MOSFET + Reed switch use?

[MrNaz]

I'm building a dive light and need to know how to use an N-Channel MOSFET to power it, as I am using a reed switch to turn it on and off which can only handle 500mA or so.

I have a 2P3S LiIon pack, 8.4V, and a 2S2P LED array. I will be using a Kaidomain driver to power the LEDs. In order to power the driver, I would like to switch it with a reed switch connected to the gate of a MOSFET. I need to confirm that what I plan to do will work, as I've never used a MOSFET before.

The reed switch is connected to the + of the battery and the other side to the gate of the MOSFET. The + of the battery is also connected to the + in of the driver. The - in of the driver is connected to the source of the MOSFET, and the - terminal of the battery is connected to the drain of the MOSFET.

The only thing that I'm not sure about are the voltages. The tutorial that I saw had a 5V signal turning on and off a 12V circuit. The design that I have described above has an 8.4V signal turning on and off an 8.4V circuit. Is this OK?

Also, what would be the best MOSFET to use, and where in Australia can I get one? Jaycar and **** Smith don't have them, so any sourcing advice would be greatly apreciated.

 

[Norm]

Google CPF Mosfet switch search

The diagram says mag switch but there is no reason why your reed switch won't work.
Diagram pinched from here.

Jaycar FETs on this page.

Norm

 

[HarryN]

Hi, life is slightly more complicated for LED uses. I happen to be going through this same learning curve on mosfets and LEDs, so please double check my comments here relative to people that actually know what they are talking about.

BTW - wikipedia has a great write up on mosfets.

First - your battery voltage for 3S2P I think is actually more like 12 volts, which is good, as the V battery usually needs to be at LEAST 1 - 2 volts higher than your LED series Vf (approx 2 x 3.5 volts = 7 volts)

There are two (or perhaps more) kinds of mosfets - N channel and P channel.

In the case of N channel mosfets, the voltage going to the gate must be at least 5 volts higher than the voltage going into the source / drain for it to turn on, and more is better. In the case of the diagram norm provided (Thanks Norm), this is accomplished by first letting the voltage drop through the lamp (basically a resistor), and putting the mosfet "downstream" - nearly at ground voltage. If the N channel mosfet were instead placed BEFORE the lamp, I am pretty sure that it would not turn on.

Once this mosfet is turned on - it then has to turn off. The way this happens, is you have to bleed off the voltage from the mosfet gate to the ground, in this case, through the 10 K ohm resistor. This is always bleeding off a little current, but very little, so it is not a big deal. Without this, the light will not turn off.

The advantage of N channel mosfets is that they have very low resistance.

In a P channel mosfet, you turn it on / off similarly, but instead place it between the battery and lamp. Instead of of applying a higher voltage to turn it on, you short the gate to ground to turn it on, then let the gate bleed up to Vbat to turn it off.

In the case of your LED driver, you can think of the driver as replacing the lamp, so it is better to use a P channel mosfet (I think). The P channel mosfets will have a slightly higher resistance than N channel, but sometimes you are just stuck.

You can thank many people on this forum, including MPF and billynomates for helping me start to grasp these concepts, which hopefully, I have articulated correctly here. Consider to read through some responses to my questions on a 6 amp driver project I am working on, posted here: http://www.candlepowerforums.com/vb/showthread.php?t=232655