity protection circuit using one MOSFET. With this more expensive solution there is virtually no voltage drop.

 

[jeffosborne]

But, what about that pesky protection diode in the MOSFET that protects it against reverse voltages? Kinda spoils the party. I suppose they are available without the diode, but all the high power, low-ohm mosfets I have include the diode. Jeff

 

[mpf]

The first (left most) P fet does what you want.
Note the diode is in the correct direction but the Fet turns on and shorts it out.

see http://www.maxim-ic.com/appnotes.cfm/an_pk/2012
or google reverse voltage protection mosfet

 

[BoarHunter]

Over the last 25 years, many people have reinvented this set up to protect against reverse polarity.
The caveat is, as stated above, power losses !

The very low voltage and high current typicals to LED drivers from NiMH or even LiIon, make these set up a waste of energy and a mechanical protection will always be the best, the simplest and no power loss !

 

[Bullzeyebill]

Over the last 25 years, many people have reinvented this set up to protect against reverse polarity.
The caveat is, as stated above, power losses !

The very low voltage and high current typicals to LED drivers from NiMH or even LiIon, make these set up a waste of energy and a mechanical protection will always be the best, the simplest and no power loss !

The foam donut found on many of our twisty lights is one of the best "mechanical" reverse polarity protections. Not really good for a clicky switch light. I think that the diode protection is mostly ok for light running at 4+ volts, and many of our lights use several cells with buck constant current circuits. I have been one of those unfortunate few(?) who, playing around with different battery types, particularly flat top LiIon, has burned out a few LEDs, and or circuits.

Bill

 

[kramer5150]

IMHO a mechanical solution is the best for the reasons already mentioned.... plus it has proven to be very reliable. I have never heard of a fenix light failing from reverse polarity.

 

[jayflash]

"Easiest?" As Illum & kramer indicated: a mechanical solution to this electrical problem seems to be the easiest and may be the cheapest. The best solution could vary, depending upon your needs.

 

[lolzertank]

But, what about that pesky protection diode in the MOSFET that protects it against reverse voltages? Kinda spoils the party. I suppose they are available without the diode, but all the high power, low-ohm mosfets I have include the diode. Jeff

The diode isn't "included" in the sense that it was deliberately in the package. It's actually very important to the function of MOSFETs.

http://en.wikipedia.org/wiki/MOSFET#MOSFET_structure_and_channel_formation

Look at the first picture on the side, showing a N-channel MOSFET in its off state. Notice how between the substrate (which is normally connected to the source) and the drain, there is a PN junction. That's the body diode. The body diode is how a MOSFET blocks current when it's off.