Ground issues...

[kolokithas]

Hi all,

I have the DeskCNC controller inside my box,i also have three Geckos 320 inside my box.I have common GND between them.Its ok or i will have to insulate the controller?I know that i can't isolate the Drives from the chassis GND because the enclosure of the gecko is latched to his power GND,so, the power supply's ground goes to the entire chassis automatically.Now i really have a question!!Even though i am related to electronics only for hobby,i didnt studied to a college or anything.I know that everything(with power) to work needs two wires to make a circuit the + and the gnd.How the gecko drives they don't need the gnd from the controller and they only need the 5volts and the step and dir signals from him?How they close the circuit with controller?If i isolate the connection between the grounds of the two devices and connect only 5volts and step/dir signals the whole thing should work?Did i miss something?how do we have a circuit here?The gecko needs the gnd of the controller but it doesnt mentioned anywhere because it is considered self-evident?As i know from my experience over the years we used to have grounded all the devices inside a metal box and have a common minus for the components inside.First for convenience and second for EMI/RFI filtering.I know the faraday shield but its applicable in some cases that common gnd causes noise i/f etc.My current setup is geckos with controller with common gnds,its ok or should i have to change anything?

Thanks for the interest...
I look forward for a reply!
Panos

[Mariss Freimanis]

Connect GND and +SUPPLY wires from your supply to each drive. Three drives, three sets of wires.

STEP and DIRECTION works this way: Current flows from your PC or breakout board +5V into the drive's COMMON terminal, thru the STEP or DIRECTION opto LED and back to your parallel port (or breakout board). It shorts the STEP or DIRECTION to PC ground to complete the current path and makes a step happen.

Mariss

[CJL5585]

The Utility ground from the utility should be firmly connected to the electronics chassis.

There should NOT be any connections from any power supply output voltage to the electronics chassis.
The voltage from a power supply going to different drives, modules, etc. should be fed from a common point. Do not loop wiring.

Different power supplies such as a 5 volt supply and a 48 volt supply should remain isolated from each other. Do not tie the commons togather.

Most electronic equipment that requires grounding to the utility or chassis ground will have a grounding lug attached for the purpose, or a tag attached with the equipment that specifically states that the equipment should be grounded.

The ONLY COMMON GROUND between different systems should be the UTILITY or chassis ground.

Jerry

[kolokithas]

Hi again,

The thing is that i use two toroidal transformers for my supply.The one (28V) supplies the three geckos(3 pair of wires).The other supplies with two secondaries my breakout board.The first is +5V for the controller and the other is+24v for the breakout board,relays etc.).Inevitably the GND of the 28v goes to the chassis through the geckos.I believe that the power transistors of the geckos send the gnd to their enclosure and then via the heatsink to the chassis gnd.Thats ok with me,i don't have any problem with that.The thing is that i cant isolate the common GND because the DeskCNC controller sends his GNd to chassis via the DB9(rs232) plug.I mounted the controller directrly to the chassis box so i can exit the DB9 connector out of the box.The only way that i can isolate the breakout board and the controller from my chassis GND is to remove the controller from the backplate of my box and put him insulated(via plastic spacers)inside the box.Why do i need to do that?Then the computer's ground will be floating because the DB9 plug will be on the air.And practically their will be no connection with the geckos except the 5v,step and dir signals.So i believe that the circuit wont work because the gnd connection between the geckos and the PC will be missing.
So.....?

[Mariss Freimanis]

The controller +5V should come from the PC 5V supply, or barring that if an external 5V supply is used, be grounded back to PC ground, not the motor supply ground.

Mariss

[kolokithas]

I SEE!

After reading carefully the post from Mariss i realised that the geckos they dont need the ground of the controller because of the optoisolator.Thats the key to the whole story.The opto isolator creates his own circuit with the controller and the other side of the opto uses the supply of the gecko("that's why we call it optoISOLATOR,you stupid man")OK OK my fault.I get the picture!So if i am right the step and dir signals are related to ground so the controller by controlling when to gnd them,he creates a circuit and so we can have a step or direction!!!
Neat stuff...

I should go to college...

PS to CJL5585. Why i should isolate the gnds between the 28v and the 5v of the controller?

[belli]

Hi Panos,

There was some discussion about seperating the 5V and 24V on the board but for this one it's not really necessary. The ground from the PC comes through pin 5 of the DB9, not along the shield or the casing of the connector, at least it shouldn't, the shield should be connected at one end only.
Perhaps you could cut a hole in your enclosure (big enough for the DB9) and plug the cable straight onto the DeskCNC board, I do.
The Gecko's have opto isolators as you mentioned and so there is no connection to ground on this side of the power supply.
The inputs only require pulling down to ground on the board and so if the switches are insulated from the machine frame, in a plastic casing like most are then this ground won't be connected there either.
For the outputs, use the relays or put an external relay/contactor/optoisolator and then this part is also isolated.

Regards
Greg
PS How's the board?

[CJL5585]

Originally Posted by kolokithas PS to CJL5585. Why i should isolate the gnds between the 28v and the 5v of the controller?

The 5 Volts is for the optoisolators. The 5 volt common is referenced back to the computer.

The 28 volts is for the relays and external functions.

When relays and external functions switch power, voltage transients occur. Do you really want these transients to be introduced into the computer 5 volt logic circuits via the common connection?

With the voltages isolated from each other, the transients are limited to the 28 volt winding.

[kolokithas]

Hi greg,

Thank you for the reply.The board is fine.Everything work as a clock.The only thing left for now is to tune the geckos with the oscillator and some minor adjustments for performance and safety...I decided to isolate the power from the controller to the rest of the box.But the DeskCNC controller sends the gnd to the DB9 casing via the support lugs thats soldered on the board(that's for the history).If i remove the controller from the box and put it over the breakout board(as it was designed to be) the whole thing will be insulated.I will have to enlarge the hole for the RS232 plug(thank god its aluminum).So lets cut the small talk and get down to business.There are so many projects and so little time!!!

Think three times,measure twice,cut once(the other side up is time consuming)

Thanks greg for the support.
If i have any problem i know where to reach my hand.
I should also thank cjl5585 and mariss.
Thanks
Panos

[Mariss Freimanis]

The whole idea for optically isolating the STEP and DIRECTION inputs is to keep the PC ground (PC case, etc.) seperate from the drive power supply ground. The drives generate a very noisy electrical enviroment on their side of the fence. You don't want it to pollute the PC side with this noise.

Secondly, anything catastrophic happening on the drive side will leave the PC side completely unscathed.

Mariss