Tying PSU Grounds and Mains Earths?

Hi all,

I'm just drawing up my wiring diagram for my printer.

I've got 24V and 5V power supply units and I intend to have a double pole relay cut the live and neutral of the supply to the 24V PSU until the duet grounds the PS_On pin on the three way external 5V connector.

Can I join the 24V ground, 5V ground, mains earth, and cable/enclosure shielding all together? Are there some power supplies that don't like this?

I will be adding earth bonding to the chassis and fusing the fans and heaters.

Thanks.

Edited 1 time(s). Last edit at 08/10/2017 07:02AM by WesBrooks.

Yes you can do that. The main complication is that if you also connect the Duet to a PC via USB, you will get a ground loop, which can be problematic. But as you have a Duet, you will probably use the web interface and not USB.

If you will be driving the relay coil directly from the PS_ON output, connect a flyback diode in parallel with the relay coil.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your feedback. I was thinking about avoiding earth loops on the system but had forgotten about the USB. I'll look out for something like this: [www.adafruit.com]

In the mean time is it best to unplug my laptop before plugging into the duet for firmware upgrade, setup, etc?

Edited 1 time(s). Last edit at 08/11/2017 01:51AM by WesBrooks.

See [duet3d.com] for recommendations. Firmware upgrades to the Duet are normally done through the web interface, so they don't usually require a USB connection.

Ground loops only affect a few users. I use USB for debugging new firmware and I have not had a problem with ground loops, but the mains supply in my office is clean and all the sockets are on the same ring main. If you are unsure, then running the PC and printer and nothing else from a mains distribution block is normally sufficient - and of course make sure that the connection between the negative VIN terminal of the Duet and the PSU is sound.

Edited 1 time(s). Last edit at 08/11/2017 02:42AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Judging by the wiring diagram if I am tying together the chassis, mains earth, and PSU output grounds to a single point then there is little point connecting both the 5V ground and V_IN ground is there? From an earth loops perspective it would seem good practice to just connect to the V_IN ground on the duet?

Edited 1 time(s). Last edit at 08/11/2017 06:18AM by WesBrooks.

Yes, connecting the V_IN ground is sufficient.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Getting myself a little confused with the earthing!

Back ground: I've set my system up with 2 24V 1 5V PSUs. One of the 24V is a little one that I had kicking about and will just provide power to some guided contact relays which will make or break the power the the second main 24V PSU. The 5V is to allow the Duet to control kill the power to the heaters and steppers in the event of a temperature error. The fans are driven from the first 24V PSU. If one of them short it will blow the fuse dropping the force guided contacts and killing the power to the heaters. If the duet needs to kill the 24V power the fans won't be shut off.

Drawings will be shared when ready, but they are messy at the moment!

Challenge: What to do with the earths and PSU negatives grounds. None of the PSUs have continuity between -ve and earth. They do have some filtering there though that causes a brief peep on my multimeter! Mains is easy, all back to a common point that then connects to the earth pin on the supply with no switching! I'll tie the chassis to this too, and ground the stepper motor bodies and any shielding the to chassis. The wiring on the duet ties the -ve side of the 5V and V_In so there is little point leaving the -ve terminals of the PSUs unconnected. The question I have is would it be better to leave the common -ve connection from the power supplies isolated from the chassis/mains earth or connect it to the earth point? I'm guessing isolated so any noise from the steppers or shielding has a harder time getting into the duet?

Thinking a little further I'm guessing that the filter or what ever it is between the -ve and earth must be there to do something useful, and tying the -ve to earth would defeat whatever that is!

This is proving to be a right mess! My ATX PSU has the -Vs on the output side bonded to the earth. The passively vented single voltage PSUs that I have are isolated. In an ideal world it looks like the answer would be leave the output isolated to minimise any AC leakage and tie any shielding to the case/chassis and mains earth. However, for one of my current projects with an ATX PSU since the ATX is earth bonded any other PSUs will automatically be earth bonded on the negative output side too.