Electronics > Beginners

Two PSUs in Series – Poor Thinking = Poor Results?

(1/4) > >>

t1d:
What I am trying to do:
Tie two PSUs in series to create a positive, GND and negative rail, while balancing the load between the two PSUs.

Why I need to do this:
I need more watts to be able to test the limits of a DC Constant Current Load circuit. And, I need "tri-state" powering capability for my lab.

Why I am using multiple diodes and resistors:
To increase the wattage they are capable of supplying.

What is going on:
To test balancing, I set both PSUs to 2.18vdc, measuring with my multimeter, because the PSU readouts are only roughly correct. I left their current settings unlimited. Then, I connected the load.

I expected to see the voltage and current readings of the PSUs to be rather similar. What the readings actually were was:
PSU #1 – 2.15a @ 1.1vdc
PSU #2 – 2.38a @ 2.1vdc

Conclusion:
The amperage seems to be roughly coordinated. The voltage does not seem to be well coordinated, with PSU #1 doing more of the work.

What I suspect:
I think either…
- The diodes are unbalanced, or
- That the diodes do not provide enough resistance to cause the PSUs to balance, because multiple diodes are being used.

I would like an explanation of what I am doing incorrectly and what I need to do to make it work as desired. Thank you for your help.

capt bullshot:
For a series connection of the PSUs you don't place the diodes in series with the PSU outputs. Your schematic doesn't make any kind of sense to me.
Normally, to protect series connected PSUs, one would place a diode in reverse direction parallel to its output (some PSUs have that internally already).

madires:
Connect + of PSU #1 with - of PSU #2 directly. It's as simple as connecting two batteries in series.

Ice-Tea:
As Madires said: think of them as batteries (note: only valid for floating supply). Say you have a AA battery. Convention has it to call one end "0" and the other "1V5". But it would be equally right to call one end 783V and the other 784,5V. The only thing you truly know is that one end sits 1V5 higher than the other. Now, say you have two batteries, you call one -1V5 and 0V and the other 0V and 1V5. As you now have an electric siystem where you assigned the same name or reference to two nodes, you need to connect them and you're done.

Zero999:
Connect the power supplies in series, with a diode in reverse parallel with each. The power supplies need to be floating, i.e.neither side connected to earth.


If you just connect the load between the +V and -V terminals, ignoring the ground symbol, the current will be shared equally between each power supply. This is due to Kirchhoff's current law which states the current trough all components in a loop will be equal.
https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws#Kirchhoff's_current_law

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod