design minus micro controller

Discussion in 'General Electronics Chat' started by clintster7, Jul 3, 2010.

  1. clintster7

    Thread Starter New Member

    Jul 3, 2010
    4
    0
    hi I am not very smart .. lol I may be in over my head ..

    I want to build an electric motor controller but only as an amp protection and need some guidance ..

    most large DC motors require controller that at bare minimum have an voltage control to keep the DC battery supply from having to many amps pulled ..(this may not be the correct terminology) sorry.

    what I am wanting to do is get a better understanding of some components used in most controllers ..

    I will give you what my project is and why it is not available on the market .

    I will give you a scaled down version of my project .. After I finally get a grasp on the blue brint I can scale it up.

    A 12v DC battery hooked directly to an electric fan for example .. I want the fan to pull all the amps it needs unless it is under extreme load .. under to much load it will pull to many amps from the battery .. I want it to have the maximum volts available but I need some way to regulate the maximum amps to the fan so I do not short circuit the max amp rating on the Battery.

    The problem is that many dc controllers are very complex and have speed (voltage control) and motor startup settings .. I need to design a simple control that all it does is give a non-adjustable amp current protection ..

    help me understand MOSFETs in parallel for first order of business . I do not need the home built amp protection to communicate with the motor
     
  2. Dx3

    Member

    Jun 19, 2010
    87
    7
    It seems to me that fuses were invented for this purpose. They do not interfere with the current flow until it becomes excessive.

    What do you want that a fuse can not do?
     
  3. clintster7

    Thread Starter New Member

    Jul 3, 2010
    4
    0
    well a fuse will be in line but I need it to hold the amps to a usable level .. From the battery side .. Say 80% of the the batteries maximum output .. An example woud be a motor that could possibly pull a few more amps from the battery than the short circuit rating on the battery..

    if the battery has a short circuit of 10 amps and I don't want the motor to have more than 8 amps . How can I put a cap on the amps avalible for use for the motor ?
     
  4. Dx3

    Member

    Jun 19, 2010
    87
    7
    I think you want a constant current supply. They allow anything to pass until the current limit, but they use up volts to do it. There is the rub.

    I'm attaching a datasheet for an LM317 chip. It contains more than one way to do a constant current supply. After you study it, you might have more questions, or you might realize that having your 12 volt battery never able to provide more than 10 volts is a real problem.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You realize, for the motor to stay at speed, while under load, it will require more amperage than no load.

    Also, If you are going to slow the motor wile still having such a load, the motor will not be able to cool itself properly under load.

    You may want to re-consider your Volts/Amps thought.

    The proper way to do it, is to set the amperage maximum, but allow the voltage to increase to allow the motor to do its thing.

    You can incorporate a DC-boost design to ramp up the voltage above the batteries 12v when the fan requires more wattage.

    You can get more wattage 3 ways. Either increase VOLTS, increase AMPS, or increase BOTH.

    If you only want 12v and 1.1amps, constantly, you would use a Constant Voltage Regulator and a Constant Current Regulator

    The LM317 that DX3 mentioned in the previous post can do both. You will need 2 of them, one for voltage one for current.
     
  6. clintster7

    Thread Starter New Member

    Jul 3, 2010
    4
    0
    that was great .. I think i have a better understanding of what they do .. now it has this statement "The LM317 is versatile in its applications, including uses in programmable output regulation and local on-card regulation. Or, by connecting a fixed resistor between the ADJUST and OUTPUT terminals, the LM317 can function as a precision current regulator. "

    I think this is what I am looking for. right ? I do not want to step down the voltage I just want to limit the current amps . am i thinking about this correct ?

    Is what they are talking about above talking about .
    ((Figure 4. Precision Current-Limiter Circuit)) on page 8 ?

    thats a lot to take in the first time I have looked at something that is so many uses . I am kinda stumped but am not giving up .
     
  7. clintster7

    Thread Starter New Member

    Jul 3, 2010
    4
    0
    that is why this is not a usual controller .. It is not a fan but I was just using the fan as an example of what I am trying to achieve.. Ill give you more information on its actual application . This is very unique in the fact that it is a motor attached to another motor .. the controller for the 1st motor is a standard controller but very expensive .. I will dish out the $$ for the first complex motor controller .. but the second motor is a booster motor that will already be near its peak efficiency but will be under some load when it is switched on ..

    So the second motor is the booster motor only needs a 20 second of max power that the battery can give ..

    when I say "max power" I meant 100% of the batteries voltage and 80% of the max amps that the battery is capable of producing I will give an example using a automotive 12 volt battery ..

    using a 12v automotive battery with a 400 amp crank rating I would want the whole 12 volts to the electric motor but only 80% of the max amp rating (320 amps ) now the voltage on the battery will start to drop as the seconds go by. this is pushing the batteries output to it 80% maximum (but no more).. its more of a "how much power can I drain out of a battery in the shortest amount of time without melting the electric motor or blowing up or damaging the battery .. thats the best I can explain it .. I need protection for the battery no mater what the motor does . but the motor will only pulse boost for a short amount of time . say less than 20 seconds but only once then the battery will be fully charged before the pulse boost motor will be used again . you can see how that motor starting amps is not needed in this application because the boost motor will already be turning not far from its max recommended rpm range ..

    I assume i could put multiple MOSFETs in parallel to achieve higher amp output ?or is it series mosfets I am lookin at ..now Im confused because Im up too late .. I ll get back to this tomarrow ..

    oh and thanks for your patience with me as this is not my field of expertise . But I guess that makes me a cross trainer .. LOL THANKS
     
    Last edited: Jul 4, 2010
Loading...