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Thread: Neon/Fans/Anything controlled from your PC

  1. #11
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    Quote Originally Posted by Andrew_C View Post
    I have no experience in using them. However from reading up they are simply transistors which accept very low current.
    Then you don't read up the right places.
    You can get power MOSFETS that can handle very high load currents. They are used in most switched mode power supplies (SMPS) like all PC power supplies and here they handle very high currents. In SMPS the MOSFETs switch at high frequencies and here a high percentage of the power dissipated in the MOSFET is actually not dissipated during on mode, but in while switicng between on- and off modes.
    But if you just use it instead of a realy and not for a PWM circiut, then then you don't have to worry much about swithing losses, here you dissipate all the power in on mode.



    QUOTE=Andrew_C;1288360]Any recommended part numbers for the MOSFET? I normally use 2N2222A transistors. Saving on components would be nice.[/QUOTE]
    You should look for N-channel power MOSFETS. The lower RDS(on) (resistance from drain to source in on mode) the higher current the MOSFET will be able to handle - and the larger and more expensive it will probably also be. The good thing is that MOSFETs for car applications only have to handle 12V, so instead of picking a MOSFET that can handle high volatages, pick one that has low RDS(on) instead.
    When you know the RDS(on) and the current you load is going to draw you can easily calculate how much power is dissipated in the MOSFET while it on.
    Let's say you have a 1 Ampere load and a MOFET with an RDS(on) of 0.1 ohm.

    You can for example look for a suitable N-channel power MOSFET here: https://ec.irf.com/v6/en/US/adirect/...N=0+4294841672
    International Rectifier is one of the biggest manufacturers of power MOSFETs.
    But try to check which N-channel power MOFETs your preferred electronic componets store sell and pick one of these.

    It's also important to chose a MOSFET with low enough Gate-Soruce threshold voltage for your input signal to be able to turn it on.
    The Gate input impedance is virtually so high that you draw zero current from your input signal. This also means MOSFETs are very sensitive damage from static electricity, especially before you have monutented it in the circuit.

    When switching inductive loads like fans, remember to ad a flywheel diode to protect the MOSFET from back-EMF.

    Read this tutorial:
    The MOSFET as a Switch
    http://www.electronics-tutorials.ws/...or/tran_7.html


    If you want to use an opto-isolator you can get MOSFETs circuits with build in opto-isolator that can handle both a.c. and d.c. loads. There are also know as photovoltaic relays: https://ec.irf.com/v6/en/US/adirect/...N=0+4294841620
    IR has a photovoltaic relay that can handle continuous load currents up to 6 Ampere DC, packed in a small 6-pin DIP package: https://ec.irf.com/v6/en/US/adirect/...ame=PVN012APBF

  2. #12
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    Quote Originally Posted by mayhembdm666 View Post
    A resistor can short...So this is still unsafe.

    For proper protection, You need to have a safe gap for the circuit..
    Resistors hardly ever short circuits, their failure mode is usually open circuit.
    But the resistor in front of an opto isolator is just as likely to short circuit as a resitor in front of a MOSFET, so you are not anymore protected from a short circuited resistor if you use an opto isolator as if you use a MOSFET.
    I used to repair electronic devices for a living for several years and short circuited resitors is a not issue at all really - in reality it hardly ever happens, I can't rember seeing short circuited resistors. Wirewound resistors are maybe more prone to this, but they are not used here, but only for some power applications.

    An opto-isoltated circuit does however offer more protection, it can protect from reverse voltage and large over voltages. But you don't need an opto isolator to simply protect from a short circuited load transistor. The opto-isolator could short circuit the inputs pins too and draw a lot of current. But a resitors in fron of the opto-isolator or load transistor could prevent overloading the output port in case of a short circuit.

    If you want opto-isolation and MOSFET you can use a Photo Voltaic Relay as suggested in my earlier post. This is a good and small solution and I think the price is about the same as a regular realy, while it's much smaller and doesn't need a transistor to drive it if your output can supply enough current for the LED in the Photo Voltaic Relay, and you don't get the clicking noise of regular relays.

  3. #13
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    Its not a common fault, But can and does happen.
    The opto is used to create safe zone that can not be shorted regardless.

    If by chance u do ever have a resistor short then the next part inline gets the full voltage, Having the emitter on the opto fail simply means it will not pass that point in either case
    2004 Holden WL Caprice Auto GENIII
    Base System = Raspberry Pi
    Everything else is pending for now as switched from a Mini-ITX setup

  4. #14
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    Ah, sorry I came across wrong. I know they can handle high load currents. I was referring to the switch current required being low - hence the sensitivity.

    I have to say you definitely know your stuff in this area and thanks a lot for your input. I am interested in the Photo Voltaic relay but I can't find a good source of them in the UK.

    I don't want to over-complicate this project. I wont be drawing any more than 2A I shouldn't think (which is what my relay is rated at). Mainly will be controlling automotive relays - I think their coils are around the 200 milliamps.

    The simplest circuit design so far with the least cost involved is the MOSFET + resistor.

    My main source of components for small projects is Maplin.co.uk as they have a store very close by to me.

    >>>>>>>>>>>>>>
    http://www.maplin.co.uk/Search.aspx?...SFET&source=15
    >>>>>>>>>>>>>>

    Is my selection limited so much as that none of them would work?

  5. #15
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    Quote Originally Posted by Andrew_C View Post
    Ah, sorry I came across wrong. I know they can handle high load currents. I was referring to the switch current required being low - hence the sensitivity.

    I have to say you definitely know your stuff in this area and thanks a lot for your input. I am interested in the Photo Voltaic relay but I can't find a good source of them in the UK.

    I don't want to over-complicate this project. I wont be drawing any more than 2A I shouldn't think (which is what my relay is rated at). Mainly will be controlling automotive relays - I think their coils are around the 200 milliamps.

    The simplest circuit design so far with the least cost involved is the MOSFET + resistor.

    My main source of components for small projects is Maplin.co.uk as they have a store very close by to me.

    >>>>>>>>>>>>>>
    http://www.maplin.co.uk/Search.aspx?...SFET&source=15
    >>>>>>>>>>>>>>

    Is my selection limited so much as that none of them would work?

    If you only need to drive automotive relays, use a ULN2003 (16-pin package) Its cheap, has built in flyback diodes and has 7 drivers in them. If you need 8 since I suppose your using a parallel port, use the ULN2803

    P.S. you are using a parallel port right.

    -dave

  6. #16
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    Yes the parallel port.

    I have looked at the data-sheet and read a few forums but I can't figure out the logic to how it works (ULN2803). I get what the diodes are for, but I can't figure out how to put it on a PCB??

    Please help?

  7. #17
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    Quote Originally Posted by Andrew_C View Post
    Ah, sorry I came across wrong. I know they can handle high load currents. I was referring to the switch current required being low - hence the sensitivity.
    Ok, I see what you meant. It's correwct that MOSFETs require virtually zero switching current, which is a good thing.

    Quote Originally Posted by Andrew_C View Post
    My main source of components for small projects is Maplin.co.uk as they have a store very close by to me.

    >>>>>>>>>>>>>>
    http://www.maplin.co.uk/Search.aspx?...SFET&source=15
    >>>>>>>>>>>>>>

    Is my selection limited so much as that none of them would work?
    I have had a look at Maplin's selection of N-channel power MOSFETs.
    What is your switing source? A parallel port?
    I would pick IRFZ24N, this should be fine for most purposes and is cheaper than some of the other power MOSFETs listed.
    http://www.maplin.co.uk/Module.aspx?ModuleNo=33843
    Datasheet: http://www.irf.com/product-info/data...ta/IRFZ24N.pdf
    VGS(th) Gate Threshold Voltage 2.0 ––– 4.0 V VDS = VGS, ID = 250μA
    This means that worst case the MOSFET will require an input voltage of 4V before it switches on. So your outputs from your PC needs to a above 4V + a smale safty margin to be sure your MOSFET switches on. You can try to measure the output voltages of the port you will use to check if they are above 4V.
    If not you will need a MOSFET whith a lover G-S threshold voltage. But I think you will be fine with IRFZ24N.

  8. #18
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    I only get 2.93 from the parallel port.

    Is there a downside to using the uln2803? as I am only switching 200ma automotive relays


  9. #19
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    Quote Originally Posted by Civic Modz View Post
    If you only need to drive automotive relays, use a ULN2003 (16-pin package) Its cheap, has built in flyback diodes and has 7 drivers in them. If you need 8 since I suppose your using a parallel port, use the ULN2803

    P.S. you are using a parallel port right.

    -dave
    Yes if you want to drive several realys ULN2003 or ULN2803 is a good idea.
    These are darlington arrays (two transistors in a row) which means they draw much less current from the input than a single transistor would do.
    If you only need to switch low current loads, you could connect them directly to the darlington array an avoid using a realy at all.

    Ypu can also buy these at Maplin.

    ULN2003A http://www.maplin.co.uk/module.aspx?moduleno=5502
    Datasheet: http://www.st.com/stonline/products/...79/uln2003.pdf

    ULN2803A http://www.maplin.co.uk/Module.aspx?ModuleNo=2395
    Datasheet: http://www.st.com/stonline/products/...6/uln2803a.pdf

  10. #20
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    Quote Originally Posted by Andrew_C View Post
    I only get 2.93 from the parallel port.
    Whit or without any load connected?

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