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Grounding question
« on: March 04, 2011, 06:34:06 AM »
Hi all,

Putting together my DIY machine and am just working on the power supply. I have built 3 driver modules - 1 per motor, and a BOB. I have just had a concerning thought whilst planning the mounting of the PSU and other boards.

My BOB is a simple affair, basically a pcb with a DB25 connector on which routes the port pins to screw terminals with LED indicators. The screw terminals from the pins connect via cables to the driver modules. I used screw terminals rather than plugs/sockets at the moment as it is obviously easier to change wiring if need be. Once Im 100% happy I will probably change the terminals for molex connectors or similar.

I have 2 power supplies at the moment, a 5V to supply the logic, and a 32V, both standard mains power supplies that come with most low voltage appliances - black box that plugs into with a cable and typical DC socket on the end. My new supply (hacked from an old audio amp) now gives out 5V regulated using an LM7805 and 36V.

The parallel port pins 18-25 are also connected to the db25 outer metal casing, which is of course then connected to the PC ground when plugged in, and therefore also connected to my mains earth. I have connected the grounds of both supplies and my BOB, and it all works fine.

Is this correct so far? should the pc ground/earth be connected to my 5V/32V ground? My new power supply will obviously be earthed, and at the moment will be sharing the common ground too, i.e. everything that is  0V will be connected, and also connected to mains earth. Should they all be connected like this or should I seperate the mains earth and DC grounds?

Hope that makes sense !



Offline Tweakie.CNC

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Re: Grounding question
« Reply #1 on: March 04, 2011, 07:37:54 AM »
Hi Calv,

A few guidelines....

Ideally you should treat earth and ground as two separate circuits.
The screen/shield of the DB25 should only be connected to earth at the computer end.
All exposed metal parts of your machine should be connected to earth (ideally at one common point).
The pins 18-25 of your parallel cable are ground and should connect to all the circuit grounds (ideally again at one common point).
Avoid any ground loops - it has to be a star or string connection which is open ended.
Avoid any earth loops - again star or open ended string.
The connection between earth and ground should only occur at one point otherwise a loop is formed.
Any shielded cables ( limit switches, estop etc.) must only have the shield connected at one end (the controller end).

These are ideal situations and not always possible but should be aimed for.

Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Grounding question
« Reply #2 on: March 04, 2011, 06:30:57 PM »

Ive made some modifications and its now all ready to go into the case, I cant do anymore now though as I need to cut out the back panel for the DB25 connector and its a bit late to be out with the dremel so I'll carry on in the morning.

Thanks again,