Announcement

Collapse
No announcement yet.

OT - PWM controller troubleshooting

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT - PWM controller troubleshooting

    A few years ago I built a yarn skein winder for the Wife. I used a cheap bangood PWM controller to power a wiper motor salvaged from my old car. It has worked great up until last week when she lost the variable speed function. It will only run flat out. Also the momentary switch I had wired to start stop it failed, but I'm not worried about that issue. I don't see them being related, but I could be wrong? I've replaced it with a toggle while I wait for another momentary switch to get here.

    The problem is trying to diagnose WHY it failed, and to see if I can fix it. I'm not the most knowledgeable with electronics, but i've been trying to learn more the past few years. I've ruled out the pot as being faulty as I've got another identical controller wired up and swapped pots to rule it out.

    I know the cheapest and best solution is probably to just replace it altogether (I've already done that), but I'd like to learn some troubleshooting skills, and further my understanding of electronics in the process. Here are some pics of the bad board. Everything looks good, nothing looks burned, all the solder connections looks fine on the back. It still works, just doesn't have any speed control. Anybody have any suggestions to start trying?





    This is the good one, and how I have it wired. Working as it should. 12v dc in on right, on/of switch at top (used to be momentary, now o/f toggle), and motor out on left. 10k pot.



  • #2
    The primary suspects would be the two devices fitted to the heat sink, what is the part No's on them?
    Also the 14pin IC

    Comment


    • #3
      One of the photos shows one device on the other side of the heats sink? Or are my eyes decieving me??

      Comment


      • #4


        And here's the other one. The marking are gone, and on closer inspection it looks pretty crappy compared to the other on on this board, and on the replacement board.

        Perhaps it overheated?

        I can't really get a good pic of the good one on the other board, it's mounted on the opposite side of the heatsinks than the broke one, and it's hard to read the numbers on it. But here is that same component on the good board.


        I'll see if I can track down a magnifying glass to read the numbers on it. But just comparing the two now, visually, it "looks" like that is the problem? Anyway to test it?

        Comment


        • #5
          Originally posted by MaxHeadRoom View Post
          One of the photos shows one device on the other side of the heats sink? Or are my eyes decieving me??
          No your eyes are fine. I didn't notice it while taking the first pics, until I tried to get detailed pics of the components from both boards.

          Comment


          • #6
            I see the 7812 12vdc regulator, the other # is unreadable, probably the PWM driver. The prime suspect.
            If you have a meter. measure the 12v output for a start. centre pin to RH pin facing it. 7812.

            Comment


            • #7
              See if the input measures short to the output. Is so, the driver is bad.

              Comment


              • #8
                There is quite a few Youtube videos and articles on a PWM design using a LM324 Op-amp if curious as to the type of circuit.

                Comment


                • #9
                  Why is this better than a simple rheostat? I am not an electrician. Edwin Dirnbeck

                  Comment


                  • #10
                    In order of probability ( a WAG, but often true).

                    1) Solder joint.

                    2) Failed switching device (one of the devices on the larger heatsink)

                    3) failed component in the gate drive. ("Might" be the case anyway, if the switching device failed.....that often kills the driver.)

                    4) Failed voltage regulator

                    5) A combination of the above, especially if the regulator fails

                    The switching device is probably a MOSFET, may be a low voltage type. The component on the opposite side is likely a diode in parallel with the motor load ("freewheeling diode").

                    See if there is a short from the motor connection to the switching device to ground (power disconnected). If so, then the switching device failed. If not, the failure may be elsewhere, something (solder joint?) that makes the MOSFET turn "on" permanently when power is applied.

                    Likely, the LM324 is driving the gate directly. These are simple devices, and the switching frequency is low enough that the power loss due to slow turn on/off is assumed to be no issue. That is not always really true.

                    Originally posted by Edwin Dirnbeck View Post
                    Why is this better than a simple rheostat? I am not an electrician. Edwin Dirnbeck

                    It's better than a rheostat because it wastes less power, and actually works better. This will give a "more" stable speed than the rheostat does, as the load changes.
                    4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

                    Keep eye on ball.
                    Hashim Khan

                    Everything not impossible is compulsory

                    "There's no pleasing these serpents"......Lewis Carroll

                    Comment


                    • #11
                      Originally posted by polaraligned View Post
                      See if the input measures short to the output. Is so, the driver is bad.
                      Which one is the input and which pin is the output . (that's where my electrical knowledge is right now.....)

                      Originally posted by MaxHeadRoom View Post
                      There is quite a few Youtube videos and articles on a PWM design using a LM324 Op-amp if curious as to the type of circuit.
                      Thanks. Yes I'm very interested in learning how these work. Ultimately I would like to design my own board that has some additional functions like being able to program total revolutions, and having counter, etc. Back when I built it, I started looking into controlling it with an arduino, but just never had time to finish it, and already had these boards handy to get it up and running for her. It's been one of those things that I got to 80% and moved on.

                      I've got a lot of learning to do before I get there though. I can never seem to find the time to dig back into this stuff. Electronics has just never came naturally to me, and always seems like an uphill climb. Without the internet I'd be completely lost.

                      Before it broke I made a simple foot switch with a momentary push button to turn on/off and she could set the speed with the pot. I'm going to change that to a foot pedal that controls the pot, and a fwd/rev switch to turn on, then she can vary the speed from 0 to full with the pedal. That would function better for her anyway, as she winding 3 skeins at a time, with a potential for 6 total if I ever finished printing the arms for the other side .

                      Comment


                      • #12
                        Originally posted by Dan Dubeau View Post

                        Which one is the input and which pin is the output . (that's where my electrical knowledge is right now.....)


                        .................
                        With the multimeter, set on a resistance range that does NOT measure diodes, if possible.... Measure from each motor pin to power supply negative. On each, measure with the probes forst in one polarity , then the other (reverse positions.

                        One position may give a low resistance in both polarities.... If so, the switching device is probably bad.

                        The switching device is the one on the larger heatsink. The barrel shaped device behind it seems to be the freewheeling diode.

                        There are three connections on the MOSFET. As you look at it, they would be "source" (negative) on the right, "drain" (output to motor) in the middle, and "Gate" control input) on the left.

                        The measurement you want is from "drain" to "source". looks like that is from negative to the second pin from the right on the 4 pin connector.
                        Last edited by J Tiers; 01-22-2022, 02:31 PM.
                        4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

                        Keep eye on ball.
                        Hashim Khan

                        Everything not impossible is compulsory

                        "There's no pleasing these serpents"......Lewis Carroll

                        Comment


                        • #13
                          Originally posted by Edwin Dirnbeck View Post
                          Why is this better than a simple rheostat? I am not an electrician. Edwin Dirnbeck
                          PWM controller is better than a variable resistor (rheostat) because it does not waste power by heating the control element. If there is a resistor in series with the motor, the resistor will heat up because there will be a voltage across it and there is also a current present.
                          So, if the resistor would be, let's say 3 ohms and the motor is taking one amp then the voltage across that resistor would be 3 volts. The power generated in that resistor would then be 3 volts times 1 amp so 3 watts.
                          The PWM takes an entirely different approach here. It switches the power to the motor on/off in a rapid fashion. The time the pulses stay on in relation how long they are off determines the speed of the motor. The difference here to the resistor approach is the power generated by the switching element. In a perfect world it is zero. There is either no current or no voltage across the switch so zero times something is zero.

                          My primary suspect of the failed unit would be the semiconducrtor used as a switch. When power semiconductors fail, they almost always get shorted. It requires quite a jolt to burn that thing open. That would also be seen outside the semiconductor.

                          I also suspect that the new unit will have the same destiny as the now deceased unit.

                          Comment


                          • #14
                            Originally posted by Edwin Dirnbeck View Post
                            Why is this better than a simple rheostat? I am not an electrician. Edwin Dirnbeck
                            A variable resistance is very inefficient due to heat loss etc.
                            The PWM is one of the many efficient ways, it produces variable width pulses that control a mean current level to the motor.
                            Looks like a fairly level saw tooth who's level varies with the voltage pulse width
                            I would suspect the Mosfet as the prime reason for the fault.
                            Measure the voltage to ensure that the 12vdc is not too high, i.e. shorted.regulator
                            Last edited by MaxHeadRoom; 01-22-2022, 02:49 PM.

                            Comment


                            • #15
                              She's already back in full production mode, so I won't get it back to test until sometime tomorrow. I found another momentary switch (door lock switch from a ~2000 Buick century) that restored functionality for her for now. The toggle switch was unusable. I can take the broken board down and bench test it later with a different motor, and poke around with a multimeter.

                              Question 1: So would the failure mode of that mosfet be to pass full current? That would logically explain why the broke board still had all function, fwd/reverse, except for the speed control pot.

                              Question 2: The mosfet is essentially a relay, right? The IC takes the resistance value from the pot, and sends on/off pulses to the mosfet to pass full current to the motor for various durations to vary the speed.?.

                              Is my basic understanding of this circuit close?

                              Comment

                              Working...
                              X