Half complete Triac conversion needing some advice.

[Dan-27]

Hello. I was unable to post in the 'welcome to the forum' bit so before diving into the conversion question I have, a little about me. I'm a keen tinkerer, building and fixing anything that catches my interest. I primarily build combat robots, but have undertaken go karts, BBQs, RC planes and lots of other projects over the years.

The problem:

I recently took ownership of a Denford Triac milling machine and am looking to convert it to Mach 3 using the existing drivers. So far following a superb forum thread on the Denford site I've added the Step and Dir lines for the X,Y and Z axis. These all work if connected to ground. I've built a simple transistor amplifier circuit to switch these 24v lines from a 5v BOB. I've got them doing individual steps by touching onto the 5v output from the BOB but I can't for the life of me work out how to get Mach 3 to output the voltage pulses I need.

As far as I know I have configured Mach 3 correctly, as per the tutorials on their site. I am using one of the generic unbranded 5axis CNC breakout boards, and can't get any outputs to switch on. I was hoping I would be able to read around 5v on one of the Dir outputs, I thought these held either 5v or 0v to determine whether the stepper turns clockwise or counterclockwise, but I'm getting 0v from every pin regardless of how I jig the axis in Mach 3.

Any help or advice much appreciated.

Thanks, Dan.

[Kitwn]

Dan,
Welcome aboard, perhaps this will help.
The outputs of your breakout board are probably open-collector transistors working like relay contacts to ground rather than to the 5v rail. The link below is to the instructions for a common stepper driver from Stepperonline. Have a look at page 4 for a diagram of how to wire it up.

Kit

https://www.omc-stepperonline.com/download/DM320T.pdf

[Dan-27]

Hi Kit

Thanks for your input. I've been racking my brain trying to work out how to get the open-collector transistors to send 5V to the Base on the transistor required to power the Step signal on the milling machine driver. I've drawn out 2 circuits, the first it the one I have already tried and it failed, but when I connected the 2K resistor directly to the 5V source on the breakout board, rather than the X step output (Pin 2) it did cause the X axis to step once. The second circuit is my version 2, do you think this would work?

I'm open to any suggestions, all I need to do is connect the X Axis step signal input on the milling machine to ground and I need the pulses from the BOB to make and break that connection.

Thanks

Dan.

[m_c]

You want the BOB output to switch to GND for the driving transistor, so the only way to acheive that with a single transistor is to have the driving transistor pulled up via a suitable resistor, then have the BOB output switch the driving transmitters base to GND.

[Kitwn]

Dan,
The open-collector on the BOB (if that's what the output actually is) is a connection to earth which may be able to control the 24v line directly, though without more knowledge of the exact equipment involved I would not want to suggest you do that without checking with the manufacturer's datasheet. The fact that an open collector does not connect to the BOBs own 5v power rail gives it the flexibility to be used with different voltages from external equipment but without detailed knowledge of your specific equipment caution is advised.

The attached is a possible circuit to try. The 10K resistor pulls the open-collector output of the BOB to +5v when the BOB output is HIGH. This turns on the external transistor pulling the mill input to LOW (about +2v in practice). When the BOB output goes LOW the external transistor is turned off and the 47K resistor pulls the Mill input HIGH. It's primarily the absence of pull-up resistors which has doomed your experiments so far, though there may be one built into the mill input. Try measuring the open circuit voltage at the mill input to find out. If it's 24v then the 47k resistor can be left out. This would effectively give you an external, buffered open collector.

Note that this is an inverting buffer, you may need to adjust settings in MACH3 accordingly (I don't use it so cannot help with where to do that).

Exact resistor values are not critical, the values chosen give about half a milliamp of current in the transistors when turned on.

Kit

Attachment 26325

[Dan-27]

Hello

Thanks for the replies. Kit unfortunately I can't open the attachment, it keeps bringing up an error message that reads 'invalid attachment specified. Would you mind trying to attach it again please?

Thanks

[Kitwn]

OK let's see if this works...

[m_c]

Kit, going by Dan's second post (relevant bit quoted below to save you scrolling up!), the drives must already have internal pull-ups, so there shouldn't be any need for a pull-up resistor to the 24V.

I've drawn out 2 circuits, the first it the one I have already tried and it failed, but when I connected the 2K resistor directly to the 5V source on the breakout board, rather than the X step output (Pin 2) it did cause the X axis to step once.

[Kitwn]

m_c,
I think you're right, I did suggest Dan check for 24v on the mill input to check. It's the lack of a pull-up on the input side that is most important here.

For those who might worry about it, two pull-ups in parallel will not cause a problem as long as the total current through the relevant transistor when on is within limits, so adding an external resistor for testing purposes will not harm anything. 2Kohms per volt (gives half a milliamp of current) is a good rule of thumb.

Kit

[Dan-27]

It's not playing ball unfortunately.

I believe I have replicated the circuit correctly, and the X axis does step once when I connect the wire from the base leg to pin 2 on the BOB, but jogging the X axis has no impact. I have also moved the wire from Pin 2 to 3 on the BOB, with no affect. Pin 2 is the X axis step and 3 the X axis direction.

The mill does have just shy of 24V on the signal input. On the picture of the breadboard touching 'X step' to 'Mill GND' causes one step.

How can I test if Mach 3 is making any impact on the BOB? I think all the results I have currently achieved could be been done with just the 5V and GND. I only have a multimeter for testing.

Also I don't think the BOB is capable of switching 24V. Aside from it being very cheap, I think the IC's in the middle handle the output switching (the 5 on the left are opto isolators) Searching 74HC245D found they are 'Octal bus transceiver's capable of handling 2 - 6v. Again way outside my electrical understanding, but I think that's correct.