# simple inverter reverse polarity protection cable project

#### edro

Joined May 5, 2016
16
I need to make a robust inverter cable with simple inverter reverse polarity protection. I need help using diodes, fuse application. Note, please, that I am a decent software engineer but flunked my first (basic) electronics class in college in 1972. Either the help you can give is really simple or really thoroughly explained. I have an electronics brown thumb having fried or "popped" more 555 TTL chips than I can remember.

Here are the calculations that I made, probably with a lot of mistakes. Please feel free to correct or even scold me! The inverter is 1000 watts, 2000 peak, so 2000/120v AC is 16 amps, times 12v is 200 watts 12 volts DC. So a 200 watt in-line fuse is important. If I want to make a cable that prevents reverse polarity hookup then I was thinking that I can get put several 3 or 5 amp diodes in parallel then put that (maybe better wording would say "those") in series with the cable. If someone connects reverse polarity I figure that the diodes would create enough resistance to blow the fuse without damaging the inverter. Then only replacing the fuse would allow it all to work again.

The other idea I found would be to use several self healing resettable fuses (PTC) in parallel to add up to 200 amps. I'm unfamiliar with these fuses, never heard of them before this week. I also saw a YouTube of a circuit using a transister circuit but it was over my head. I need something simple that I can build myself!

Do you have any advice to give me on this idea? Do you think this will work? Any better plans that I can implement?

Thank you!
Ed

#### mcgyvr

Joined Oct 15, 2009
5,394
Whats wrong with a keyed plug that only goes in one way?

#### crutschow

Joined Mar 14, 2008
32,874
If the inverter output is 1000W then the input will be more than 1000W due to converter inefficiency.
This means an input current of more than 1000W / 12V = 83A.
You could use fuse in series with a diode in parallel to the input.

If you want to avoid fuses, you could use a MOSFET in series as a low-loss diode.
Here's an article on that.

#### edro

Joined May 5, 2016
16
Great idea, but the cables have large battery clips so a person could possibly put the battery clips on the wrong battery post. Some of the batteries I have, it's difficult to see which is positive or negative because it's black on black lettering, and there's sometimes crud over the lettering, so it's just difficult to see. Even when the posts are well marked it is possible for some moron (such as I am sometimes) to connect it the wrong way before realizing it.

#### edro

Joined May 5, 2016
16
Thank you crutschow. Evident that you know more than I do. I went to the link you provided, the article on how to use a "MOSFET in series as a low-loss diode"... brain freeze looking at the article, no idea. I understand a battery and a diode. The MOSFET article is over my head. Maybe you can help me with a simple schematic.

Here is what I can understand. (graphic below). Can you post the part number for the right size for my inverter? A link to the correct part would be even better. Some websites have so many variations for parts.

I'm not even sure my graphic (Visio 2003) is correct or not.

Ed

Last edited:

#### Dodgydave

Joined Jun 22, 2012
10,946
Your diode is wrong, it needs to go the other way.

Last edited:

#### edro

Joined May 5, 2016
16
If the inverter output is 1000W then the input will be more than 1000W due to converter inefficiency.
This means an input current of more than 1000W / 12V = 83A.
You could use fuse in series with a diode in parallel to the input.

If you want to avoid fuses, you could use a MOSFET in series as a low-loss diode.
Here's an article on that.
ah HA! I looked up "MOSFET reverse polatity schematic" and found this:

[image deleted]

Please help me to see that this will work -- will it will allow current to go to the inverter but if the battery is hooked up incorrectly, it will not allow current to flow? I would still put a fuse in between the battery + and the 'D' pin on the MOSFET. Will reverse polarity blow the fuse or will it just not pass the current?

What size specification MOSFET do I need to protect a 1,000 watt inverter?

And about the fuse, I don't know that a 83 amp ANL fuse is made. Would 100 amp blow before damaging a typical 1000 watt inverter? Also, what about the 2000 surge watts that the inverter allows? Would a 100 amp fuse blow prematurely if 2000 watts load happens for 1 or 2 seconds?

Thank you again!

Last edited:

#### edro

Joined May 5, 2016
16
Your diode is wrong, it needs to go in series with the fuse,...
That makes more sense. Yes. Thank you.

#### crutschow

Joined Mar 14, 2008
32,874
Your diode is wrong, it needs to go in series with the fuse,...
The diode is in the correct position, just the wrong polarity. You want the fuse to blow if there's an reverse connection.

If you put the diode is series then you will have the large forward loss at the high inverter current.

#### crutschow

Joined Mar 14, 2008
32,874
ah HA! I looked up "MOSFET reverse polatity schematic" and found this:

Please help me to see that this will work -- will it will allow current to go to the inverter but if the battery is hooked up incorrectly, it will not allow current to flow? I would still put a fuse in between the battery + and the 'D' pin on the MOSFET. Will reverse polarity blow the fuse or will it just not pass the current?

What size specification MOSFET do I need to protect a 1,000 watt inverter? .................
The MOSFET acts like the series diode. It conducts well in the forward direction (the same direction as the substrate diode) and blocks in the reverse direction.
You want a MOSFET with a low ON resistance (no more than a milliohm) as even that will dissipate over 6W at 80A and will require a heat-sink.

If the ground of the inverter is isolated from the vehicle chassis, then it would be better to use an N-MOSFET and put it in the minus lead, since N-MOSFETs have lower ON resistance for a given size chip as compared to P-MOSFETs.

#### tcmtech

Joined Nov 4, 2013
2,867
Great idea, but the cables have large battery clips so a person could possibly put the battery clips on the wrong battery post. Some of the batteries I have, it's difficult to see which is positive or negative because it's black on black lettering, and there's sometimes crud over the lettering, so it's just difficult to see. Even when the posts are well marked it is possible for some moron (such as I am sometimes) to connect it the wrong way before realizing it.
View attachment 105504
Buy a red and a white paint pen and draw the (+) in red and the (-) in white on the batteries.

If there is enough light to see the battery lugs to connect the clamps to there is enough to distinguish a red (+) from a (-) white.

#### edro

Joined May 5, 2016
16
Buy a red and a white paint pen and draw the (+) in red and the (-) in white on the batteries.

If there is enough light to see the battery lugs to connect the clamps to there is enough to distinguish a red (+) from a (-) white.
Another fine idea but it doesn't address some of the complications that exist - which I neglected to explain:
1. The batteries are not already hooked up to anything thus there are no existing color coded connections. (I look to see which is connected to chassis ground but that's an assumption that the chassis is a negative ground..)
2. The batteries are various, not in my possession, not mine to paint red, white, or any other color indicators.
3. And, (this one is the most relevant): I'm not there. Someone else (less able to distinguish anything other than there are two connections to make), they are the ones that I'm trying to prevent from making a reverse polarity mistake.

Still, thank you for the idea. A fine one barring these complications.

#### edro

Joined May 5, 2016
16
The diode is in the correct position, just the wrong polarity. You want the fuse to blow if there's an reverse connection.

If you put the diode is series then you will have the large forward loss at the high inverter current.
Thank you for clarifying!!! And good for Dodgydave to edit their post!!! I was a little too quick to just take you all at your word 'cause I'm such a newbie..

#### edro

Joined May 5, 2016
16
The MOSFET acts like the series diode. It conducts well in the forward direction (the same direction as the substrate diode) and blocks in the reverse direction.
You want a MOSFET with a low ON resistance (no more than a milliohm) as even that will dissipate over 6W at 80A and will require a heat-sink.

If the ground of the inverter is isolated from the vehicle chassis, then it would be better to use an N-MOSFET and put it in the minus lead, since N-MOSFETs have lower ON resistance for a given size chip as compared to P-MOSFETs.
Wow. I understood your first sentence. But from "You want a MOSFET with a low ON resistance..." and so on it's pretty much greek to me. It isn't that I don't want to learn something here but I'm pretty newbie in electronics.

I can tell you this:
- inverter will be isolated from the vehicle chassis.
- adding a heat-sink won't be a problem.

Here are my questions:
- will this simply not pass the current along if connected with reverse polarity (open circuit) OR
- will this stop the current, creating so much resistance that the battery fuse will blow?

I have only a simple understanding of a transistor, it can pass current or not depending on the input (base? last time I thought about it was over 40 years ago, in 1972)

#### crutschow

Joined Mar 14, 2008
32,874
......................
Here are my questions:
- will this simply not pass the current along if connected with reverse polarity (open circuit) OR
- will this stop the current, creating so much resistance that the battery fuse will blow?
...............
The MOSFET will completely stop any reverse current.
If you stop the current then the fuse won't blow.
Fuses blow when there's too much current (low resistance), not with a high resistance.

MOSFETs fully turn on when the gate voltage is greater than the source (typically +10V Vgs for standard N-MOSFETs and -10V Vgs for standard P-MOSFETs).
The are fully off when the gate voltage equals the source voltage.

Last edited:

#### edro

Joined May 5, 2016
16
The MOSFET will completely stop any reverse current.
If you stop the current then the fuse won't blow.
Fuses blow when there's too much current (low resistance), not with a high resistance.

MOSFETs fully turn on when the gate voltage is greater than the source (typically +10V Vgs for standard N-MOSFETs and -10V Vgs for standard P-MOSFETs).
The are fully off when the gate voltage equals the source voltage.
Got it. I did some "research", really just some YouTube tutorials, and wow! MOSFETs are really cool! So I understand now how they work, how to "hook it up", and all is well. Just one thing now: I cannot understand the details on the data sheet. Would you be willing to tell me the Jameco part number for what I need? And how many I need? --do I put them in parallel to cover the amps that I need to pass through? To refresh, application is a 1,000 watt inverter hooked to a 12 volt battery ... 83 amps, double that for peak.

Ed

#### edro

Joined May 5, 2016
16
Or, do I just need one, and keep it cool, 25 degrees, right? can use a 12v fan, + alum heatsink.
Just need a Jameco part number for right-sized one for me, please.

#### crutschow

Joined Mar 14, 2008
32,874
This IRF2804PBF is the lowest ON resistance (2.3mΩ) MOSFET that Jameco sells.
It will dissipate about 15W @ 80A so would need an appropriate heat sink.
Adding more MOSFETs in parallel would reduce that--
2 in parallel would halve the power and 4 in parallel would dissipate ¼ the total power.

#### edro

Joined May 5, 2016
16
This IRF2804PBF is the lowest ON resistance (2.3mΩ) MOSFET that Jameco sells.
It will dissipate about 15W @ 80A so would need an appropriate heat sink.
Adding more MOSFETs in parallel would reduce that--
2 in parallel would halve the power and 4 in parallel would dissipate ¼ the total power.
Considering the cost, why wouldn't all inverters be built with this built in? Seems pretty plain to me. I'm making 2 of these "idiot-proof" cables (a term my dad used to say a lot, and I think that was because of me, boo hoo). I only have one inverter but I'm looking to purchase a pure sine wave inverter.

Thanks so much crutschow! I'll take a photo when I'm all done and post it here.