Using FETS for polarity protection and soft start up..

[mrpackethead]

I've recently been working on a project where ensuring that the Power input to it was *always* correct. Given the cabling setup, it could be possible for an inadvertent crossover to be made, and the results would be quite catastrophic.. The exact circuit doesn't really matter, but it operates on DC with about 4A flowing.. THe system is housed in a waterproof sealed box, so keeping it cool is very important as well.

So, polarity protection is simple right, just put a diode in series? Lets have a think about that.

A silicone diode with 4A passing through it, is going to get pretty hot.. its going to drop .7V @ 4A, thats 2.8W! In a small box thats going to get quite hot.. Not an option..

So, what options could we use instead?

[Barnabybear]

Why not fuse the input and use a diode in parallel in the reverse direction - if the polarity is reversed the diode will conduct above 0.7 volts and blow the fuse. Not very flash but it's cheap and will save the other stuff.

[JHinkle]

Removed so training can continue.

How can I remove the attachment?

Joe

[JHinkle]

I've recently been working on a project where ensuring that the Power input to it was *always* correct. Given the cabling setup, it could be possible for an inadvertent crossover to be made, and the results would be quite catastrophic.. The exact circuit doesn't really matter, but it operates on DC with about 4A flowing.. THe system is housed in a waterproof sealed box, so keeping it cool is very important as well.

So, polarity protection is simple right, just put a diode in series? Lets have a think about that.

A silicone diode with 4A passing through it, is going to get pretty hot.. its going to drop .7V @ 4A, thats 2.8W! In a small box thats going to get quite hot.. Not an option..

So, what options could we use instead?

I'm sorry. I replied as if you were looking for a solution. Re-reading your thread - it appears you were starting a "teaching" thread;


Joe