help with multiple power supplies in project

Hi! Could someone help me with some power supply questions?

Attached is the picture of what I am trying to do.

My project has 3 separate power supplies. Power supply 1 is for a motor. Power supply 2 is for a tape head. Power supply 3 is a small wall wart to provide external power to the microcontroller. I want to put all 3 supplies in the same housing.

  1. Do I need to connect all 3 DC GNDs together? Power supply 2 came with the housing and already has its DC GND connected to the chassis. Right now the other new 2 are not connected to the chassis.

  2. My new control panel has jacks and pots on it. Should this panel be conductive and connected to the chassis or which GNDs? My jacks and pots are using AGND on the microcontroller. So maybe this panel should stay isolated from the chassis and other grounds?

  3. Power supply 1 has the three prongs to the AC outlet. The other power supplies only use two prongs to AC. Do I need to connect the AC GND on power supply 1 to the chassis?

  • Basically I just need some help with the overall grounding system of what you see in the picture. Thanks so much for the advice!!

Although I am not familiar with the grounding system in the US, normally the third 'Earth' connection on the plug is the protective conductor designed to detect leakage and should be connected to the metal housing of devices and power supplies for this purpose. To maintain isolation from mains, the negative terminal of the power supply is not connected to the housing .

Its is not clear how the various control signals operate in your project. For example are there any opto-isolators? In order to maintain common reference you would normally connect all GNDs together (-ve). However, you have to be careful since one PS has an earth connection and the OTHER has a negative connected to the chassis.

I would :

  1. Disconnect (if possible) the negative from the chassis of the 2nd PS
  2. connect the chassis to the earth connection of the mains.
  3. Connect all ground together.
  4. AGND is the analog ground of the microcontoller in case you are using the analog inputs. I would connect this to ground too.

Hope it helps

rrboyd: Hi! Could someone help me with some power supply questions?

Attached is the picture of what I am trying to do.

My project has 3 separate power supplies. Power supply 1 is for a motor. Power supply 2 is for a tape head. Power supply 3 is a small wall wart to provide external power to the microcontroller. I want to put all 3 supplies in the same housing.

1) Do I need to connect all 3 DC GNDs together? Power supply 2 came with the housing and already has its DC GND connected to the chassis. Right now the other new 2 are not connected to the chassis.

2) My new control panel has jacks and pots on it. Should this panel be conductive and connected to the chassis or which GNDs? My jacks and pots are using AGND on the microcontroller. So maybe this panel should stay isolated from the chassis and other grounds?

3) Power supply 1 has the three prongs to the AC outlet. The other power supplies only use two prongs to AC. Do I need to connect the AC GND on power supply 1 to the chassis?

  • Basically I just need some help with the overall grounding system of what you see in the picture. Thanks so much for the advice!!

I would have two Grounding Bars in the project, an AC Grounding Bar and DC Ground Grounding Bar

AC Grounding Bar: All metal in your project should to be individually connected to the AC Grounding Bar. The green wire from the Grounded AC Power Cord should be connected to the AC Grounding Bar. This is for your SAFETY.

DC Grounding Bar: I would ground each negative power supply individually to the DC Grounding Bar. This is for a common DC Voltage Reference in your specific project, as needed.

Then run a "bonding wire" from AC Ground Bar to the DC Grounding Bar.

This assumes that all three of your power supplies have isolated outputs. Proper Amp / Speed rated fuse or CB on AC Inputs? Proper Amp / Speed rated fuse or CB on DC Outputs?

Does Power Supply #2 and #3 have a Polarized or Non-Polarized plug? Polarized plugs should be Fused and Switched on the Hot Side.

Personally, I would have one 3-Prong Plug and one AC Cord for the entire project and then make all AC connections inside the box.

Watcher: Although I am not familiar with the grounding system in the US, normally the third 'Earth' connection on the plug is the protective conductor designed to detect leakage and should be connected to the metal housing of devices and power supplies for this purpose. To maintain isolation from mains, the negative terminal of the power supply is not connected to the housing .

Its is not clear how the various control signals operate in your project. For example are there any opto-isolators? In order to maintain common reference you would normally connect all GNDs together (-ve). However, you have to be careful since one PS has an earth connection and the OTHER has a negative connected to the chassis.

I would :

  1. Disconnect (if possible) the negative from the chassis of the 2nd PS
  2. connect the chassis to the earth connection of the mains.
  3. Connect all ground together.
  4. AGND is the analog ground of the microcontoller in case you are using the analog inputs. I would connect this to ground too.

Hope it helps

Hi, Watcher. Yes in the USA that third connection on the plug is the earth ground.

All the outputs from the micro-controller are opto isolated. The inputs are not.

mrsummitville: I would have two Grounding Bars in the project, an AC Grounding Bar and DC Ground Grounding Bar

AC Grounding Bar: All metal in your project should to be individually connected to the AC Grounding Bar. The green wire from the Grounded AC Power Cord should be connected to the AC Grounding Bar. This is for your SAFETY.

DC Grounding Bar: I would ground each negative power supply individually to the DC Grounding Bar. This is for a common DC Voltage Reference in your specific project, as needed.

Then run a "bonding wire" from AC Ground Bar to the DC Grounding Bar.

This assumes that all three of your power supplies have isolated outputs. Proper Amp / Speed rated fuse or CB on AC Inputs? Proper Amp / Speed rated fuse or CB on DC Outputs?

Does Power Supply #2 and #3 have a Polarized or Non-Polarized plug? Polarized plugs should be Fused and Switched on the Hot Side.

Personally, I would have one 3-Prong Plug and one AC Cord for the entire project and then make all AC connections inside the box.

Hi, Mrsummitville.

How do I tell if each power supply is isolated? #1 & #2 each have a fuse, but #3 doesn't. #3 is just a small wall wart.

2 and #3 have non-polarized plugs.

Could I just have one AC power cord and switch with the fuse from #1? This would power all three psu's.

  • Watcher is saying to keep the AC GND chassis isolated from the DC grounds. But you are saying to add a bonding bar connection. Not sure which one to do.

Thank you both for the help!!

Watcher is saying to keep the AC GND chassis isolated from the DC grounds. But you are saying to add a bonding bar connection. Not sure which one to do.

If you connect AC Ground, that is Protective Earth (known as PE) conductor, to the DC ground, ie the negative rail, you are effectively loosing isolation from the mains wiring of the building. This is not necessarily a bad thing as long as you know what you are doing and you have it in mind later on when you do testing and measurements with the power switched on.

The same configuration is applied for example with the oscilloscope, where the shielding of the test probes is connected to the mains earth and therefore when you do testing on circuits you have to keep it in mind and avoid current loops through the mains earth.

Bench power supplies usually have an external removable link between the negative rail and PE (earth) which you can remove when needed for this purpose.