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  1. #11
    Duff drawing removed, please ignore
    Last edited by EddyCurrent; 19-10-2014 at 03:33 AM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  2. #12
    Miller's Avatar
    Lives in Hillsborough, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 2-3 years. Has a total post count of 9.
    Great stuff EddyCurrent! I also need a power supply and that's pretty much the hardware done with. The motors are 1.5A for X and Y, and respectively 3A for Z axis. They are all rated at 30VDC, which I take it it's the max voltage?

    Anyway, what is the optimum voltage to feed the stepper drivers? I can get a a nice deal on a quality 24-28V 10A power supply.

  3. #13
    No I think Eddy's wrong.

    You want to connect to the PWM+10V(pin3) and 0V(gnd) to (Pin2) the 10K resistor is just showing the POT resistance. You are controllong the 0 to 10V signal from PWM on the BOB so don't need it.
    But Like John said best check if the Spindle controller 0 is floating or at mains potential.? Putting to Gnd on the BOB could fry it.?

    Edit: correction to pins
    Last edited by JAZZCNC; 18-10-2014 at 04:34 PM.

  4. #14
    Quote Originally Posted by JAZZCNC View Post
    No I think Eddy's wrong.
    needs a 3rd opinion now.

    found this in mean time, it uses 5v with resistor but same idea with 10v, also I'm sure everyone realised that in my earlier post I purposely left out run signals etc.

    Last edited by EddyCurrent; 18-10-2014 at 04:59 PM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  5. #15

    The key phrase is written in the thick black box round the speed controller schematic. It says:

    WARNING. The electronics is at mains potential do not earth!

    This controller, like mine, is a thyristor type with all the control circuitry run direct from the mains. It also needs a dc input from 0 to 10 v on pin 3 relative to the 0 V on the controller to vary the speed. There is no sign of a low pass smoothing filter to convert PWM to the dc control voltage.

    To control the speed from the BOB you will need an opto isolator driven from the PWM output, which will safely transfer the PWM to the high voltage environment of the speed controller. Then you need a simple low pass filter to smooth the PWM, to generate the control voltage. The was a circuit on the Mach 3 boards a little while back, I'll try to track it down.

    To reiterate, if you connect the 0 V on the BOB to the controller as shown in EddyCurrent's picture, on switching on the mains will be applied through a rectifier to ground. If you're lucky the fuse will blow, but rectifiers blow up faster than fuses! Guess how I know?
    Last edited by JohnHaine; 18-10-2014 at 06:25 PM.

  6. #16
    Ok, I found the circuit, it's on the Mach discussion forum called something like "PWM spindle circuit advice" by superagurk. I'm sure one could make something simpler though, but I haven't yet tackled the job of controlling my spindle from Mach.
    Last edited by JohnHaine; 18-10-2014 at 06:47 PM.

  7. #17
    I should have downloaded the drive manual first. I can see that there is a 400i version that is isolated. Pretty naff I would say, why make a drive like that ? are we saying the potentiometer is at mains voltage ? are they even designed for that ? I never expected the chassis to be live and the symbols for 'common' show they are connected to the chassis.
    It just shows how you can be lead astray when you are used to working on proper industrial equipment.

    Sorry for giving you duff information Miller, I'll remember to read the proper manual first in future instead of relying on gear I've worked on in the past.
    Last edited by EddyCurrent; 18-10-2014 at 07:38 PM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  8. #18
    Lots of pots designed for mains isolation, that's how all TVs used to be. There is nothing wrong with designing a drive like this, as long as the speed control is manual with an isolated pot. My KB drive is industrial, they have a whole range of them.

    Miller, important to note that the circuit in the Mach forum has to have its own mains power supply to maintain isolation. The trouble is that it needs a supply above 10 volts to get 10 v max output.

  9. Quote Originally Posted by JohnHaine View Post
    I think you're being a bit categorical here Irving! You can't rely on a limited slew rate to do the smoothing for you. And your comment on ground referencing isn't helpful because by definition "signal ground" is what the control inputs are referenced to! The question is, can the 0v on the controller be safely connected to the signal reference on the vfd?
    John, not categorical but maybe I'll accept I was being too generic. I hadn't read whole thread, but was responding to specific posts. I can rely on VFD not needing external capacitor cos it works for me and many others. Most VFD have internal low pass filters anyway.
    And the diagram posted by the OP shows a VFD.

    However, it now transpires that's all irrelevant to the OPs requirements.

    I agree with Eddy, to show COM as 'chassis' but have it at some unspecified high voltage is wrong in my book.
    Last edited by irving2008; 18-10-2014 at 10:46 PM. Reason: Ii

  10. #20
    OK, I've sort of been inspired by this thread to work out a simple circuit to interface the PWM from a BoB to a non-mains-isolated speed controller such as the Sprint or KE type thyristor units. This circuit does not need an auxiliary power supply, it runs from the 10v available from the controller itself. It uses an ST Micro dual "rail-to-rail" op amp to hopefully get the full 10V control range. It isn't tested yet but offered just as an example, but I think it should work.

    I've just noticed an error in my labelling - on the input side the pin labelled "0V BoB" should go to the output you have selected for the PWM speed control, not the 0V rail!

    On the controller side, it looks like for the Sprint box that Miller has, +10 V comes from terminal 1; the control input ("0-10V to controller") goes to terminal 2; and 0V is on terminal 4.

    This could easily be made on a small piece of strip board, but take care with the IC2 connections as there will be mains voltages on pins 5 and 4 so make sure they are well isolated from 1 and 2.

    I suggest configuring Mach for about 25 Hz PWM base frequency. One problem you may find is that the low speed end where the machine may not start until the speed is turned up a bit from zero; or may run very slowly even though set to zero. It's quite complicated to replicate the function of the "min speed" pot in the controller.

    I do need to get round to adding speed control from Mach to my Novamill, so this should get tested sometime! But any questions or feedback welcome.


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    Last edited by JohnHaine; 19-10-2014 at 10:33 AM.

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