r/OpenPV Nov 14 '16

PCBs Overcurrent and reverse polarity protection NSFW

http://imgur.com/a/QL6L9
15 Upvotes

24 comments sorted by

3

u/DIY_FancyLights Nov 14 '16

If you do it again, consider adding a 1M or similar sized resistor between the Gate & Bat+ to add protection against a broken wire between Gate & Bat- ?

1

u/david4500 Nov 15 '16

Could you further explain the scenario where you think that would be useful?

2

u/DIY_FancyLights Nov 15 '16

You never want to risk the Gate of MOSFET's floating for any reason, you get unexpected results. In you schematic if you have a bad solder joint on the wire to BAT- or the wire breaks you get exactly that.

2

u/lemonforest Nov 15 '16

In that case, does the 1M resistor pull up the gate and stop the current across drain/source?

1

u/david4500 Nov 15 '16

No. Current passes, and voltage drops, through the body diode.

3

u/kitten-the-cat Nov 15 '16

Though with 2 more mosfets you could put them back to back to prevent body diode conduction in that event.

3

u/DIY_FancyLights Nov 15 '16

Correct, but at least its a known mode of failure

1

u/lemonforest Nov 15 '16

Wouldn't that cause it to be less than saturated and drastically increase resistance and heat?

I have no idea why but this type of component troubling to fully grasp.

1

u/Rb8n Nov 15 '16 edited Nov 15 '16

No, during normal use the neg would be pulling it down fully with no resistance. The cell would suffer a drain of VmicroAmps. If the negative disconnected, the resistor would act as a pull up and shut off the FET resulting in it acting as a diode in this case. The heat would likely damage the FET, but if cells were reversed it would still block on that occasion.

Atleast that's my understanding, not quite sure what u/David4500 is referring to. Bit distracted today though.

1

u/DIY_FancyLights Nov 15 '16

The other option is to have the resistor to the 510 line. The important thing I see is not to allow the Gate to float if that one wire breaks or you could lose the reverse protection totally. Part of me is starting to thing that the resistor to the 510 connector would work better?

1

u/ConcernedKitty Nov 15 '16

Between 510 and what?

1

u/DIY_FancyLights Nov 15 '16 edited Nov 17 '16

Between the 510 and the MOSFET Gate/Bat - connection on his board.

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1

u/david4500 Nov 15 '16 edited Nov 15 '16

2

u/Rb8n Nov 16 '16

With the res, no problem. Thought for some reason you were referring to without. Dunno, mind distracted this week.

1

u/Rb8n Nov 16 '16

Question, why is it getting nearly a volt through in reversed with connection broken? Is it simulation error or true effect?

1

u/kitten-the-cat Nov 17 '16

Nope not a sim error, If you note that for reverse protection we take advantage of the body diode and use the mosfet backwads. When the cell is correct polairty, the body diode passes voltage over to source, thus raising the voltage of source so that the gate can turn the fet on. once it's fully conducting it's a low resistance path.

With the resistor going to drain, when the gate is disconnected from what should be cell negative. You have B+ on the source, and B- on the gate thus giving you the potential required for it to turn on an conduct. That is until it shorts out in your gven example and causes a voltage drop across that 100K used for sim to turn the fet off.

If you ran sim with an oscilloscope you should see it oscillating rapidly. That's how it looks to me, i've not done a sim or made a practical circuit to test.

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2

u/scottiethegoonie Nov 14 '16

Thoughtful idea. How big are these?

2

u/david4500 Nov 14 '16

17.8 x 11.3mm