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  • #16
    Can you not just use a bridge rectifier on the tach output so it's always the correct polarity.

    If the diode drop is an issue, there are precision rectifier circuits on the web.
    Paul Compton
    www.morini-mania.co.uk
    http://www.youtube.com/user/EVguru

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    • #17
      Interesting idea Paul, thanks. KB is specific about the 7V/1000 rpm for the tach input and that's the spec for the Servo-Tek tach I'm using. Dunno how much voltage drop I'd get using a bridge as you suggest or what effect the lower voltage would have. I suspect it would prevent adjusting the speed properly.

      I looked up precision rectifier circuits and that's a no-go for me. Way over my head.
      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

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      • #18
        Now I understand. We are not talking about a real tach with a pulse train output. It is a simple speed sensor that gives you a DC signal that is proportional to the motor speed. Paul Compton's suggestion gave me an idea. But I need to see an actual schematic diagram, including how it is powered, to proceed with it.
        Paul A.
        SE Texas

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

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        • #19
          Originally posted by Paul Alciatore View Post
          But I need to see an actual schematic diagram, including how it is powered, to proceed with it.
          There is no power to it per se, it is a simple DC brushed P.M. generator.
          It is a 'real tach'! Just analogue.
          Max.

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          • #20
            A bit of voltage drop from a bridge rectifier will cause a speed offset between the requested and actual speed. That shouldn't be an issue except at very low speeds.

            Using Schottky diodes in the bridge circuit would keep any voltage drop to a minimum, but they should be rated for the peak output voltage of the tachogenerator.
            Paul Compton
            www.morini-mania.co.uk
            http://www.youtube.com/user/EVguru

            Comment


            • #21
              The output of the tach in question is I believe 7v/1krpm, so that = 142rpm/v, the drop on a bridge is around 1.2v - 1.3v, I would imagine that at low rpm that may cause some hunting due to the dead spot?
              Max.

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              • #22
                Not sure at what RPM the lower voltage would affect real world performance but I suspect it would be a big problem dropping over a volt across a bridge as Max calculated. My lathe is direct drive and the low end is where I hope to gain stability using the tach feedback function. No reason to mess with the tach if it doesn't work well at low speed.

                I did a little poking around this morning and my B.O.B has a pin 16 output terminal paralled to the onboard reverse relay's activation circuit. I can use that output to trigger a reed relay to switch 12V to the tach polarity reverse relay coil when the reverse command is sent. The B.O.B will supply 28 mA @ 5V so I should be safe with the reed relay's 20mA/5V coil. Is that bad electro-magical Ju-Ju switching a relay with a relay?
                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

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                • #23
                  Originally posted by MaxHeadRoom View Post
                  If you wanted another relay for the tach when the reverse occurs, you could tap across RY1 coil, it is a 110vdc relay.
                  By the way, I meant to thank you earlier for that tip Max as it's a good'un. If it weren't for the KB being all snug and wired into its new encosure I'd remove the A.P.R.M. and see about tapping into the coil circuit as you suggest. That'd probably be the best solution.
                  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


                  • #24
                    Originally posted by MaxHeadRoom View Post
                    There is no power to it per se, it is a simple DC brushed P.M. generator.
                    It is a 'real tach'! Just analogue.
                    Max.
                    Ah ha! Last piece of the puzzle. Great.

                    Paul Compton's bridge rectifier will work with just a bit of a modification to make up the Voltage lost in the bridge. Here is my suggested circuit:



                    The simple addition of a 1.5 Volt cell in series with the sensor line will make up the constant, 1.5 Volt loss in the bridge. So you get a rectified sensor signal that is back to the proper level. A single AA cell will probably last a long time, depending on the current draw of the motor control circuit. Change it every year or two to be sure it does not corrode and leak.

                    One caution: The sensor input will never drop below 1.5 Volts so if that is a problem, you may not be able to use this. But I suspect that that represents a very slow speed and you will not be operating that slow.

                    Oh, sorry, some may label this Voltage generator a "tach" but I just can not bring myself to do that.
                    Paul A.
                    SE Texas

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

                    Comment


                    • #25
                      That's purty clever Paul, thanks!

                      I didn't hook up the generator (do you like that description better?) to the control but did check how many spindle rpm equals 1.5V output and it's 192 rpm. I don't know if I'll be going that slow in reverse. Won't know that until i get into threading steel.

                      One nice thing I learned that I hadn't checked before was that the direction doesn't affect the voltage. 192 rpm in reverse gave me dead on 1.5v too. Apparently Servo-Tek did their homework on brush timing.

                      I am sure that I don't want to leave a constant 1.5V feed into the control 24/7 though. Dunno if it'd hurt but it just doesn't feel right.

                      I dug up some proto board and am sticking together a little reed relay circuit to switch the polarity reverse relay. If that doesn't work I may give your circuit a try.
                      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

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                      • #26
                        Originally posted by DICKEYBIRD View Post

                        Apparently Servo-Tek did their homework on brush timing.
                        They have been making DC tach's for over 50yrs, so they have probably got it right by now!
                        Max.

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                        • #27
                          Originally posted by MaxHeadRoom View Post
                          They have been making DC tach's for over 50yrs, so they have probably got it right by now!
                          Max.
                          Yep, I could tell immediately it's a quality unit. Got real lucky & darn near stole it on ebay.
                          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


                          • #28
                            You might be able to use an opamp as a full wave precision rectifier, that should eliminate the voltage drop of the diodes.

                            http://sound.westhost.com/appnotes/an001.htm

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                            • #29
                              Originally posted by macona View Post
                              You might be able to use an opamp as a full wave precision rectifier, that should eliminate the voltage drop of the diodes.

                              http://sound.westhost.com/appnotes/an001.htm
                              That was one of my suggestions, but Dickeybird isn't comfortable with building electronics and you may well need specialist high voltage op-amps. We've not been told just how high the motor is being revved.

                              I'd just try numging a bridge in to see what effect it has. If low speed control is a problem, I'd have a search and see how low a voltage drop Schottky diode I could find (with a high enough breakdown voltage). Some Schottky diodes are as little as 0.15v conduction drop.
                              Paul Compton
                              www.morini-mania.co.uk
                              http://www.youtube.com/user/EVguru

                              Comment


                              • #30
                                Originally posted by EVguru View Post
                                ...but Dickeybird isn't comfortable with building electronics...
                                Ahh, the British gift of understatement.
                                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

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