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Which transistor to switch power?

Discussion in 'Power Electronics' started by Wilderness bob, Sep 26, 2021.

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  1. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    Hey everyone,

    I thought this was a beginner forum, I'm very much a beginner and I have a very basic question. I need to switch a max of 2A @ 6Vdc and thought I would need a jfet based on my understanding, but I haven't been able to sort through the filters on the digikey site. I thought I could narrow it down by the case type, find a case type for the mounting style then power range and go from there. On digikey, they jump from mA to 35A and really high voltages. I can't find any pages on the google box that corrilate case type with power, which I thought was the main basis for different cases. Could someone suggest the correct filters or maybe an actual transistor?

    very much appreciated
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    This is a start but you need to provide more info.
    A drawing of the circuit would determine what you are trying to do.
    For example, what is driving the switching element.?
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    A typical relay circuit might be better.

    Martin
     
  4. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    this is obviously a very simple diagram. the servo will draw a max of 1800mA stalled at 6V.
     

    Attached Files:

  5. Bluejets

    Bluejets

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    Oct 5, 2014
    Normally servos are not "switched off" as they need power and a signal to hold position.
    Other than that, micro will drive a logic level mosfet via a resistor or two.

    Once again, you have to provide more info if you want any detailed answer.
     
  6. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    i realize that, and in this case while "off" the servo will be a position that is mechanically locked so it won't need to rely on power to hold it. it will be off most of the time, a week at at time, that's why i want to switch the power off to the servo when it's in the rest position, no need to have any unnecessary power draw while not in use. it's actual resting position might change ever so slightly because of thermal expansion of the linked components etc. so i don't what the servo to be constantly trying to compensate. so, it's going to be a MOSFET because of the power draw? as for the resistors, where and why? to limit the current at the base?
     
  7. crutschow

    crutschow

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    May 7, 2021
    Do you need to switch the high (power) side or can you switch the low (ground) side?

    What is the on/off signal control voltage from the micro?

    Will the two control signal lines from the micro both be low when the servo power is off?
     
  8. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    I can switch either
    Up to 5v
    Yep, though there is only one control, the other is a feedback signal from the servo just to verify it’s actual position. I didn’t need to include that in the sketch, just ads confusion.
     
  9. Bluejets

    Bluejets

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    Oct 5, 2014
    You really need to provide more information on what it is you are trying to do.
    Include any and all devices, power supplies , micros etc. etc.

    Servos do not need a position feedback to the micro as it is all taken care of within the servo itself.

    When you say "up to 5v," what is it exactly as some micros will run on 3v3 or less and you may need to drive any transistor or mosfet in a different manner.
     
  10. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    at the rest position, the servo will come up against a hard stop, and though the servo knows it's position relative to what the micro tells it to be and where the internal pot is, it doesn't know that it should stop when it meets the hard stop. also at this hard stop the system will have completed the function and will be idle for a week or more. the linkage is such that when it's in the rest position there is a mechanical hold so the servo doesn't need to do it. hence i want to power down the servo until the next cycle. the feedback is really to ensure that i know the servo is in the correct position and that it will trigger an alarm if not.
    it's being controlled by an Arduino Nano 5V, everything powered by a 6v 2A wall wart. other than the servo and a couple of LEDs and buzzer, that's it.
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    6v will not be enough for the Nano regulator and too much (5V max) for the Vcc.
    Sounds like you need a gear motor with a limit switch at the end stop rather than a servo.
    Using the servo and it's positioning abiliity is obviously a wasted exercise.
    You could of course, rip the guts out of the servo including the feedback pot centre and stop mechanism but that also would be a waste.
    There are many cheap gear motors around with a vast array of output speeds and voltages.
    Simple enough to program a mosfet to shut off when the motor reaches a limit ( switch or optical device)
    Depends where the motor has to start again from.....
    Again...not enough information.
     
  12. Wilderness bob

    Wilderness bob

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    Sep 26, 2021
    Apparently a nano will run on 6v min. Not sure I understand why it will run happily from the USB with only 5v, does the USB bypass the regulator? Any way, I don’t understand about the Vcc, the nano output is 0-5v regardless of the vin. I have to disagree about the servo, it is ideally suited to the task. Quick response, goes to an exact position as instructed and in this case has built in feedback to verify that position. No messing around with mechanical switches that require carful alignment and as things ware, including the switch, may need adjustment. Geared motors can be difficult to control endpoints accurately, there can be a certain inertia as the motor winds down. One more part to fail. Honestly, cumbersome, finicky and crude. So why not use a MOSFET to control the power to a servo rather than a geared motor? So, while I appreciate your feedback on the project, I am not asking for a critique. I have thought about this for a long time and many alternatives. This is the route I would like to go, for my own reasons, all I need to know is how I can control the power to the servo with a nano, whether that is with a transistor, or a series of them maybe is what I have appealed to this forum to find out. You keep asking for more info, well then, I have given the exact components that I’m working with and their configuration voltages and loads, if you need more info it would help if you could me more specific. I really don’t know enough about electronics and terminology so I’m alright with a hand holding.
     
  13. Bluejets

    Bluejets

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    Oct 5, 2014
    Your 6V is feeding into a regulator via RAW, the regulator needs overhead to operate correctly, usually 1.5v.
    The usb is 5v and feeds to the Vcc.
    With a mosfet on the servo and having to provide some kind of limit to shut it down to stop overriding a certain position completely overrides the need for the servo positioning, hence the gearmotor.

    Other than that, if you cannot be told, then the mosfet will provide an easy switch from the micro.
    Use an N-channel logic level mosfet and operate as a low level switch. Provide a pull down (maybe 10k ) on the gate and a series gate resistor to the micro of say 220 ohm, both 1/4w will do.
    For the limit, use INPUT_PULLUP and wire your switch to ground.
    Don't forget to add either hardware debounce capacitor or software debounce.
     
  14. Wilderness bob

    Wilderness bob

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    0
    Sep 26, 2021
    I might need to the wart to 9v then use an external voltage regulator to get it down to at most 7.5v for the servo.
    The MOSFET is not to control the position, as you said, the servo provides the means. It is simply to reduce wear, power consumption, and heat buildup while the servo is in the rest position for extended duration. There is a built in mechanical hold with my linkage while in the rest position, but unless my PW is exactly synced to my linkage, the servo will struggle to correct. While a gear motor and two switches would eliminate this issue, I feel the the servo solution is actually simpler, more accurate, more durable and reliable, fewer parts, less cost, more compact, and for me, more fun. I’m playing with a microcontroller, why not use it? My servo has a feedback wire built in and it’s even waterproof, supposedly, not that it’s relevant. So it’s all in one nice little package. A few simple lines of code, one compact servo, a power supply and current sensor and I can get it to react to the sensor output any way I like
    , confirm the servo is in the correct position, trigger an alarm if it’s not, and reduce power consumption when idle. Pretty nifty little setup if you ask me. Both solutions are viable and would work. I feel that the servo suits my application better, one difference between them Is that the servo doesn’t NEED a MOSFET whereas the geared motor does, it’s just a feature I’d like to employ.
    No, I won’t be TOLD, but I am alway open to reason. You are very passive aggressive and while I thank you for the begrudgingly shared information, I would ask that if you can’t keep your ego in check, to refrain from contributing to any future posts of mine.
     
  15. Bluejets

    Bluejets

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    Oct 5, 2014
    Well regardless of your opinion or what you might or might not be used to , you cannot put the mokkers on any comment here.

    I've been trying to sort out just what it is you are trying to do and show the best way to go about it.

    If you need others to chime in with a similar opinion then so be it and I'll leave you to it.
     
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