reed activated mosfet switch diagram

[SemiMan]

Thanks for the help so far!

It's true, I'm not that concerned with price if we are talking about a few dollars. I looked at the digikey links and they seem to be a smaller package size as the NTE (around 1/2 inch) but the TO-220 is listed as 6mm...which is about 1/4". How hot would these get in practice? I'm hoping to not heat sink them at all in terms of ease of installation.

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Giving you are putting it inside of the flashlight where convection is going to be limited, I would not go much beyond 0.5W but since the goal is to put power into the LED and not the FET, go with as low of resistance as is practical (cost, etc.). Also keep in mind that you have to look at max on resistance which is often specified at room temp and then adjust for maximum temperature. Hence for this reason keeping the FET cool by keeping the on resistance low makes a lot of sense.

For the ones I specified, even on a single cell, I would estimate the RDSon at 0.01 ohms or so worst case assuming a gate voltage of 3.7. Let's call it 0.015 just to be careful. That gives you about 6 amps, but I would keep it well under this to ensure the part stays cools and the RDSon stays low. Keep in mind you can parallel them to reduce the on resistance.

Semiman

[archer6817j]

Hey if anyone is looking for reed switches I found these on ebay and really like them. They are very sturdy and they have SMT pads instead of wires that are easy to solder since you don't have to bend the wire leads. I have no affiliation.

http://shop.ebay.com/switchkingone/m...&_trksid=p4340

[archer6817j]

UPDATE: do not use the NTE2839 MOSFET as indicated on my drawing

Especially if you don't know anything about electronics...like me. I had this in a 2s 18650 light (P7) that draws about 2.8 amps. Fortunately I noticed that the MOSFET got 'really' hot before it set anything on fire. It was melting wires that it was touching. I got lucky and opened the light, it was working intermittently, and there was smoke inside from melting insulation.

You are going to want to use this MOSFET, which was recommended by CPF member, SemiMan: http://search.digikey.com/scripts/Dk...=497-9093-5-ND

I installed in place of the NTE2839 and it barely gets warm to the touch. Awesome!

[MikeAusC]

The NTE2389 seems to have a typical gate threshold of 3V, but with min and max of 2V1 and 4V.
Also, the threshold voltage is where the FET starts to turn on, not where it's completely on.

Depending on luck with the individual FET, it is possible that the circuit might not work.

Words of Wisdom !!!

Junction transistors need less than one volt to turn on fully, but do draw some current.

FETs draw zero current for a DC switch, but only certain ones will switch on fully using the 3.0 volts from a single LiIon cell at the end of its discharge. If it doesn't switch on fully, it has a voltage drop which produces heat in the FET.

[fyrstormer]

Why worry about 1 milliamp being used by the resistor? That's easy: because it's wasted power. The power being used to drive the LED is not wasted because it's doing useful work -- or at least, the power that is wasted by the LED producing heat is beyond the control of the designer, but the power being wasted by the resistor is within the control of the designer, so it's a valid target for improvement.

Don't ask "why try to make it work better?", ask "why not try to make it work better?" "Why not?" is the question that drives progress.

[darkside501st]

Why worry about 1 milliamp being used by the resistor? That's easy: because it's wasted power. The power being used to drive the LED is not wasted because it's doing useful work -- or at least, the power that is wasted by the LED producing heat is beyond the control of the designer, but the power being wasted by the resistor is within the control of the designer, so it's a valid target for improvement.

Don't ask "why try to make it work better?", ask "why not try to make it work better?" "Why not?" is the question that drives progress.

Wow, I have been looking for a solution to my problem for three days. While this is an older thread and you brought it back from the dead I might not have seen it otherwise. I just wanted to say thanks to everyone that posted all the helpful information here.

I am working on a project using a fade-in/fade-out circuit for LEDs. I am building some repulsor props for a friend of mine who has an Iron Man costume. He wanted to use some reed switches to activate the LEDs but when I tried one it got stuck in the closed position. I am not sure what caused it. The rating on the switch is 500mA. My circuit draws about 50mA so I don't think that it was excessive current. Anyways, I thought that isolating the switch from the circuit through a relay was the answer. I was looking at some relay circuits when I came across this forum/thread. I am just a beginner working with circuits and components. I would not have thought about using a Mosfet instead of a relay. So thanks again for all the help given in this thread. I am going to start some experiments now.

Well, I did some more experiments and unfortunately using a mosfet as a switch won't work with my circuit. My circuit needs to maintain a connection to the ground in order for the capacitor to drain through the LEDs and produce the fade-out effect. I am not sure if it is possible to connect the mosfet in this manner.

This is my circuit in case anyone wants to take a look over it and tell me if they see a way around this issue.



My power source for this circuit is 4 AAA batteries at 6v (6.5v w/fresh batteries). I am using a BC548 at Q1 and a BC327 at Q2. I am also using a different power transistor, 2N5296, at T1 but I don't even know if I need that. The max current for the LEDs will be 240mA - 720mA max depending on the LEDs I use.

It was mentioned earlier in this thread that a DPST reed switch would be a good idea but I don't quite understand why or what the purpose of that would be. I would love for someone to go over that with me.

Then, I am just wondering if there is a way to use a mosfet in this circuit or if there is something else I need to do to protect the reed switch from being damaged. It is possible that it was a defective switch but I am afraid of messing up another one.

My friend already has SPST reed switches and I have to activate this LED circuit and a sound module at the same time. I was thinking of looking for a DPST or DPDT relay to activate both at the same time. Otherwise I would have to use two mosfets, right? The sound module will be running off of the same power source; but I am not connecting power to it, I am closing the switch to activate the sounds. I know a relay would do that but I don't know about the mosfet. It seems that the power across the switch has to be at a certain voltage for the IC to play the sound. I can't just send power from the battery to the other side of the switch. I have to send power from one side of the switch to the other. Would the mosfet 'corrupt' that power/voltage signal to play the sound?

One thing I did find while looking at how to protect the reed switch was that most companies selling them recommend using a relay and on DC circuits to wire a diode (1N4004) parallel with the relay. I don't understand the mechanics of what is happening there or why the resistor/mosfet setup in this thread works without it.

Any help would very much be appreciated. Thanks, Mike

[darkside501st]

Ok so it turned out that using the Diode 1N4004 was all that was needed. I hooked it up in parallel with the load with the cathode connected just after the switch and the anode connected just after R5 on the negative rail. It worked like a charm... I tested it for several minutes without any issues.