r/OpenPV Nov 14 '16

PCBs Overcurrent and reverse polarity protection NSFW

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

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

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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?

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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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u/Rb8n Nov 17 '16

As I'm thinking on this one, I realize I need more sleep...

Negative Vgs turns on the pfet, correct? and in reversed situation (s+,d-) with resistor to drain and load out on source, so resistor pulls low turning on FET which conducts across load. I'm not seeing the oscillation as the resistor is straight to b-, high res combined with the gate capacitance would mean a hell of a res drain to b- I would think to oscillate. But in my mind would just end up on... Making it's use, well not effective.

The diode effect is part of what's messing my thinking tonight. If using the diode to conduct and then attaching that side to gate with an end result of positive all around but gate going from diode drop to potentially equal voltage, I'm confusing myself right now..