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Thread: looking for some help

  1. #1
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    Default looking for some help

    Hello
    I am trying to design a 4 channel dc ssr that will be used with 12 v or 5 v DC only

    as I am in the UK the standard 512-FQPF13N06L MOSFET 60V N-Channel QFET Logic Level used in the designs on the wiki are hard to get over here and most websites state they ship from the USA.

    would this part be inter-changeable?

    IRF540 n channel mosfet

    here is the date sheet

    http://www.st.com/web/en/resource/te...CD00003275.pdf

    Regards

    Stephen
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  2. #2
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    Default Re: looking for some help

    Yes, you can use it as a switch.
    The gate voltage spec has a range of 2V to 4V when the switch will start to turn on. This range can be at the higher end for your units, causing your Arduino to not fully turn the lights on. In this case you may need a driver transistor between the Arduino output and the mosfet's gate. Hence the 12V supply would be better to ensure a good switch on state.
    How to implement it can be varied. Some use an opto-isolator, some don't.
    An opto example is here .. https://arduino-info.wikispaces.com/...PowerFetSwitch
    There may be other designs you can find, to reduce parts count, if you search for it.
    Or you could experiment on your own - if you have the basic tools to evaluate your results.

  3. #3
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    Default Re: looking for some help

    if I used version 1 schematic would i need to change the resistor voltage between the opto and transistor

    http://www.doityourselfchristmas.com...y_Instructions

    I am happy to go with out the optos but if it is going to struggle I will just use them
    2016 pixel power RAF Wittering soon to be RAF Akrotiri
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  4. #4
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    Default Re: looking for some help

    Quote Originally Posted by kartingted View Post
    if I used version 1 schematic would i need to change the resistor voltage between the opto and transistor ..
    No, you may leave them as they are. You can also try the circuit as is, but use the IRF540 for the mosfet and see if it works for you.
    You didn't indicate your lights' power supply, nor current load you plan to drive.

    The mosfet is operated as an ON-OFF switch. The higher the gate voltage the harder the mosfet turns on, until it saturates. It takes anywhere from 2V to 4V to start conducting, according to the data sheet. The gate voltage to cause saturation has a variation from device to device and thus can't be given accurately. You can use the transconductance graph to get a ball park value.

    If your mosfet gets hot, or the lights don't reach full brightness, it may indicate that the mosfet is not turned on hard enough. In this case you should use a 12V power supply to drive the opto transistor so that gate of the mosfet will get close to 12V of drive.

  5. #5
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    Default Re: looking for some help

    My strips are 12v dc and run around 1.5 amp max

    I have some 5v strips but I'm not sure if I'm going to use them this year,

    If I use version one design with a 12v regulator on it I should be ok and will get full brightness
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    Default Re: looking for some help

    Yes, it should.

  7. #7
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    Default Re: looking for some help

    Thank you

    now time to design in eagle should have it done by tonight

    thank you for your help Lightup
    2016 pixel power RAF Wittering soon to be RAF Akrotiri
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  8. #8
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    Default Re: looking for some help

    You need to look at the i-v curves in the datasheet to see how well the part will work in your app. I can't do it myself now because my phone won't display pdf files. Since the part is likely in the same family as the irf530 mentioned in an earlier thread, it probably needs more than 4V to get to a sweet spot on the i-v curve.
    Phil

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    Default Re: looking for some help

    Quote Originally Posted by P. Short View Post
    ... it probably needs more than 4V to get to a sweet spot on the i-v curve.
    Yes, the mosfet turn on is listed between 2V (min) and 4V (max). Hence the 12V drive supply would be a better choice, as recommended.

  10. #10
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    Default Re: looking for some help

    If you look at the 'Output Characteristics' graph, you will see that the part will typically pass only about 2A with a 4V gate-drive. I'd imagine that it's a lot worse with a 2V gate drive. It should be noted that this is a 'typical' value, not a guaranteed value. The 'Gate Threshold Voltage' parameter listed in the 'ON' section of the datasheet is a largely worthless characteristic, as the 'ON' state is defined there as passing only 250uA of current.

    I think that to guarantee good results the design should go for at least 5V of gate drive. There are many ways to achieve this, although I think that the design using opto-isolators is the safest way.
    Phil

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