The electrons have to go through the N-channel MOSFET from source to drain, not drain to source. The drain has to be positive relative to the source. You can turn the MOSTFET around to get that, but then the internal diode will conduct (to protect the MOSFET) if you connect the power backwards, so there won't be any protection for the computer anyway.

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What's an additional VIA among friends, anyhow?

Unless I've missed something important, then Maxim Integrated disagrees with you, specifically Figure 3.

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Well, perhaps the MOSFET can handle small reverse voltages like that where the internal diode would go on anyway if the transistor were not turned on; but that's not the way the transistor is designed to be used. I worked in VHF & UHF power transistor applications engineering, mostly with MOSFETs, at a transistor manufacturer in the mid-1980's. I didn't work in the department that designed the wafer process and the silicon geometries, but having done many successful amplifier designs with them, I am a bit familiar with MOSFETs. I've also designed lower-frequency ones (non-RF) into our company's power supplies, including switching regulators. OTOH, there is an unconventional way I use bipolars in our audio designs, a way that most designers would think would never work; so I know there could be some elusive tricks. The data sheet for the IRLML6344 is at http://www.mouser.com/ds/2/196/irlml6344pbf-938034.pdf . Figures 10a and 14b show the direction it is designed to conduct in. The drain is positive with respect to the source in normal operation.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Don't get me wrong, I'm not saying you don't know what you're doing. I'm the one who has no idea what's going on with mosfets. What I am saying is that I have two seperate camps of knowledgable people (maxim, instructables & stackexchange vs yourself & the datasheet) saying opposite things. Therefore I do not know which way to go.

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Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

Electrons will happily go either way, as long as the gate-source voltage is above the threshold.

There might be several ways to handle the protection problem. I'll try to give it some more attention when I have more time. The simplest would be to just put a diode in series with the positive side, but then you'll need the power supply to be slightly higher than the target operating voltage; and if you want to operate something like an A/D or D/A converter off the same supply, you'll probably want it to have its own reference, even if it's only an 8-bit converter. We do have portable products for private pilots, for the ones flying ultralights or vintage or other aircraft which may have no electrical system, and in that case I do put 1N5817 diodes in series with the 9V batteries; but there's a switching regulator after that that kicks the voltage up to a tightly regulated 12V for the audio circuits and another that takes the 12V down to 5V for the microcontrollers. It's a little different for units that are powered by the aircraft power (which may be anywhere from 11V to 30V, plus spikes). The next simplest way to handle the protection problem might be to put a fuse in series with the positive and a diode from there to ground, so in the rare occurrence that you do connect it backwards, the fuse blows.


It depends on the internal construction. MOSFETs are usually not symmetrical inside, and the dimensions and doping of the source and drain sides are different. Even if it works, the gain may be super low in the reverse direction, or you may run into other problems. I suppose the way to find out for a particular part number is to try it, since the data sheets don't characterize the operation in the reverse direction.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Basically, that is accomplished by using the body diode in a MOSFET for this purpose. And then you can turn on the MOSFET to reduce the voltage drop you'd otherwise have. This is also called an "ideal diode circuit". Here's one


That's why the gate threshold is measured relative to the source voltage. And that's the reason why you usually don't see the MOSFET being using the "wrong way", because it puts the source side on the higher voltage, and that makes it harder to get the gate even higher. But once you manage to get the gate voltage high enough, it doesn't matter which way the current flows.

You can't just swap source and drain on most MOSFETs, any more than you can swap emitter and collector on bipolars, even though NPN backwards is still NPN, and PNP backwards is still PNP. They don't work well when put backwards, because the N on one end has a different level of doping from the N on the other end, and its dimensions are different as well. Will it work? Sort of, but not well. Gains, saturation resistances, breakdown voltages, etc. will be different, and generally not in a good way.

When you can't find adequate answers for a particular application, just experiment and take a lot of data under all the anticipated conditions (and a little beyond), before implementing it in something that will be costly if it doesn't work out as hoped.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

But you don't swap the source and drain, because you always keep the gate voltage referenced to the source. That's a crucial part of the idea.

Also, keep in mind that we're only using the MOSFET in fully saturated mode.

update: deleted - had errors

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_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Garth,

you are right, one should not trust somebody elses simulation, which I unfortunately did. The internal diode was definitely pointing the wrong way in my example. Sorry for causing confusion.

However, the power in Alarm Siren's diagram is sourced from right to left, which added to the confusion. So the MOSFET is pointing the right way there.

Just a little rant: Have I mentioned that I hate these subdivided schematics, where things are sorted by the space they ocupy, not by how they relate to each other? Power in the middle and oriented right to left! Just great! The board layout is probably more readable than the schematics.

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It would be a big help to post the schematic in monochrome.

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I'm sorry that my schematic has upset you. I actually think its quite readable (though I suppose I am biased!) though I agree the power section is orientated in the "wrong" direction relative to what one normally expects. I can assure you that, at least at current, the board layout is definitely a lot less readable, seeing as its not yet finished!
I had to make it all fit on one sheet because I've yet to be able to figure out how to get Kicad to do multiple sheets properly, hence all the squishing.



I can do that for you, but may I ask, in what way it will help? All the information is the same.

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Want to design a PCB for your project? I strongly recommend KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

An explanation can be found here. Aside from vision issues that may make it difficult for some to read a schematic in color, it is my opinion that color does nothing to make it easier to understand the schematic.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!