Inverse polarity protection

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Dear All:

How can i protect a 5V circuit from inverse polarity.

I don't want to use diodes to avoid voltage drop.

any ideas?

Thanks.
Alfredo Mendiola Loyola
Lima, Perú

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My preference is a series diode, with the diode feeding into the power regulator. Then the regulator Vout can be 5.0 V or whatever is desired.

If you don't mind the board space, parts, and increased complexity you can use an active circuit, which I"m sure someone else can post.

If it is a low power project you could put a fuse in the V+ power line, say 100 mA or whatever is adequate for your project. Then put a diode, reverse biased, across the power line. If someones connects the power leads up backwards then the diode will conduct, heavily, essentialy a short circuit, and HOPEFULLY pop a fast blow fuse before the diode dies from exceeding its maximum foreward current rating. Pick a hefty diode.

Certainly NOT the best solution, not one I'd use for a comercial product, but better than no protection on a one-off project.

Hey, it is a poor man's Plan B, not my first choice, which is the series diode mentioned above...

JC

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Non-reversible power plugs?

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Actualy, the shunt diode with a series fuse is standard procedure in the 2 way radio industry. Kilobuck Motorolas or a $40 cb. As long as a "mechanic" didn't leave out the fuse when the radio is swapped to another vehicle, the technique is dead reliable. The diode ALWAYS fails as a short circuit and pops the fuse. Works like a charm. In a low voltage circuit with no headroom it's the only way.

Rick

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You can use a mosfet. Please note this configuration is a little tricky (its not in common source mode). Also you will fry the mosfet if it has a source-gate diode.

V1 is your 2 wires input. The GND symbol is now your circuit reference ground. 100R is where you put the load you want to power.

This approach is not suitable for all applications.

edit: found a good link for you to read:
http://www.geofex.com/article_folders/mosswitch/mosswitch.htm
oddbudman

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Last Edited: Thu. Jan 8, 2009 - 07:34 AM
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Ale_BKK wrote:
Non-reversible power plugs?

You underestimate the ingenuity of mankind. :roll:

It gets a bit expensive in terms of components and board real-estate, but if you can have a bridge rectifier and filter capacitor on board, before the regulator, then your wall wart can be of either polarity, even AC. :D

If you think education is expensive, try ignorance.

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@DocJC:
What do you mean with:

Quote:

My preference is a series diode, with the diode feeding into the power regulator.

Simply 5V - Anode - Cathode - circuit? That'll cost us like 0,7 V, doesn't it?

[edit]
ah, I got it. You place it the diode before the regulator. Like:
Anode - Cathode - RegulatorIn

That way we don't have any current flowing. But we do have 5 V at every point which is directly connected to GND, don't we? That would concern me a bit.
[/edit]

Also, I'm not sure what you mean with

Quote:
If it is a low power project you could put a fuse in the V+ power line, say 100 mA or whatever is adequate for your project. Then put a diode, reverse biased, across the power line.

Here's a higly professional picture ;)

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Very good, Nephazz. Both the circuits in your picture are wrong. Takes genius to do that. :roll:

If you think education is expensive, try ignorance.

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[no sarcasm, ironie or whatever]
yes, I suck at analog electronics. I'm pretty sure that #1 works, though.

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Nephazz wrote:
[no sarcasm, ironie or whatever]
yes, I suck at analog electronics. I'm pretty sure that #1 works, though.
As a fuse destroyer yes. As protection agains inverse polarity, no.

+     Fuse
o----|-----|---+------
               |
              ---
In             ^       Out
              / \
              ---
               |
o--------------+------
GND

Stealing Proteus doesn't make you an engineer.

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If you use a Transil diode in Arnold's circuit, you've got both,
reverse polarity protection as well as overvoltage protection.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Nephazz wrote:
[no sarcasm, ironie or whatever]
yes, I suck at analog electronics. I'm pretty sure that #1 works, though.

Just teasing you a little. ;) No offense intended. We've all had our bad days. :roll:

If you think education is expensive, try ignorance.

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why is my circuit 1 bad? Diode+Fuse will be like 0 Ohm at reverse voltage. Woosh, mass current kills the fuse before my Mega takes damage.

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Nephazz wrote:
why is my circuit 1 bad? Diode+Fuse will be like 0 Ohm at reverse voltage. Woosh, mass current kills the fuse before my Mega takes damage.

What happens after the fuse blows? Where does the reverse voltage go?

If you think education is expensive, try ignorance.

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To prevent needless repairs and/or returns, consider a "polyfuse" instead of a conventional fuse.

Tom Pappano
Tulsa, Oklahoma

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Use a polarized connector. Whether it's the shape of the housing, or if it's keyed, any moron will need to be pretty determined to connect it the wrong way. Look at a PC's power supply, or consider a DIN-type plug -- ever try connecting a mouse or keypad the wrong way? :D

--Rich

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*doh*, thx emuler. I do blow the fuse but, well^^

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The MOSFET Circuit shown above is good, at least for voltages from about 4 to 15 V.

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emuler wrote:
Ale_BKK wrote:
Non-reversible power plugs?

You underestimate the ingenuity of mankind. :roll:

Ah, good point.
Every time a fool proof system is made the world creates a better, more destructive fool to deal with it.