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Author Topic: GROUNDING - what is the correct way to ground a CNC machine?  (Read 46996 times)

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Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #30 on: January 13, 2014, 10:30:30 AM »
Ah so a knob-head then. Why suggest anything at all in the first place??? - forums are better off without people like you.
« Last Edit: January 13, 2014, 10:32:20 AM by Rimmel »
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #31 on: January 13, 2014, 10:51:34 AM »
If you want a bank of I/O optos, look at the Opto22 racks, they have anywhere from 4-48 opto boards with various termination types and also 5v or 24vdc control as well as all degree's of AC/DC voltage inputs.
Plentiful and cheap on ebay.
N.

Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #32 on: January 13, 2014, 02:34:19 PM »
If you want a bank of I/O optos, look at the Opto22 racks, they have anywhere from 4-48 opto boards with various termination types and also 5v or 24vdc control as well as all degree's of AC/DC voltage inputs.
Plentiful and cheap on ebay.
N.

Thank you very much.

Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #33 on: January 13, 2014, 03:29:26 PM »
I do like the look of the LTV847 though as it has 4 opto relays on it. The anode/cathode/collector/emitter parts is easy enough to understand, just struggling with how to get the 24v down to 5v for the input. Looking at the datasheet the max is 6v input. Found a couple of diagrams but they seem to be 5v circuits using a 470ohm resister.

Offline Hood

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Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #35 on: January 14, 2014, 04:59:52 AM »
OK I have been researching and come up with a diagram, but before I connect it could someone have a look and tell me if I'm anywhere near? (don't want to blow my breakout board up).



The forward current should be limited to 40mA@24v. Now this is where I have no idea - will the LTV-847 be ok at that? I don't understand the info in the datasheet

http://www.alldatasheet.com/datasheet-pdf/pdf/86746/LITEON/LTV847.html

Then again I've read to use a 3.3k resister and don't need a diode????  ???

thanks
« Last Edit: January 14, 2014, 05:10:41 AM by Rimmel »

Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #36 on: January 14, 2014, 05:18:19 AM »
Apparently a replacing the resistors with 580ohm will give me 20mA@24v

Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #37 on: January 14, 2014, 02:21:32 PM »
Built it, tested it, working great. Used 590ohm resistors in the end which gives me around 20mA@24v.

Thanks to all (well nearly all) for the help. Mucho mucho appreciated.

Rim

Offline simpson36

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #38 on: January 18, 2014, 09:14:50 AM »
There is a lot of difference between inexperience and laziness. 'People like me' have a great deal of patience for inexperience. We also are often the inexperienced ourselves as no one person is expert at everything. Laziness is a different animal.

Since you have put in some effort, I will fill in some blanks for you. You have already figured out that you do not actually 'convert' 24V to 5V, so no need to cover that.

You do not need a diode unless you are driving a load that will create a surge that needs to be dumped. Mechanical relays have coils which generate surge when they are shut off and the field collapses.

You will encounter diagrams that show a device identified only as a 'relay' with a diode installed around it. This is typically not necessary with solid state devices.

The IMPOTANT current spec to design to is the MAX load on the DRIVE I/O and to be aware that some drives list max PER pin and some list a TOTAL for all pins. Obviously, do not exceed either. At 20ma, undoubtedly you are below the max on your drive, but be aware of the total if that is how your drive is rated. The LTV can handle enough current to fry most drives, so just limiting enough to protect the chip is not enough. Your choice is very conservative so you are good to go.

I use a 400 ohm power resistor on my Mitsu interfaces. You may want to consider the type of resistor you are using as it will probably get very hot. Power resistors can handle the current indefinitely, provided you space them up a bit off the PC board.

Take the 5V + and the 5V ground off the BOB. Do not use the PC power supply or any floating source for the 5V signal source.

Check the BOB inputs for the presence of pull-up or pull-down resistors and  choose your active high or active low appropriately. This info will be in the documentation that came with your BOB. Often you can choose the state with jumpers. You can wire the 5V side of the LTV for either.
 
If that last statement is not clear, just call 1-800-KnobHead and I'll see if anyone can help you . . .  :)



 
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #39 on: February 18, 2018, 07:04:38 PM »
I copied most of the following from another post of mine on the forum:

OK, 'grounding'...'(earth)ground IS NOT equal to DC common. They do not mean the same thing, unfortunately we all throw around the term 'ground' very loosely. 'Ground' is this context refers to 'Earth Ground' which is a safety device. The incoming mains voltage is referenced to ground (through the 'ground' rod(s)) and so is your equipment. This provides a path of least resistance to shunt the voltage away from important things like people in case something goes wrong. 'Ground' SHOULD NEVER carry any current, it IS NOT a common return path for all circuits. That is the job of DC common, and your system may have more than one DC common which is not a big deal. Generally speaking you should not tie your DC common(s) to (earth) ground anywhere. Some power supplies may do this internally with a small resistor to pull the DC common up off the ground plane noise.

The problem is that folks think 'ground' is this universal reference for EVERYTHING in a system and try to measure voltage to it from any given point, which is wrong. Think of it this way, if I were to nail three pieces of wood together at odd angles and ask you to measure their length how would you do it? Would you pick the bottom of the closet piece of wood and measure from there to every other piece? Or, from the floor (ground) to each piece? Nope, because that would not tell you a thing. You would run your measuring tape from end to end on each piece of wood (so your reference is the beginning of each piece of wood and you are measuring from that reference to the end of the wood to find its length.) Measuring voltages is the same idea, you are measuring from a reference point (common) to some other point in the same circuit (same piece of wood).

It is very important that you tie your machines (Earth)ground to your mains earth gound, even if you have a local ground rod. This prevents nasty shocks from things being (earth)grounded at diffrent physical locations. A differnece of 100' could create a 50V potential difference.

Hi Jeff. I realise that this is a very old thread but it's highly relevant to the issue I have. I have just bought a brand new CNC router from Pacific Tooling (via Roger Webb). The router has arrived (from China) and mostly looks ok. I haven't fired it up yet for a few reasons but one reason is that I have noticed that there is no earth from the mains power to the chassis of the machine. I believe that I can fix this very easily by running a suitable earth wire from the rear of the mains socket on the machine to any part of the machine chassis firmly fastened of course.

Having read the above thread, I would just like some reassurance that this cannot affect any of the DC components or cause any interference. I would have assumed that the factory would have done the earth as a routine and necessary part of assembly. The fact that they haven't makes me wonder if there is an appropriate reason for the absence of a mains earth.

Your thoughts would be much appreciated