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555 pwm controller.

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Discussion in 'Project Construction Technologies' started by HellasTechn, Mar 4, 2018.

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  1. Mar 4, 2018 #1
    HellasTechn

    HellasTechn

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    Hello friends.

    A colleague gave me a nail drill that was damaged (rpm stuck to max). I found the controller chip shorted and can not find a replacement anywhere (local stores-ebay-aliexpress).
    It uses an 1Amp transformer to bring 220V down to 18 and a bridge-capacitor circuit to make it DC.

    I thought of using an LM317 but that dissipates way too much heat to be used in such small box.

    So i came up with the idea of using a 555 in free run mode as pwm controller and feed the output to a power transistor that would then drive the motor.

    Any thoughts , ideas ?
     
  2. Mar 4, 2018 #2
    Minder

    Minder

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    For one, you rarely need regulation for PWM/motor control, the demand cannot be that high if only a 1 amp Txfr?
    The drill motor must draw way more than that.
    A little more explanation of what you have and wishing to control is needed.
    You can obtain 555 controllers on Ebay for a couple of $$'s.
    M.
     
  3. Mar 4, 2018 #3
    HellasTechn

    HellasTechn

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    i was thinking to build the circuit pictured below.
    not really. Anyway the transformer is indeed rated 1A.

    Yes i already have a few.

    To put it simple i need to control the speed of a motor that works with 18Vdc using a potentiometer. The current consumption will vary depending on the motor load (since it is intended to work on nails) but it will not exceed 1A since the transformer is rated 1A.

    The 555 pwm looks a simple and cheap solution for me. i guess that the power transistor heat dissipation on a small heat sink under 1 amp will be ok for use within a small plastic box.
     
  4. Mar 4, 2018 #4
    Minder

    Minder

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    Using a 1amp supply for a device that may draw 1a or more is using an undersized supply IMO.
    This is not a large supply, I would have used at least a 5a supply for margin.
    If you already have some of the suggested PWM controllers then it will have a power Mosfet in it.
    Also they normally come with smoothing caps.
    So basically all you need is a Txfr and a bridge.
    Basic PWM. http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
    M.
     
  5. Mar 4, 2018 #5
    HellasTechn

    HellasTechn

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    The original power supply was 1A so that makes me think that it will not draw more than that. (note that the motor is also rated 18Vdc)
    What i do not know is what impact will the new circuit have on current being drawn by the motor.

    So if i understand correctly your advice is to use an actual pwm controller not a 555 as a pwm controller.

    what is Txfr ?
     
  6. Mar 4, 2018 #6
    Minder

    Minder

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    The one i linked to IS a 555 PWM controller, one most common way of implementing it, as does the cheap ebay versions.
    Txfr shorthand for Transformer.
    Incidentally What was the original IC,?
    M.
     
  7. Mar 5, 2018 #7
    HellasTechn

    HellasTechn

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    Ho i am sorry. i didnt really pay too much attention on the title.

    I dont remember it right now but tomorrow i will let you know since i am away and dont have it with me.

    This circuit you linked should be fine with the about 22V that DC from the bridge-capacitor right ? I know that the 555 accept 18V max. should i add some resistor load to stabilize the supply voltage to 18V ? Or better use an 7805Vreg to feed the 555.
     
  8. Mar 5, 2018 #8
    kellys_eye

    kellys_eye

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    No. The motor will try to draw what it needs even to the detriment of the transformer (overloading it, blowing any internal thermal fuse etc).

    If you have a transformer that has an output that EXCEEDS the drill requirement THEN the drill will draw 'what it needs' and the transformer will survive.
     
  9. Mar 5, 2018 #9
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Assuming that this transformer is what was supplied with the drill, then a simple PWM controller based on a 555 should be fine.

    When you say "nail drill", I'm assuming it is some sort of device used for cosmetic purposes by a "nail technician". I can quite easily see that something like this would be low power.
     
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  10. Mar 5, 2018 #10
    WHONOES

    WHONOES

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    Try the attached circuit for a very simple 555 PWM motor driver using an N channel MOSFET. In the circuit diagram, the motor is represented by R1. It should have diode across the motor with the cathode connected to the + supply.
     

    Attached Files:

    HellasTechn likes this.
  11. Mar 5, 2018 #11
    Minder

    Minder

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    Because the 555 circuit will be fairly low current and stay steady enough, You can use a simple series resistor a cap and and 15v zener to drop the 555 voltage.
    M.
     
    HellasTechn likes this.
  12. Mar 5, 2018 #12
    WHONOES

    WHONOES

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    Up to a point. But you must bear in mind that the FET gate pulse current will have a fairly high amplitude which would necessitate a fairly low impedance volt reg circuit. For my circuit, I would recommend, for a 24V supply, a 100Ω resistor and 3 to 5W 12V zener and at least a 220μf cap.
     
  13. Mar 5, 2018 #13
    Bluejets

    Bluejets

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    Beats me why you don't simply replace the original crook chip.
     
  14. Mar 5, 2018 #14
    WHONOES

    WHONOES

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    I think that HellasTechn claimed that he had tried to find a replacement but could not. You must be an Aussie with that turn of phrase!
     
  15. Mar 5, 2018 #15
    Bluejets

    Bluejets

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    Yes, but not a convict decendant, plumber calls me the Viking
     
  16. Mar 6, 2018 #16
    WHONOES

    WHONOES

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    I've got rellies in Oz.
    Like your attitude.
     
  17. Mar 6, 2018 #17
    HellasTechn

    HellasTechn

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    Yes it is the original transformet. the one that came with the device.
    Exactly.
     
  18. Mar 6, 2018 #18
    HellasTechn

    HellasTechn

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    I can not find it anywhere !

    The thing is that the person who asked me to fix it, does not want me anymore to do it because he found a used one for like 15$. I am going to build the circuit anyway because pwm controllers are new to me and i want to play around a little bit.

    THANK YOU ALL FOR YOUR HELP !

    I'll get back here for help later on !
     
  19. Mar 6, 2018 #19
    Minder

    Minder

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    So what was the original part No.?
    M.
     
  20. Mar 6, 2018 #20
    HellasTechn

    HellasTechn

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    :) I have not forgotten about that. I will take a look tomorrow at work and let you know on friday.
     
    Minder likes this.
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