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  • Question about diode application.

    Still messin' around with the DC motor/speed control tach generator and have another question; this time about diodes. Original thread: http://bbs.homeshopmachinist.net/thr...tach+generator

    I now have a simple DPDT relay wired in your basic polarity reverse config. in the output leads of the DC tach generator. The relay coil needs to be energized to reverse the output polarity only when the the spindle motor is running in reverse. I could control the relay with yet another relay controlled by Mach or I was thinking of energizing the relay coil from the output side of the KB (reversing) speed control. It outputs max 90 vdc in reverse polarity (obviously) when the motor is running backward. The KB manages dynamic braking/reverse extremely well and "knows" how to stop smoothly from full speed then reverse direction safely with no danger of electrons getting confused & blowing up something due to the operator's input errors. That's why I'd like to slave the tach-gen reverse polarity relay directly to the output of the KB. That way there'd be no danger of the tach's output being reversed whilst the spindle is spinning down.

    Would it work if I was to put a "clever" diode or 2 on the motor output leads of the KB to energize the relay coil? By clever I mean one that could pass a max of 12v at a 150 mA or so and be set up to power the coil in one polarity direction only. The relay coil is rated 12v/75 mA.

    I know not of these electro-magical devices but am sure ye grizzled sorcerers of said magic will know if this is feasible or if I just need a good dope slappin'.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    Here's the dilemma, you want to reverse the tach to ensure that the +ve output is always connected to the +tach terminal when the motor is in the appropriate direction, this excludes paralleling the reversing relay as the motor, may or may not be in reversal at that point?
    IOW as the motor traverses through reversal, the tach output will be very low or near zero, one method to detect the point of reversal is a op amp/comparator circuit that is sensitive enough to detect the small change in tach voltage reversal and immediately pick up a relay that reverses the board connections when motoring in that direction.
    Max.

    Comment


    • #3
      Originally posted by DICKEYBIRD View Post

      Would it work if I was to put a "clever" diode or 2 on the motor output leads of the KB to energize the relay coil? By clever I mean one that could pass a max of 12v at a 150 mA or so and be set up to power the coil in one polarity direction only. The relay coil is rated 12v/75 mA.

      Diodes don't exactly work like that. They do have a breakdown voltage, above which they begin to conduct. If all you want to do is drop a signal from 90V to 12V for a relay, there are a variety of ways to do that. If you are looking for a simple solution and don't care about isolation, then you could accomplish that using a zener diode and a resistor. The zener diode is in parallel with the relay coil and the resistor is in series. If the polarity of the signal will change, you will want another diode.

      But I don't know jack about these speed controls. Based on what MaxHeadRoom says, it sounds like there may be more to the story.

      Comment


      • #4
        To pursue the op amp idea, do a search for LM311 zero cross over, there should be whole raft of circuits, they will drive a small relay directly.
        Max.

        Comment


        • #5
          Put a diode in series with the relay coil to only conduct when in reverse mode and use a series resistor to run the relay from 90 volts. From your data, the relay coil resistance must about 160 ohms. At 90 volts the resistance that would draw 75ma is 1200 ohms. Subtract the 160 and that leaves 1040 ohms in series to run the relay at 90 volts.

          IF your input voltage varies with speed, you could use a voltage regulator IC like a 7812 or 7912 to get your voltage constant at 12 volts--but your input must be a few volts higher than 12 to get it to regulate.
          Last edited by mikem; 05-25-2013, 03:51 PM.

          Comment


          • #6
            Hmmm, great info thanks all! More head scratchin' to do. Probably safer to go back to plan A; triggering the tach signal reverse relay from the B.O.B.'s spindle reverse relay output & make sure to put an M5 spindle stop & a short pause in the code before switching directions.

            As usual, my so called clever ideas require more cleverness than I have.
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

            Comment


            • #7
              Incidentally how are you handling the M5 stop? If using braking, is the inhibit on only at motor rest? if so when is the inhibit applied?
              Max.

              Comment


              • #8
                Good question Max, I'll have to get back to you later. I'm at *!^$!*(!) work today & getting hammered.

                If I remember correctly, I'm switching 1 KBCC terminal on for forward, another terminal on for reverse and both terminals off is off/brake. I'll ck. later when I get back to the shop to be sure.
                Milton

                "Accuracy is the sum total of your compensating mistakes."

                "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                Comment


                • #9
                  I have never used that particular model, so looking at the manual it would appear that maybe the inhibit is activated in the braking mode, which may make sense as all control would be removed from the motor and the brake resistor connected across the armature?
                  Max.

                  Comment


                  • #10
                    That sounds right. I don't think I'm using the enable or the inhibit terminals yet. May not need to? There's a schematic for the A.P.R.M. module and the KBMM speed control in the manual but I wouldn't know what to look for.

                    I put the big (6" 4-jaw) chuck on it a while back just to test it. I spun it up to 1400 rpm and entered M5 with my fingers on the brake resistor. It came to a quick stop and I could feel the resistor get warmish.
                    Milton

                    "Accuracy is the sum total of your compensating mistakes."

                    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                    Comment


                    • #11
                      If you wanted another relay for the tach when the reverse occurs, you could tap across RY1 coil, it is a 110vdc relay.
                      It may be OK in this case, if the inhibit occurs during/before reversal.
                      It is OK to reverse the tach if the motor is inhibited.
                      Max.

                      Comment


                      • #12
                        I re-read the other thread and was surprised to find that the tach leads had to be reversed to run in the other direction. But, whatever. Normally a tach provides pulses and any polarity of the leads would only be for power to the tach. But, I don't know everything and what works, works.

                        I have to think that the designers of this speed control must have had some scheme in mind for reversing the motor direction. Have you tried to contact them and ask? Perhaps there is a jumper or input or something for this situation. I say this because reversing the tach leads when the motor is turning in the opposite direction seems like a bad way to do things. I mean, what if you flip the reverse switch and the motor is still going forward? Do you reverse the tach leads immediately or do you have to wait until the motor actually stops and reverses? Then, how would you detect that reversal? This just does not sound right. Or is the operator expected to turn the motor off and wait until it stops before applying reverse power? That also does not sound like a good design.

                        I think you need to know more about how the circuit is designed to work before you make any further changes. Do contact the manufacturer.
                        Paul A.

                        Make it fit.
                        You can't win and there is a penalty for trying!

                        Comment


                        • #13
                          The issue is that this is not a version such as the 4 quadrant version where the direction is changed without physically reversing the armature leads, as the manual states, if using the armature reverse option in conjunction with a Tach, the +v tach polarity output has to be maintained at the + tach input, IOW the drive does not automatically recognize a tach reversal, as it would be done in the 4 quadrant version or your typical spindle/servo controller, where either a dual bridge or H bridge motor control exists.
                          Quote:
                          Note: If control is used on a reversing application, the tach wires must also be reversed so that positive (+) is always connected to B or T.
                          To set control for tachometer feedback.
                          Max.
                          Last edited by MaxHeadRoom; 05-25-2013, 07:29 PM.

                          Comment


                          • #14
                            Originally posted by Paul Alciatore View Post
                            I have to think that the designers of this speed control must have had some scheme in mind for reversing the motor direction. Have you tried to contact them and ask? Perhaps there is a jumper or input or something for this situation. I say this because reversing the tach leads when the motor is turning in the opposite direction seems like a bad way to do things. I mean, what if you flip the reverse switch and the motor is still going forward? Do you reverse the tach leads immediately or do you have to wait until the motor actually stops and reverses? Then, how would you detect that reversal? This just does not sound right. Or is the operator expected to turn the motor off and wait until it stops before applying reverse power? That also does not sound like a good design
                            Thanks Paul. I'm guessing this control design was approaching the end of it's life cycle when it came out and the 4 quadrant designs Max mentioned were right around the corner. The DC tach generator feature (again I'm guessing) is old technology and the modifications to the design to solve the polarity issue weren't practical.

                            Coincidentally, I bumped into a fellow on another forum that's working with exactly the same setup I have and he's already requested some tech info from KB. I hope he gets some good info from them.
                            Milton

                            "Accuracy is the sum total of your compensating mistakes."

                            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                            Comment


                            • #15
                              Originally posted by DICKEYBIRD View Post
                              I'm guessing this control design was approaching the end of it's life cycle when it came out
                              I don't think its so much at the end of the life cycle, as KB has always made the 'simpler' types of DC motor controllers, but I believe the add on board for reversing and braking was to cater to those that needed a little extra over and above the basic KB-125 etc.
                              For those that want reversible drive with also tach reversing, there is the KBRG-225D etc.
                              And then of course there is the PWM drives also.
                              Max.

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