Do I need a signal diode (like IN4148) or a rectifier diode (like IN4001) to add to the +ve line to protect against polarity reversal? Would the available supply after the diode be 0.6V less? Is there any way of avoiding this? Thanks, everyone
John Popelish wrote: [snip...]
[snip]How does one go about protecting the gate? I heard about this trick in passing from one of those "Bob Pease" online lectures, and was intrigued, but he just said 'fet', and didn't go into detail, so I never too the time to figure out what it was about.
It seems like a useful technique. I guess just ensuring that the gate doesn't go outside it's max rating limits is enough.
It also seems like somebody would offer one of these as a discrete protection device, with built in limiting for the gate, and datasheet characterization for max voltages, Ron, currents, etc. Or, do the normal specs work for this backward connection?
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
At rated current, it may be more like 1 volt less. If you use a schottky diode it will be about half as much loss. If you use a mosfet hooked up backwards (turned on channel in parallel with the body diode when correctly connected), it can be considerably less than that, but you may have to complicate the circuit a bit to provide spike protection for the gate. What supply voltage and load current are we talking about?
-- John Popelish
Sorry, Rich, but I don't think your answer is very useful...
Unless I'm misunderstanding him horribly, he's looking for info on what to use as what I've always heard called "steering diodes", so that no matter which way a battery/battery pack gets hooked to the circuit, the polarity is correct, not info on keeping a dead cell from reversing a pack.
Royal: What I've always done (in non-weight-critical applications) is "over design" the power supply (Add an extra cell, ferinstance) appropriately to compensate for the diode drop that will be involved, and then plug the power (Either AC or DC...) into a bridge rectifier's inputs. Put DC on the inputs of a bridge, and it's going to put out DC of the right polarity on the output pins regardless of which way the supply voltage is hooked up. Free bonus: If your circuit doesn't have a problem handling AC ripple, you're also set to just hook up the correct voltage AC (from an ultra-cheap wall-wart, for instance) to the same pins you'd hook the battery to. If it does have a problem with ripple, a couple of caps and coils "downstream" from the power input can easily clean things up to tolerable when used with AC, yet sit there quietly doing nothing if the device is being fed DC.
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I just had to design one of these for a device that runs on 4 AA cells. I added 100k in series with the gate to the normally positive battery clip, drain normally negative clip and source to circuit negative rail and a 6V8 zener, anode to source, cathode to to gate. Since people may be touching the cells as they touch the battery clip, I had to allow for body capacitance discharge into the clips without zapping the anti reverse fet switch. Put the cells in backward and you have only 100k in series with a forward biased zener across the battery.
-- John Popelish
I would probably use a 1N5817 schottky in series. At this current, the drop is only about 0.2 volts.
-- John Popelish
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