HP DPS-1200 FB problems when running in series

[jarppi]

Hi,

I bought a few HP DPS-1200 FB power supplies to power my chargers. I was able to get them running by following the guide in here: http://www.rchelination.com/setting-...-power-supply/.

I also isolated DC ground (AC ground left as is) from one of them when trying to build 24v power supply, but something seems to be messed up as I don't get full 24v, but about 18v instead. I hope someone more knowledgeable than me would be able to tell what is going on and how would I be able to fix it.

So, I have 2 PSUs:

• PSU-1: DC grounded normally
• PSU-2: DC isolated to create floating PSU

When I run each of them individually, I measure about 12.2v output from each one of them. When I connect them in series and place them on top of each other, PSU-1 being the first one with it's (+) connected to PSU-2 (-), I get only about 18v. However, if I put them next to each other so that cases are not touching, I get about 24v.

I did some measurements that hopefully help in pinpointing the problem.

PSUs on top of each other:

• PSU-1 ground -> PSU-1 case: 0 ohms
• PSU-1 ground -> PSU-1 (+): 12v
• PSU-1 ground -> PSU-2 ground: 9v !!! (so, about 3v lost in the wire between PSU-1 (+) and PSU-2 ground)
• PSU-2 ground -> PSU-2 (+): 12v
• PSU-1 ground -> PSU-2 (+): 18.5v

PSUs next to each other, not touching:

• PSU-1 case -> PSU-2 case: about 3v (fluctuating) (this puzzles me a lot as both cases are connected to the same AC ground and still there is difference)
• PSU-1 ground -> PSU-1 (+): 12v
• PSU-1 ground -> PSU-2 ground: 12v
• PSU-2 ground -> PSU-2 (+): 12v
• PSU-1 ground -> PSU-2 (+): 24v

I also tried using two other power supplies to do the same, but same thing happens, so I assume the PSUs are not defective.

I found some posts describing similar situation, but didn't find an explanation and/or solution to the case. I would be very happy if I can find some help in here as this just doesn't make any sense to me.

[TeroS]

Looks to me that your floating on PSU2 is not right since you get the wrong voltage when cases touch.

[jarppi]

Thanks for the reply TeroS. Any idea how should I proceed in investigating the issue?

I don't get any reading with my multimeter between the PSU-2 ground and the case. I have used plastic screws with plastic spacers on all three places where the board is connected to the case. The only place where anything except plastic is connecting the case is the AC ground.

Is there something else I should still look for?

[TeroS]

What is the exact type of the PSU's? How have you fixed 12V outputs on PSU's, is there any possibility to any solder-issues to GND there? Please describe how you have them connected in serial and where you measure 18V?

I have four HP 1200 FB (3 floating) and I get pure 48VDC out when all cases are touching. Model is "DPS-1200FB A"

[TeroS]

Just chatted with some friends using the same PSU. Some of them have the symptom as you but when PSU is under load the voltage have always been ok. Have you tried under load?

[jarppi]

Cool, I was hoping this might be related to load, but didn't dare to test it as the symptoms are kind of strange.

Unfortunately, I am upgrading my charging equipment PSU-first, so I don't have a charger with 24v input. At the moment, I only have iCharger 106b+ and that is rated only up to 18V. I have to figure out if I have any other device in the house that would take 24V...