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Two PSUs in Series – Poor Thinking = Poor Results?
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t1d:
An apology with benefits, I hope...

Background
As I mentioned, I am wanting to tie my PSUs, because I need more wattage to test my DC Constant Current Load. I posted about the load project about one year ago.

At that time, I researched about tying PSUs in parallel to acquire more wattage and posted that under a separate thread:
https://www.eevblog.com/forum/beginners/how-to-connect-two-psus-to-get-more-current/msg1967069/#msg1967069

I only remembered the findings of that research very vaguely, which led to my questions in this thread. I totally forgot that I had made a complete thread about tying PSUs... Well, in parallel, that is... My questions here are about PSUs in series, but I think all of the solutions are the same as for parallel.

In my sleep, tonight, I remembered something of the first thread and searched for it in my posting list. I found it.

The Apology
I think that the answers in my first post apply to series connections, as well as parallel... Meaning that I may have wasted your time on this thread. I own that.

I apologize for wasting your efforts and creating confusion. I am sorry. Please forgive me.

The Benefits
Several folks have said that an unbalanced condition in tied PSUs can not exist. The referenced post explains how it happens and offers practical solutions. The video at Post #6 does a good job of showing what is going on and how to solve it. So, perhaps for the folks that did not know that an unbalanced condition can happen, there might be the benefit of learning about it.

There is also the benefit of understanding that the diode solutions proposed here only address protecting against reverse power, not balancing. Maybe there is the opportunity of learning there, too.

I hope these benefits do exist, because I feel so bad about creating a wild goose chase. Again, my sincere apologies.

I will go back through the original thread and try the solutions. I will post my findings and, if the proposed solutions do not work, I will ask more questions. Until then, thank you for all your help.
madires:
If you want to parallel PSUs for more current you could use MOSFETs controlled by dedicated power oring ICs instead of Schottky diodes to lower the losses, especially for larger currents.
Zero999:

--- Quote from: t1d on October 20, 2019, 10:29:04 pm ---I printed out Zero999's schematic, so I would not make any mistakes when reconnecting the wiring. I followed the instructions and checked all connections for accuracy.

Without the load connected, I checked all the voltages. Everything was as expected. ~2.2vdc, ~-2.2vdc, and ~4.4vcd.

I then connected the load. The readings were as follows:
PSU #1/V2 = 0.25vdc @2.15a and the constant current LED illuminated. Remember, the CC is set to max on both PSUs.
PSU #2/V1 = 2.10vdc @2.47a and the constant current LED not illuminated.

One PSU is a Tenma. The other PSU  is a BK Precision. Both are rated 30v/3a.

So, something still seems to be incorrect, or misunderstood. What do you think?

--- End quote ---
Your power supplies have two modes of operation and constant voltage and constant current. They will switch between one mode and the other, depending on the load and settings. If it's set to 10V and 1A, then loads with a resistance of under 10/1 = 10 Ohms will result in switching to constant current mode and loads above 10Ohms will result in constant voltage mode.

If you want to use the power supplies in constant voltage mode, which was my assumption, they need to be connected in series, as per my schematic.

If you want to use the power supplies in constant current mode, then connect them in parallel.

The problem is, if the power supplies are not designed to work in parallel, they can become unstable when they're connected in parallel and switch to constant voltage mode. What will normally happen is the one with the higher voltage setting will provide all the current and the other one will do nothing.

Your experiment failed because you connected the power supplies in series, with them set to constant current mode. If you set the current limit on both power supplies to 3A, with a total voltage of 4.4V and a load resistor of 1.5 Ohm, it will work, because the current will be 2.93A, which is under the limit of 3A, therefore the power supplies will be in constant voltage mode.
james_s:
Two power supplies in series or parallel both get you double the wattage of one power supply *if* conditions are right. In order to get the extra power out of the series arrangement, you need a load that can take advantage of the higher voltage. If your load cannot accept a higher voltage than can be delivered by one power supply without drawing more current than can be supplied, then having multiple supplies in series offers no benefit.

In series the max voltage is added, the max current stays the same.

In parallel the max voltage stays the same, the max current is added.

In both cases the max wattage is added provided both supplies are delivering their max voltage at their max rated current, whether this is possible with a given configuration depends on the load.
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