Best Way to Prevent Reversed Polarity
SailerMan
Posts: 337
What is the best way to make sure that if someone hooks 12v up to a 12 v circuit that it doesn't get damaged if the polarity is reversed. Let's assume that the connectors are not polarized to physically prevent it.
Comments
assumong that 12v is applied the diode will cause a forward biased voltage drop of approx 0.7v - not an issue if you are just supplying a reg. after that ... or 11.3v·may be perfectly·acceptable.
rgds,
John
edit: made circuit a bit more accurate ...
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'Necessity is the mother of invention'
Post Edited (QuattroRS4) : 12/5/2007 6:04:51 PM GMT
Not all regulators do provided this feature, but·protection is quite common as a feature·for automotic automotive electronics because some mechanic is likely to put a set of battery cables on wrong and blow out everything unprotected.
I have tried reversed·diodes·across the inputs, but... without a fuse they just burn up on the first error and leave the circuit available to harm on the second go around. You can put a fuse in line with them and have the diode rated higher amps than the fuse, but the same problem is still there if you don't inspect the fuse before turning on power.
I have put diodes in line which work fine, but... you get an additional 0.7 voltage drop and a waste of more batteries. It is okay for wall warts though.
And just using tiny 250ma fuses in line [noparse][[/noparse]and hoping they will blow before serious damage occurs] are absurd because they add enough resistance to cause a voltage drop and they blow at the slightest error.
I hope that helps. I really like the LM2940-x much better than the 78xx regulator.· Read·its·PDF.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
Post Edited (Kramer) : 12/9/2007 9:12:34 AM GMT
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Regards,
Jim
1. If everything runs off a regulator, use one that's reverse voltage protected, as noted above.
2. If you can tolerate a 1.2V drop, use a bridge rectifier to "right" the polarity.
3. If you can tolerate only a 0.6V drop, use a series diode.
4. If you can't tolerate any drop, use a fuse or polyfuse (PTC thermistor) in series with a hefty diode wired "backwards" across the voltage input.
5. If the operating current is low enough, an incandescent lamp in place of the fuse in #4 also works.
-Phil
Addendum: 6. Use a p-channel MOSFET as described here.
Post Edited (Phil Pilgrim (PhiPi)) : 12/5/2007 6:27:25 PM GMT
Here is my Schematic.
http://forums.parallax.com/attachment.php?attachmentid=50114
-Phil
Looks like you could use a diode bridge and not worry about the voltage drop.
There's lots of high end equipment that is not reverse polarity protected...well, it is...but in a low-tech way!
Use a connector that doesn't allow for an improper connection (don't use those headphone type single jacks with the tips and collars....use a multi-pin molex connector of some type).
cheers
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<FONT>Steve
What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
Lee Travino after the second time being hit by lightning!
Then looking at your schematic the 12v supply is not used else where merely as a supply to lm2575 regulators which have a stated input range from 7-40v ... so a simple series diode would work - however since these regulators can supply 1amp each - ensure that the current capabilities of the chosen series diode is sufficiently large enough..
Rgds,
John
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'Necessity is the mother of invention'
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Post Edited (metron9) : 12/7/2007 4:41:49 AM GMT