Small 3v driver (info request)

[Tw15t3r]

Wow.... interesting how this all has developed over a short period of time. Following this thread has also helped me learn somethings.

Btw, where do you go to reference the parts number for the type of parts you need?
Like how did you find out that Q1 should be a FDN302P?
Any website or something?

[edit]Shouldn't an N-channel shut off the system in that case for Q1 since the negative side of the battery is connected to the base? A P-channel is correct I thought?[/edit]

 

[Viracocha]

I went for a p channel as that was what rkcstr used in the micro-drive to switch on the positive line with negative input.

Also, i've included that mosfet because the cheapest place i've found to get them is
http://cgi.ebay.co.uk/40pcs-FAIRCHI...hash=item350104200357&_trksid=p3286.m63.l1177
so if i didn't use them up for some of these drivers they'll be wasted. The only other place i've found (outside of digikey) with a decent (over 200mA) SOT-23 mosfet is charging 50p each with £5 min postage costs. digikey is out for me as i live in the uk and because everything is shipped from the us the minimum postage is £12!!!  

To find the parts i just find a shop which can supply them (not digikey ) and see what they have in stock.

 

[kernelpanic]

I went for a p channel as that was what rkcstr used in the micro-drive to switch on the positive line with negative input.

You can use that p-channel there.But you have to reverse your source and drain. Mosfets are controlled by the gate to source voltage. So upon closer examination of your circuit, even putting an n-channel mosfet there for Q1 wouldn't work. (Since the gate-source on Q1 will be too low through the zener-resistor network.) I've munged your diagram further to show what I mean...

cheers,
kernelpanic

 

[Viracocha]

So basically, swop the source and drain around. I used the image from the circuit design program directly, need to read up on mosfet diagrams

 

[Tw15t3r]

For Q1, u'd probably want a depletion type MOSFET. This is why:

There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device.

And in this case, u'd want a P-channel. Then, what will happen is that for the normal configuration, the negative is connected to the gate, allowing conductivity. If the battery is connected the wrong way round, then the positive will feed into the gate switching off the transistor.

That is.... in theory....

 

[kernelpanic]

For Q1, u'd probably want a depletion type MOSFET. This is why:
[quote author=Wikipedia link=1222777857/20#20 date=1223284948]There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device.

And in this case, u'd want a P-channel. Then, what will happen is that for the normal configuration, the negative is connected to the gate, allowing conductivity. If the battery is connected the wrong way round, then the positive will feed into the gate switching off the transistor.

That is.... in theory....[/quote]

You don't want a depletion mode mosfet for Q1. With the switch off you want no current to travel between drain and source. An enhancement mode mosfet will protect the circuit fine. Depletion mode mosfets are generally used for dual-rail powered circuits...

cheers,
kernelpanic

 

[Tw15t3r]

Oo... ok! Learnt something. Thanks!