Understanding the MC-60 treadmill motor controller schematic

Discussion in 'The Projects Forum' started by chay, Nov 19, 2012.

  1. chay

    chay Thread Starter New Member

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    I'm working to learn how to build my own motor controller for what I envision to be an open source treadmill walking desk.

    After playing with a number of arduino circuits and applications, I felt ready to move up in horsepower to treadmill-sized PMDC motor controllers, but found there's not yet much available that I could find by way of tutorials and helpful information for designing circuits at this amperage and voltage.

    So I decided to study the motor controller on a used Proform treadmill that I inherited. It uses an MC-60 motor controller. On this forum, I found two relevant mc-60 schematics, one from Tom Rollins ("trolley1"), and the other from "SgtWookie".

    Tom Rollins commented with regard to his schematic that it might be missing some resistors, so I thought I'd engage my own learning with his schematic in the background to see if I could reproduce his schematic on Eagle CAD, which I also wanted to learn, by following his reverse-engineering process on my own PCB. I did that, and filled in names and values to devices for which I also downloaded data sheets. I found what I believe were a couple of mis-labeled devices, and a missing trace. I've attached my hopefully improved schematic here, and I blogged about the schematic and my process here.

    These are two circuit-level changes from the Rollins' schematic (also attached) to mine. I've more than triple-checked them. I think I've got them correctly.

    1) In the attached mc-60v4 schematic (mine) there's a start-up sub-circuit, I believe, centered on U2_A (under the speed control pot) connected through R20 to leg L (Low)(this is only faintly labeled on my schematic, I see). On Tom Rollins' schematic, he shows that to be connected to leg W (Wiper).

    2) Between R7 near U2_D (below the LED SCR Trigger) and R9, the trace on my schematic including R10 does not show on Rollins' schematic.

    At any rate, now I'm to the point of trying to better understand design elements of the circuit. I have a load of amateur-level questions, but I'll start with just three.

    1) I understand how a full bridge rectifier converts AC to DC. The conversion here, as I understand will be pulsed to A+ anywhere from 0-120VDC. I understand how SCRs work. That given, how does 120VDC get stepped down to a steady 12VDC for the rest of the circuit? I see the circuit returns from the rectifier through D5, R14, between D4 and D% and through R9 (5KOhms). This probably is simple, but I've not figured it out for myself yet. Is the zenar diode, D10 key, I wonder?

    2) What does D12 do in the rectifier that's helpful? Why is it there?

    3) What do D4 and D3 do, and why are they connected to the AC input?

    Understanding this would be for me a great start.

    If anyone wants a copy of the Eagle CAD file or datasheets, let me know. Thanks in advance.

    Attached Files:

  2. Jony130

    Jony130 Senior Member

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    I just did a quick look at the diagram and yes D10 + R9 "create" 12V for almost all electronics component in the circuit.
    http://en.wikipedia.org/wiki/Zener_diode#Uses
    Also D4 and D3 (D1 and D2 also) are the rectifier diodes for "Zener voltage regulator".
    D12 - I'm not so sure yet, but maybe this diode is to reduce the motor Inductive kick.

    Also you have a error in your diagram. Why R16 (1K) resistor is in series with motor? How can this work?
    And why this resistor is connect via R13/R13 to SCR gates?
    Last edited: Nov 19, 2012
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  3. chay

    chay Thread Starter New Member

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    Thank you Jony130.

    Hmm.
    With regard to D4 and D3, I see. That is so interesting. I had been assuming that Q2 and Q3 did that. Thank you. So the opto-isolator, U2 is between the two half bridges of the rectifier.

    You wrote:
    I don't know. But I just pulled out my PCB and put the multimeter to it. There's definitely continuity for R16 between A+ and R12/R13. Check and double check. The PCB was working fine when I pulled it off the treadmill, too.

    With regard to getting 12V, now I'm understanding R9 (5K) to drop 120V to 24V and the Zenar diode, D10 just drops any voltage over 12V to ground, right?

    Can I ask you about other parts of the circuit, or should I start a new thread to do that?
  4. Jony130

    Jony130 Senior Member

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    I'm 100% sure that this is a error only on the diagram.
    It's impassible that a small R16 1K resistor can survive the current that is drown by the motor.

    Why R9 drops only from 120 to 24V?? Simply we have 12V across D10.
    You don't need to start another thread. Ask question in this thread

    Attached Files:

    • a.JPG
      a.JPG
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  5. chay

    chay Thread Starter New Member

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    Thanks again, Jony130. That was really helpful, and in particular, the image you shared was very helpful to me.

    I revised the schematic to reflect your insights, I think. Did I get it right? In addition to adding a trace from C17/C18 to the A+ side of R16, I searched for and found a different Eagle CAD schematic representation of the opto-isolator.

    The attached schematics are the same, just different scales (800px and 1000px wide).

    [Oops. I see that I labeled the SPEED CONTROL POT as 0-120VDC. I know it's actually 0-12VDC. I'll fix that.][Done.]

    mc-60v5.800w.jpg

    mc-60v5.1000w.jpg
    Last edited: Nov 20, 2012
  6. chay

    chay Thread Starter New Member

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    I don't know if electronics experts regularly do pseudocode, but I wonder if you, Jony130, or anyone else could give me a pseudocode-level explanation of the sub-curcuit in the loop from about U2_C and above?

    I've double checked the orientation of D5 on my PCB. The schematic seems right, but that orientation confuses me as I try to decipher that loop in the circuit. Any help there?

    Thanks.

    mc-60v5.1000w.jpg
    Last edited: Nov 20, 2012
  7. Jony130

    Jony130 Senior Member

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    Do you mean this part of a circuit (see attach file)



    Sure we can help see this picture
    [​IMG]

    Attached Files:

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  8. chay

    chay Thread Starter New Member

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    Yes! Thank you so much for that picture. I get that part, now. I love it.

    Yes, too. If you had energy to give me a pseudo-code-like explanation of of this part of the circuit you provided, that would be wonderful.

    I've been studying op-amps in my own attempt to unpack that, but so far I've gotten confused on the feedback around U2C, and combined effects the op-amp chain. U1C also confuses me.

    Thanks!

    MC-60MotorControler.allAboutCircuits.PNG
  9. Jony130

    Jony130 Senior Member

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    As for the op amp try read this "tutorial"
    http://forum.allaboutcircuits.com/showpost.php?p=444315&postcount=9
    Also I want ask you about R1 resistor and Q4. Are you 100% percent sure that R1 is connect between Q4 base and diode D9 anode? Because in this configuration I don't see how this circuit supposed to work. It seems to me that R1 resistor should be connect between Q4 base and GND and additional Q4 base is connect to D9 anode.
    Last edited: Nov 22, 2012
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  10. chay

    chay Thread Starter New Member

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    Holy cripes. You are so right, Jony130. That was screwed up. Jeeze, you're good.

    See attached. This is what it looks like to me, now. I double-checked this part of the circuit on the PCB this morning. Does it look right to you? R10 connects the wiper to U2_A(3). R7, C5, and R4 form a loop at U2_D. R1 seems right between Q4 and D9/R9. Man, I don't know how I got that so wrong.

    Thanks for the op-amp tutorial link. I'll do some more studying.

    mc-60v6.png
  11. Jony130

    Jony130 Senior Member

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    Please double check C5 (U2_D).
    Also R1, because now Q4 base-emitter junction is connect parallel to D9 with PNP emitter connect to the lower voltage. So how can this BJT work?
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  12. Externet

    Externet Distinguished Member

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    R16 value and some traces cannot be right as shown on schematic at first post..
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  13. chay

    chay Thread Starter New Member

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    My self-confidence is approaching zero, but I think I may have it now.

    Looking at the Q4 pnp transistor, I think I had the collector and emitter backwards. I believe pin 1 on the transistor goes to 12v; not pin 3. If so, Rollins had it backwards, too.

    I reviewed R16 and found what looks now to me like multiple errors. It seems like I had U2 backwards, too. Ugh. Does this work?

    Thanks. Thanks loads.

    Btw, in this version I included a layer where I've collected some observations about the op-amp side of the circuit. Most of them, I took from someone's commentary that I found early on in a google search. Others are mine. Any feedback on that would be more than welcome.

    View attachment TransistorPNP.2N4403.pdf

    mc-60v7.png
    Last edited: Nov 22, 2012
  14. chay

    chay Thread Starter New Member

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    Today in mc-60v8, I found, traced, and added couple of passive devices not found on Tom Rollins' schematic, and I corrected U2 posts 1 and 2 by switching them, and corrected name and value for C3 (U2_D), and corrected the name for R19 near the wiper.

    Added: R10 near Q4
    C5 near LED speed control

    I also added a little commentary to the IR compensation sub-circuit that reflects my research to understand how that works.

    Any corrections or insight are more than welcome. I'm still working to understand important parts of the circuit like the current limit sub-circuit and how ramp-up/ramp-down acceleration works.

    mc-60v8.png
  15. Jony130

    Jony130 Senior Member

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    Good job chay , now the only think you need to swap the emitter with collector.
    And now the Q4 can work as a "zero crossing detector".
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  16. Jony130

    Jony130 Senior Member

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    Hi
    I want to show you how current protection circuit work.
    Here you have a simplified diagram.

    [​IMG]

    As you can see R28 act like a current sense resistor. He convert motor current into voltage VR28 = Im *0.01Ω .
    U1B act like a voltage comparator. He compares R28 voltage with "reference voltage". This reference voltage is determine by R51, R44, R46. And is equal to:

    Vref1 ≈ 12V*(R46/(R46 + R51||R44) ≈ 0.11V

    And as long as motor current is smaller then Vref/R28 = 11A nothing interesting happen in the circuit.

    But if motor current is higher then 11A so the voltage across R28 is greater than Vref. The U1B comparator is trigger and change his output state from low to high state. This high state is transferred to U1A output and to U2B input.
    This change U2B output voltage from high to low. And this causes that the left site of a R51 resistor is now connected to low state. And this change Vref voltage to

    Vref2 ≈ 12V*(R46||R51)/(R46||R51 + R44)≈ 12V*R46/(R46+R4) ≈ 60mV

    This small change in Vref voltage means that the motor current must drop below Vref/R28 = 6A to restore normal operation of the circuit.
    So as long as Vref2 < Im*R28 circuit is in current protection mode.

    [​IMG]

    I hope that this small description helps you.

    Attached Files:

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  17. chay

    chay Thread Starter New Member

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    Wonderful. Thank you.

    I understand this part of the circuit much better, now, and I have much to chew on. Thank you, again.
  18. stevevogel

    stevevogel New Member

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    I have forgotten a lot over the years but am trying to fix one of these that will not drive the motor at full speed. I am thinking that it is possibly a leaky capacitor as those electrolytics tend to fail with age. Also, the longer I leave power on the faster it will go, sort of like something slowly charging up. Possible C8 or C10.

    The input at "W" goes from 0 - 11 volts. But with the input at 11V, the output to the motor starts at 0V, goes to 17V and then very slowly has made its way up to about 45V. ("W" is the wiper on the speed control pot).

    Not having a full ground makes it tough to get a scope on it to see whats going on. And I don't remember how to float the scope ground so as not to do damage.

    Any thoughts?

    Thanks,

    Steve
  19. spilperson

    spilperson New Member

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    First off, thanks for Chay and Jony for the thread, which is very close to my needs.

    I am building a variable speed wood lathe using a PMDC motor from a treadmill. I think I can use an MC-60 for the control, but I am not clear on the limitations of that controller.

    Does anyone know the motor voltage and current limits for the MC-60?

    Chay or anyone else, do you have any other documentation for the MC-60? I have spent several hours on google in the last week just looking for a data sheet, a manual, anything, but no luck. Anything, whether official or unofficial would be greatly appreciated.

    If it helps, I have two motors. First is a 1.5 HP used motor, max 17 amps at 90 Vdc. Second is a 2.5 HP, can't recall the max amp draw but max Vdc is 130 for that one. I am thinking I won't be able to easily get to 130 VDC, but I am ok for now if I chop off part of the top end of the motor.

    Thanks in advance,

    Frank Spillman
  20. vrainom

    vrainom Member

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    @spilperson:

    The downside of using an mc-60 board is that it's a threadmill control, so it's not designed to give full power to the motor because who could keep up with that!

    I might be wrong but I think the most you can get out of it is about 50v average and there's no friendly way to set the current limit. You might consider a proper dc drive like a kbm, minarik, leeson or the like.

    Edit: There's still some errors in the op's schematic that are not present in Rollins' version. I made a schematic of one of these boards years ago and mostly agrees with Rollins'.
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