Reverse Polarity Protection

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Sorry for a bit off topic.. I already have a working board for an automative app for which i intend to do some reverse polarity protection using very minimal discrete components.. Is it bad to protect the ground pin using a diode like...

diode
----|>|-----Negative terminal.
|
GND(Board Gnd)

And same for the POSITIVE WITH the diode reversed ??

I think that's not a good solution.. Well, no MOSFET's because working voltage is 12 to 24v. and most MOSFETs' VGS is below that.. Right ?? I don't have space for the bulky ones....

I'm looking for some cheap and simple and also taking less space solutions.... The board is already made, but whenever a reverse polarity is applied, a few tracks are burned and the board is rendered useless.. And the worse thing is that, there is a couple of antennas connected to the casing which in turn will be connected to the chassis of the vehicle. So that ground is shared as well.... I'm confused.. The experts, here are good in those things, i know .... Any Help ??

Also for the PIC 18 series, 80 pin chips, does it really help things improve if we isolate the AVdd and Vdd together with AVss and Vss ?? If then, why ?? Can I have different Vdd and AVdd levels ?? I think it's not possible. I tried it once..

Thanks....

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Probably the best description of protection in the automotive environment is given in the datasheet for LTC1625. See chapter "Automotive Considerations: Plugging into the Cigarette Lighter" on the page 16.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I've tried the circuit shown in Figure 8 (page 17). The problem is
that if there is a large over-voltage condition that lasts more than
a short time, the TVS will blow. When they blow, they actually
short out. We had a 3AG fuse, probably 3 amps, in front of the
main protection diode and that fuse blew and prevented the device
from smoking. So I cut out all the TVS devices in the remaining
boards. Fortunately, we hadn't shipped any units yet.

I'm not sure what to do for transient protection now, for a product
that plugs into a cigarette lighter.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

Brian S · Joined: 06 Sep 2005 Posts: 13

You can use a diode bridge for automatic polarity selection, with a few caveats. Connect the DC input to the "AC - AC" terminals of the bridge, the "+" bridge terminal to your circuit's + input and the bridge "-" terminal to your circuit's Gnd. You can supply the "AC-AC" terminals with DC of either polarity (or even AC), and the proper polarity will be applied to your circuit. There is no direct connection to ground while using this system, so you may have to supply some extra bypassing for RF applications, but probably not when you have an off-board antenna fed with coax.

Use a bridge of suitable current and voltage rating. Note that you'll have an extra diode drop of voltage loss with the bridge...if that's a problem, using Schottky bridge will reduce the drops.

For "fuseless" protection, follow the bridge directly with a PTC Resettable fuse, and a Sidactor or similar as a spike protector....this network is for overvoltage/overcurrent protection only, as the bridge will prevent reverse polarity. If the Sidactor fires due to voltage spike, the PTC will open briefly to limit current, then reset itself when the surge goes away. If the PTC opens due to your circuit failing, it will reset once the failure of your circuit is cleared.

I don't know the Patent Status of this system.

Good luck!

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

How old is that patent? We were doing that with marine electronics in the mid 1980's so the patent is either close to expiring, or was never valid anyway.
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PCM programmer · Joined: 06 Sep 2003 Posts: 21708

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

Thanks, one less thing to worry about. I had no clue on patents' lifespan.

However I am just wondering how many other trivial solutions (like long-pressing, double clicking a push-button: US Patent No: 6,727,830 etc.) we should go roundabout in our designs just not to infringe any valid patents, and how many other trivial patents infringements will be unknowingly still left ticking in our products.

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Thanks all..

We indeed had the TVS in the circuit. But, as PCM pointed out sometimes it does fail and we don't use fuses and that made the whole of the board useless.. As for the diode bridge mentioned by BrianS, it's a good solution, fail safe.. But takes some space which, unfortunately we can't afford to spare.. Four diodes is a bit big.... moreover voltage drop is a no-no here since it is a cause of concern too....

okay somebody got something else ??

Thanks..

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You should be able to protect your board with just a simple 1N4002 diode.
Put it inline with incoming +12v supply. The anode goes to the battery,
and the cathode goes to your voltage regulator.

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Thanks PCM, but that diode does exist in our board.. the problem is when you apply reverse polarity, the antenna ground is connected to the chasis of the vehicle(shared). So this diode can't protect the short circuit.. I would like to protect the ground in someway or the other.. I'm wondering how to..

Thanks.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Describe the antenna circuit. The only antennas that I'm aware of
are the ones for car radios. I don't think they connect to ground.

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

When all else fails uses a DC-DC isolation converter

cstan_02 · Joined: 24 Jul 2005 Posts: 13

Thanks PCM. Antenna is from my application.. The app that I'm describing has atleast two antennas connected from the board.. The casing of the unit is aluminium and the antenna connectors are attatched to it.. So thus there is a problem of the board ground directly sharing to the vehicle chasis ground.. So if reverse polarity is applied, it will burn the tracks.
Any Suggestions ??

DC-DC isolators i think should be fixed externally right ?? But here we cannot afford anything external.. And should be as small as possible so as to fit in existing board.. But also current drain is a concern here..But when everything fails and that's a good option..

Thanks..

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

I still don't understand why you can't just put a bridge rec. in there or even a single diode. Can you explain why again?

Why not just use 2 diodes on each terminal. If they connect it wrong, it will not work.