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Thread: reed activated mosfet switch diagram

  1. #31

    Default Re: reed activated mosfet switch diagram

    Quote Originally Posted by archer6817j View Post
    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.

    .
    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

  2. #32
    Flashaholic* archer6817j's Avatar
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    Default Re: reed activated mosfet switch diagram

    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

  3. #33
    Flashaholic* archer6817j's Avatar
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    Default Re: reed activated mosfet switch diagram

    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!

  4. #34
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    Default Re: reed activated mosfet switch diagram

    Quote Originally Posted by uk_caver View Post
    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.

  5. #35
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    Default Re: reed activated mosfet switch diagram

    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.

  6. #36
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    Default Re: reed activated mosfet switch diagram

    Quote Originally Posted by fyrstormer View Post
    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
    Last edited by darkside501st; 08-04-2012 at 06:26 PM. Reason: added some information and questions

  7. #37
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    Default Re: reed activated mosfet switch diagram

    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.

  8. #38
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    Default Re: reed activated mosfet switch diagram

    i work with this switch ,but with a 1s lithium battery it does not work
    can somebody help me please
    greetings luc
    http://maxlight.biz/img/1g.jpg
    Last edited by landries; 11-28-2013 at 07:04 AM.

  9. #39
    Flashaholic* uk_caver's Avatar
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    Default Re: reed activated mosfet switch diagram

    The gate threshold voltage for that FET (2V0 to 4V0) is too high for reliable switching from a 1C setup.

    What kind of load current are you switching?

  10. #40
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    Default Re: reed activated mosfet switch diagram

    10A at 3.7V

  11. #41
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    Default Re: reed activated mosfet switch diagram

    nobody to help me with this problem
    greetings

  12. #42
    Flashaholic* uk_caver's Avatar
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    Default Re: reed activated mosfet switch diagram

    Briefly put and somewhat simplified (someone please correct me if I'm missing something), the threshold voltage is the gate voltage at which a MOSFET begins to turn on.
    More voltage is needed to turn the FET 'more fully on' (graphs of 'transfer function' in a datasheet should illustrate that).

    Threshold voltage varies with temperature, and for a given device, will also vary significantly between one device and another - datasheets might specify some combination of typical, minimum and maximum threshold voltages, but they aren't consistent in which of those figures they quote.

    Therefore if you want the FET to be adequately turned on with a ~3-3.5V gate voltage, I'd be tempted to choose a FET where the maximum gate voltage was specified in the datasheet, and that maximum was low.

    Also, you do want a fairly low 'on' resistance if you're looking at a 10A load, to avoid wasting voltage and heating up the device.

    Given how many MOSFETS there are around, recommending a particular one is tricky - I don't know how 'popular' particular devices are, and therefore how likely they are to be stocked by particular distributor.

    If I was buying from my supplier (Farnell):

    I'd start by narrowing my search to N-channel devices in TO-220 packages (if I was happy with a device that size)
    Then select devices with a threshold voltage below ~1.5V (that's likely to be a 'typical' figure for the device, not a maximum).
    Then I'd probably order by threshold voltage and work through the list looking at low resistance devices, looking at the linked datasheets to see how various devices compared.

    Bear in mind that the 'on' resistance is often quoted for a higher gate voltage - at lower gate voltages the resistance will be higher and so you will want to allow for that.

    For example,
    http://www.irf.com/product-info/data...irl3303pbf.pdf
    came up as a possible device, but the datasheet only gives a minimum threshold voltage (1V) and looking at the graphs for a [presumably 'typical'] device, something like 3.5V at the gate would be needed for a 10A load.
    Similar devices from the same manufacturer also only specify a minimum threshold voltage, so I'd tend to ignore them as well.

    However,
    http://www.st.com/web/en/resource/te...CD00001901.pdf
    looked possibly rather better. (typically capable of 25A load at 2.5V gate voltage)

    Just to double-check, I upped my threshold voltage limit to 2V, and ordered the results list to show devices where the 'on' resistance was specified at 5V rather than the more common 10V, since that was likely to show devices tailored for low gate voltages.
    Doing that, I found:
    http://www.nxp.com/documents/data_sh...K954R4-80E.pdf
    Which I had initially excluded since the threshold voltage was higher (1.7V), but that was the typical voltage rather than the minimum, and the range was tighter - the maximum figure wasn't much higher (2.1V), and was rather lower than the maximum for the previous device.
    Typical max current at 2.5V gate voltage seemed slightly better as well.

    Of the three, I'd probably go for the last one, or some similar related device.

    I'd probably buy a device or two to play with and measure what the voltage drop across a particular one was at full load and a nearly-empty lithium cell - given how much devices can vary, there could be meaningful differences between one and another.

    However, those variations can also make recommendations based on experience tricky - someone could come along and say 'Just use an XYZ1234 - it works fine' and then you buy one and it doesn't work for you, just because they were luckier than you.

  13. #43

    Default

    Though to-220 may be easier to work with, for low voltage I would suggest not limiting yourself to that package and consider at least dpak/d2pak smt devices. Given the likely low heat I would even look at other smt devices and just solder wires to the pins. Going smt will give you 10 times if not 50 times more options for mosfets with the specs you need.

    Semiman

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