PWM Design Request

Discussion in 'The Projects Forum' started by pwill48124, Mar 12, 2010.

  1. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    HELP!!! I'm new and am in over my head. I have a new ETA 24vdc 20 amp power supply. I'd like to build a PWM based controller to control a 135 watt 24vdc motor. If someone could provide a schematic I'm sure I could build it with Radio Shacks help. I'd like to have 110 ac going into the 24vdc power supply and the 24 vdc going into a separate module with a pot to vary the speed on the 24 vdc motor. The motor is PWM compatible. MANY THANKS!!!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Why do you want to control the speed?

    What will the motor drive?

    Do you need it to rotate in both directions?
     
  3. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    I'd like to use the motor (with PWM control) on a mini-lathe where variable speed is a must unless you want to change gears/pulleys/belts/chains/etc. I understand that the 24v being pulsed gives higher torque than just using a rheostat to alter the voltage on an ac or dc motor. The motor need only run in one direction. I had a cheap Chinese ac to 24 vdc PWM controller on the motor and it ran fine until the unit died - smoked the 8 amp fuse with no indication of which component may be the culprit. I then procured a different 24 vdc converter. Unfortunately this converter has no pulse width capability. Thankyou for your interest.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    It may be possible to combine this...

    [​IMG]

    with this...

    [​IMG]

    You will need a swamping diode across the motor, something like what is used across this relay...

    [​IMG]

    Am I being as clear as mud?

    The 555 will need the voltage cut down from 24V to 12V.
     
  5. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    Please bear with me. I graduated from college in 1971 with a BSME. Re: the HIGH POWER LED Fader:
    1. Is Vcc 24 vdc from the converter?
    2. I'm OK with U2a,b,c,and d being part of LM339.
    3. U1a and b are 556s.
    4. What really has me baffled is the component (?) with the diodes and resistors. Is ths a single unit or built up of individual components? If individual components, could you please identify values? Also, the output is confusing. I don't know what it is. It looks variable.
    5. The other resistors and capacitors all seem well identified.
    6. Where is the output? Is it the component I don't recognize with G,0, and S nodes?

    Second Component:
    1. I wouldn't have a clue how to combine with the first element.
    2. Is Vcc 24 vdc?
    3. Is output across Square Wave Out and PWM Out? They aren't two separate inputs to another component?
    4. I'm OK with the components, including 1/4 of LM339. However could you identify Q1 and D1? Values? Part Nos?

    Swamping Diode
    1. Is the diode portion the pieces across nos 8 and 3 on component 556?


    Bill,
    Your help is really appreciated!!!!!!!!!!!!!! I can't thank you enough. I think I'm on my way.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    pwillnumbers,
    You say that your motor is 24v, 135w. Do you know what the stall current is?

    When your "Chinese ac to 24 vdc PWM controller" died, was the motor under heavy load or in a stall condition?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Attached is a PWM circuit I've been fiddling with for a day or so in simulation, not with actual components.

    The PWM range is not all the way from 0% to 100%, but it's fairly close; perhaps 3% to 97%.

    The LM317 regulator outputs around 13v. Power dissipation in the regulator is quite low, as the 555 timer and gate driver circuits only draw 25mA on average.

    C2 should be located as close to the IN terminal of the LM317 as possible.
    R5/R6 establish the output voltage of the LM317.
    C5 keeps the regulator more stable, and prevents overshooting the target output voltage on start-up.
    C4 is the timing capacitor for the 555 circuit. It's usually tied to ground instead of Vcc, but doing it this way prevents a slow MOSFET gate charge on start-up, which would cause high power dissipation.

    C1 keeps the 555 control voltage stable. R9 compensates for the transistorized 555's output limitations; the Darlington in the output (pin 3) prevents the output from rising above Vcc-1.3v. Adding the 22k pull-down resistor adjusts the trigger/threshold levels to be 1/3 and 2/3 of the 555 output instead of Vcc.
    D4 and D5 control the charge and discharge path for C4, the timing capacitor.
    VR1 is a 100k potentiometer. Adjusting VR1 controls the PWM percentage.
    R8 prevents component stress in case VR1 is turned all the way to one end or the other. It must not be omitted.
    R7 limits current to the push-pull amplifier.
    R1 and R2 control the current through D2 and D3. 1N4148/1N914 diodes are a reasonably good match for the Vf of the base-emitter junctions of the 2N2222 and 2N2907 transistors, providing they are thermally coupled. This means physically attaching the diodes to the transistor cases; epoxy such as J-B Weld should work OK.
    R3 limits the maximum gate charge/discharge current, which is important to prevent "ringing" on the MOSFET gate. It should be located as close as possible to the MOSFET gate terminal, and have short leads.
    M1 is an N-channel power MOSFET, Vdss=30, Rds(on)=3.3mOhms, Id=75A, Qg=130nC in a TO-220 case.
    D1 should be more like a MUR3020PT, which is a common cathode dual 15A fast recovery rectifier.

    Both the MOSFET and D1 will need heat sinks.

    With the timing components shown (C4, VR1) the frequency of the PWM signal is about 6.2kHz. It will likely be audible. The frequency was selected as a compromise between the limitations of the transistorized driver of the MOSFET gate vs gate charge (and keeping MOSFET power dissipation reasonable) and keeping the motor current reasonably constant. I do not know what the actual inductance of your motor is; 5mH is a value I used for a size 23 stepper motor. Your motor's inductance is likely much higher.

    As I first stated, it works in simulation. Getting it to work with actual components could well be a horse of a different color.
     
    Last edited: Mar 13, 2010
  8. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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  9. pwill48124

    Thread Starter New Member

    Mar 12, 2010
    8
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    This reply is to SgtWookie
    Many thanks for your interest. Your PWM range looks good to me from what I know. Your write-up is clear and concise. I don't understand it all but .... Your schematic looks clear enough for even me to follow; however, I do have some questions:
    1. On the lower left, the circle with +, -, V2, and 24V. What is that?
    2. Top left, Vs in. Is Vs 24 vdc?
    3. Is Vcc out same as Vcc before capacitor C4 and pin nos 8 and 4 on the LM555. Also same as before R1 and transistor 2N2222?
    4. Vs at L1 D1, is this the actual pulsed current to the motor? I see bottom left ground. Would this be the other lead to the motor?

    Please excuse my lack of knowledge.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Did you see the 1st comment on the allelectronics site? About the mounting holes not being blind, and if your screws are too long you can short out the motor? Just checking...
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    That represents your 24v power supply. The negative terminal is referenced to ground (0v), and the positive terminal is a node named "Vs". It is connected to the other nodes that are named "Vs" as well.
    Yes.
    Yes.
    No, that is a direct connection to your 24v power supply. It has the "Vs" node name.
    24v is always supplied to the positive terminal of the motor. The power MOSFET M1 switches the ground to the motor on and off. D1 is a "flywheel" diode; when M1 turns off, it gives the motor current a path back through the motor. Without it, very high voltages would appear on M1, likely destroying it.
    I am not quite understanding what you are trying to ask. All of the inverted triangles (like below V2's - connection) are ground symbols; they are all 0v.

    That is why this site exists; to share knowledge ;)
     
  12. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    Yes Sir. I almost made the mistake but backed off when I hit some onexplained resistance. I bet alot of people have ruined their motor. These should be blind holes not thru holes.
     
  13. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    SgtWookie,
    Your responses to my questions are appreciated. I didn't realize that the pulsing was on the ground side. I almost feel like I'm ready to visit the local Radio Shack store. I'm sure glad I stumbled onto this website! MANY THANKS!!
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    You can get many of the parts at Radio shack.
    You will not find the MOSFET nor D1 at Radio Shack. They only stock the IRF510 MOSFET, which is woefully inadequate for your purpose. You will have to order them from a place like http://www.Digikey.com
    [eta]
    Radio Shack sells a couple of transistor assortments, 15 pieces in each pack. Get one PNP and one NPN assortment.
    Or, if you wish, simply order them from Digikey when you place your online order.

    I suggest Digikey because they will ship SMALL orders via USPS 1st Class Mail, which can save you money.
     
    Last edited: Mar 13, 2010
  15. pwill48124

    Thread Starter New Member

    Mar 12, 2010
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    Many thanks to those who responded to my request. I have found a local 'expert' who has agreed to help me with working with the schematics provided. I REALLY appreciate what I have gotten from this website. I will continue to 'look in' and see what I can learn.
     
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