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Old Aug 26, 2015, 05:03 PM
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Dell NPS-750BB A - float the dc ground

Hello everyone,

I just received two of these power supplies (NPS-750BB A). Does anyone know how to float the dc ground in order to use them in parallel?
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Old Aug 27, 2015, 12:58 AM
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You don't need to float the DC ground to use them in parallel.

Do you mean you want to use them in series? To produce 24Volts.
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Old Aug 27, 2015, 07:28 AM
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Yes. I mean in series to produce 24V. Sorry about the error.
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Old Aug 28, 2015, 04:35 AM
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I've been replacing the two M3 screws behind the main output connector with nylon ones. Washers between the board and the case at that location, also nylon compete the job. To actually do this you will need to remove the cover and then the cooling fan. Make sure you test that the negative DC terminal and the case are isolated before using.

As with any mains powered device if you are not sure about what you are doing don't attempt this.
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Old Aug 28, 2015, 01:44 PM
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Quote:
Originally Posted by Jason50 View Post
I've been replacing the two M3 screws behind the main output connector with nylon ones. Washers between the board and the case at that location, also nylon compete the job. To actually do this you will need to remove the cover and then the cooling fan. Make sure you test that the negative DC terminal and the case are isolated before using.

As with any mains powered device if you are not sure about what you are doing don't attempt this.
Additional information. Only float the ground on the 12V to 24V power supply. After making the series connection check the grounds with an ohm meter before applying power.

Some insulation across the output terminals to prevent something from contact them would be in order too.

Glen
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Old Aug 29, 2015, 02:19 AM
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Ok. I replaced the two M3 screws with nylon ones and added nylon washers to the top and bottom of the board. Checked for continuity and I now have no continuity between the -12V and the case.
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Old Aug 29, 2015, 03:59 AM
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Once it's back together it's good to go. As already mentioned , use that one as the "high" side. With the other units DC still grounded if anything does go wrong you still have the mains earth protection.

Once you have all the connections sorted, hot melt glue works well to insulate everything. It's also easy to get off again if need be.
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Old Aug 29, 2015, 05:55 AM
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Sanity check? Your picture shows no continuity between the chassis and a green wire.
Is that wire the mains ground? If so I am confused. I thought that the mains ground to chassis should still be in tact. I thought only the -12V to mains ground should be isolated.

Can someone clarify? Have I misinterpreted the picture. Am I incorrect in my understanding of the correct way to isolate the -12V?

Glen
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Old Aug 29, 2015, 11:46 PM
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I think the main ground is located on the other side of the supply.
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Old Aug 30, 2015, 12:17 AM
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Yes the mains ground goes directly from where the mains enters to the case. This is at the back of the supply and is correctly identified in your photo.

The case should remain earthed at all times, it's just the DC that is isolated.

The mains earth continuity can easily be tested with a multimeter. Just check between any conductive point on the case and the earth pin on the mains connector.
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Last edited by Jason50; Aug 30, 2015 at 12:20 AM. Reason: Add info
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Old Sep 30, 2015, 11:10 PM
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Looking to grab a couple of these in the next day or two. Which PSU do I want to have the DC ground isolated? If I understand correctly I want the second PSU (the one I am taking the +24v from to my charger input + input) correct?
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Old Oct 01, 2015, 05:16 PM
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Yes, that's correct. Isolate the high side supply which is the one where the +24V output is taken from.
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Old Oct 02, 2015, 09:31 AM
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Yes, that's correct. Isolate the high side supply which is the one where the +24V output is taken from.
Thanks Jason. One other quick question or two before I slam this thing back together. The black wire in the above photo that connects to the top grounding screw, I want to keep that in contact with the board correct? Just isolate it from the bottom with two nylon washers and screws correct?

Also is there anything else I should be playing with before I button it back up like voltage pots etc? I see vr601 and vr602 located midway down the board.
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Last edited by Mugenski; Oct 02, 2015 at 09:37 AM.
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Old Oct 02, 2015, 06:14 PM
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Yes that's what I do with the black wire. I saw that someone else just left it disconnected and wrapped the end with electrical tape, apparently had no problem.

You don't need to adjust the pots. I think VR601 will allow you to shift the output voltage, but not much. The supply I tested with could reduce to about 12.2V and up to 12.8V. I don't know what VR602 does, I'd guess it is for current limit or some sort of protection cut off.

I have the fan speed set to minimum. Unless you add your own fan control there are only two choices, flat out or very low. I've tested at 40 Amps continuous current, measuring temperatures and it will comfortably operate all day. If I needed more current for long periods I would want the fan speed increased.
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Old Oct 02, 2015, 11:24 PM
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Yes that's what I do with the black wire. I saw that someone else just left it disconnected and wrapped the end with electrical tape, apparently had no problem.

You don't need to adjust the pots. I think VR601 will allow you to shift the output voltage, but not much. The supply I tested with could reduce to about 12.2V and up to 12.8V. I don't know what VR602 does, I'd guess it is for current limit or some sort of protection cut off.

I have the fan speed set to minimum. Unless you add your own fan control there are only two choices, flat out or very low. I've tested at 40 Amps continuous current, measuring temperatures and it will comfortably operate all day. If I needed more current for long periods I would want the fan speed increased.
Thanks Jason, I played with the jumpers on the PSU and the fan is nice and quiet but still moves air. I don't think I'll need anymore speed. For the time being I'm only going to be using one of the PSU for a smaller charger. Only 14 bucks CAD for the pair so I couldn't turn it down. I'll stick them both into a hard case all prepped for the day that I need the power. just 30A on a single for now

I use to have an old fan controller around but I think I gave it away. Something to work on when I need that 1500w down the road
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