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Thread: Help with Mosfet

  1. #11
    Hey thanks for the link NTE that is pretty cool.

    I think I will make a thread to document this attempt at a dumper once I get under way with it. So far It is only an idea, waiting on parts. I do understand the need for the resistor at the gate to pull it down. I was able to switch a bulb with the arduino that was driven by a 12 volt source and used a very small NPN transistor. This is the approach I have thought I would take but until I start actually bench testing who knows. I am thinking if I can already switch a 12 volt bulb than I can replace the bulb with a high power fet and then dump the cap from it. Basically a small transistor to switch a larger one for the heavy load that the cap presents, well more than one but that is the idea in a nutshell. Today I am going to try and drive the arduino from a rectified generator coil, as I said before just the board and the little npn pulls 50-60 ma so I would love to drive that from the machine rather than a supply battery. Who knows I may just smoke the darn thing but no pain, no gain huh.

    Depending on what I can pull from a generator coil I may be able to switch the mosfet with that too, if not perhaps I can pull that from the primary. I will just have to try different things to see what may be possible.

    Thanks for the advice everyone. I will post some progress as this develops. ----Bob

  2. #12
    For anyone who is also trying to understand MOSFET operation I have found a very nice paper dealing with parallel configurations and matching issues. This paper also helps me understand the general operation.

    http://www.irf.com/technical-info/appnotes/para.pdf

    ---Bob

  3. #13
    Quote Originally Posted by BobZilla View Post
    Hey thanks for the link NTE that is pretty cool.

    I think I will make a thread to document this attempt at a dumper once I get under way with it. So far It is only an idea, waiting on parts. I do understand the need for the resistor at the gate to pull it down. I was able to switch a bulb with the arduino that was driven by a 12 volt source and used a very small NPN transistor. This is the approach I have thought I would take but until I start actually bench testing who knows. I am thinking if I can already switch a 12 volt bulb than I can replace the bulb with a high power fet and then dump the cap from it. Basically a small transistor to switch a larger one for the heavy load that the cap presents, well more than one but that is the idea in a nutshell. Today I am going to try and drive the arduino from a rectified generator coil, as I said before just the board and the little npn pulls 50-60 ma so I would love to drive that from the machine rather than a supply battery. Who knows I may just smoke the darn thing but no pain, no gain huh.

    Depending on what I can pull from a generator coil I may be able to switch the mosfet with that too, if not perhaps I can pull that from the primary. I will just have to try different things to see what may be possible.

    Thanks for the advice everyone. I will post some progress as this develops. ----Bob
    To better understand MOSFETs operation I recently did this experiment:
    I used a MOSFET, a 10K pot, an amp meter and an optional load.

    The pot connects to VCC and Ground, the wiper to the gate of the MOSFET.

    The amp meter connects to VCC and the MOSFET Drain. (load optional)

    MOSFET Source connects to ground.

    Start adjusting the pot until the current starts to flow, Note the voltage at the gate when the current barely starts to flow. This will be around 3-4volts but varies with the MOSFET.

    When the MOSFET is not fully open it acts like a variable resistor, expect the MOSFET to get burning hot in seconds if you have no load in the circuit. If VCC is 12v and your load drops 10v the mosfet will need to dissipate (2v x current flow) watts.


    A couple years ago tried pulsing a SSG style coil with an arduino and a NPN, the arduino kept freezing up and would need to be reset. Adding an optocoupler solved this problem for me.
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  4. #14
    Great advice, thanks!

    I have been reading up quite a bit on both mosfets and bjt's. I am quickly realizing that there is much more to understand than I thought previously.

    I expect that I will be tinkering for quite awhile before I can really get to the business of making a circuit behave properly. I will probably burn up quite a bit of components in the process but it's all part of the learning process.

    BTW:
    I did try driving the arduino from a rectified generator coil and a regulator. It worked but at least in my setup it was not really practical. I had to increase my wheel load too a point to get the speed required and in doing so I think it would be better to just pull from the primary directly. It was neat to tinker with but not practical. Maybe if I dialed in the coil length, core size and wire size to produce exactly the right voltage and current it may have an advantage but for my initial testing with a coil I had laying around it was not gaining me anything to drive it this way.

  5. #15
    Quote Originally Posted by John_Bedini View Post
    Just a tip of advise if your working on this cap dump charger. You need the resistor 10K to pull the gate off or the fet will stay on. The SG is ideal you just need to understand how to use it. It can be made to run at 10 to 100 cycles but remember that is divided by two. Pin 10 of the SG can be made to regulate the battery. The best data sheet is the TI somewhere I have the original RCA data sheet that tells you much more then what is now on the net. You should run this in inverted mode you just need to switch the fets around. Many things can be done with SG 3524 that most people do not understand, but then again I'm an Analog Engineer and have been so for 40 plus years. SCR will work and you need nothing to turn them off if you run inverted mode with them, you can not run them above 30 cycles or they will not turn off, think about what I said here.
    John B

    One more thing I want to bring up is that if your going to run the SG at high voltage use a resistor zener diode combination. also zeners can be used on the gates on the fets for over voltage.

    Attachment 2626
    Hi John,

    You are a huge inspiration for me and thank you so much for all the awesome info you have provided.

    Regarding the Comparator MOSFET Cap dump, I am able to get the MOSFET to switch on and off ok when I test it to just turn a light on and off but as soon as I hook it up to my caps I can't seem to get the MOSFET to fully turn off even with 0V at the base.

    Also, is it imperative to use photo flash caps? Right now I have 4 6800uF Photoflash caps that seem to be working well. Just wondering.

  6. #16
    Also, I should mention I do have the 10K resister from gate to source aswell

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