# connecting two 12V power supplies together

Discussion in 'Electronic Design' started by panfilero, Mar 6, 2008.

1. ### panfileroGuest

Hello,

I'm trying to figure out a way to have two power signals coming into a
circuit and the output remaining the same.... what I mean is that I
have two inputs where I need to connect 28V DC, and I should have the
same result whether only one of my 28V inputs is plugged in or both
are plugged in... I'm going to try and feed these inputs into a 5 Volt
regulator.... if I connect both the positive connections of these two
inputs into the regulator.... would that be ok? Like connecting to
battery or power sources in parallel? Something feels funny about
that to me... and I'm not sure if it makes sense to connect two power
sources together like that.... my gut is saying no. But I'm not sure
why.

Any thoughts on this is much appreciated,
thanks
J.

2. ### Fred BloggsGuest

Power sources are not usually called "signals." You will be okay by
diode ORing them...

3. ### Greg NeillGuest

It's unlikely that both 28V supplies will be
precisely the same voltage (due to design
differences, component tolerances, etc.),
thus you will end up with an undesirable
current loop between the supplies.

What is the purpose of having two 28V supplies?

4. ### panfileroGuest

I think that's what was bugging me... that the two power supplies
aren't going to be clean regulated 28V sources... so I'm a little
rusty on my electronics theory but there is something that leaves me
uncomfortable about doing that.... just connecting them in parallel
like that. I'm thinking a Diode OR type solution would be the way....
basically I'm trying to pass the DC power on to somewhere else... but
the DC power may come from two different sources and either source (or
both) may be plugged in at any time, but I just need to pass that 28V
on through regardless of which source is connected.

much thanks

5. ### Guest

One way is to "diode or" the two 28V supplies. Connect the diode
anodes to each of the supply outputs. Tie the cathodes together and
to the input of the 5V regulator. Whichever supply has the highest
output voltage will supply the load. This works best when the 28V
supplies have current limiting. This ensures that when the dominant
supply reaches its current limit, the other supply will take over.

6. ### zGuest

yeah, if you're not concerned about sharing the load equally so as to
maximize the output current available, just OR them and whoever's got
the most voltage will carry it; being off constituting a very low
output voltage.

7. ### panfileroGuest

DIODe Or Solution sounds best... the only con about using that is the
voltage drop across the diode, if at all possible I'd like to get the
whole voltage across...

much appreciate the responses

8. ### Guest

Yeah, the diode drop can be a problem. You can minimize it by using
Shottky diodes.

9. ### zGuest

yeah, this is getting confusing. the title says two 12v supplies, the
text says 2 28v supplies to be regulated down to 5v, now we're trying
to avoid any voltage drop. ?

10. ### panfileroGuest

Thanks, for the relay idea.... a switch wouldn't work cause it has to
be automatic.... and the power does more than just go to a regulator
so getting the entire 12V passed is a concern, that's why I'm looking
for something other than the Diode-OR.

11. ### panfileroGuest

Ahhh.... sorry about the 12V/28V confusion..... that part is a bit
arbitrary to me, there will be situations where I'm passing 12 and
other situations where I'm passing 28. And the 5V regulator.... it's
in the circuit as well but the power does more than just provide the
input for the regulator.

Thanks

12. ### Fred BloggsGuest

And what will you do for us if we tell you, other than disappear?

13. ### JosephKKGuest

You could use some electronics and big power FETs.

14. ### Fred BloggsGuest

View in a fixed-width font such as Courier.

..
..
..
..
.. VPC
.. .-----+-----+-----------------------------------.
.. | | | |
.. | | | |
.. | |B1 |B2 |
.. | --- --- L
.. | - - O
.. | | | N-CH2 A
.. | | | VN1 D
.. | | '-----+---D S ---. |
.. | | | G | |
.. | | | | | |
.. | | \ ------- | VNC |
.. '-----|-------------|ARBITER|--+----------------'
.. | / ------- |
.. | | | |
.. | VN2 | G |
.. '-----------+---D S----'
..
.. N-CH1
..
..

Depending on the application, the arbiter could take any one of several
forms, and more than a few of those require detecting a low batt
threshold, and that is complicated when the battery is relatively arbitrary.

15. ### Jim ThompsonGuest

Except that your circuit doesn't work. The devices need to be
"back-to-back" in series, as in this previously posted circuit...

http://analog-innovations.com/SED/PerfectDiodeForChargerIsolation.pdf

(Body diodes _must_ be considered as sneak paths.)

...Jim Thompson

16. ### Allan HerrimanGuest

Hi Jim,

Fred's using N-ch mosfets with the drains on the negative inputs. The
body diodes can't contribute to reverse current flow.

The only thing he loses (by not having back-to-back fets) is input
overvoltage protection and the ability to "turn off" an input.

A similar arrangement would work for P-ch fets on the positive inputs
with a controller such as the LTC4416.
See, for example, figure 5 in this datasheet:

Regards,
Allan

17. ### Fred BloggsGuest

The circuit is a standard diode-OR when the FETs are off. Start from
there...

18. ### Fred BloggsGuest

This would be the extreme case for a 24V system:
View in a fixed-width font such as Courier.

..
..
..
..
.. VPC
.. .-----+-----------------------------------.
.. | | |
.. | | + |
.. + |B1 |B2 |
.. --- --- 20V L
.. 29V - - O
.. | | - A
.. - | | VN2 D
.. | '--------|<|-------. |
.. | | |
.. | | |
.. | | VNC |
.. | +----------------'
.. | |
.. | |
.. | VN1 |
.. '------------------------'
..
..

19. ### Fred BloggsGuest

Here's a bad example of an arbiter, part of one anyway
View in a fixed-width font such as Courier.

..
..
..
..
.. VPC
.. .-----+-----+-----------------------------------.
.. | | | |
.. | | | |
.. | |B1 |B2 |
.. | --- --- L
.. | - - O
.. | | | N-CH2 A
.. | | | VN2 D
.. | | '-----+---D S ---. |
.. | | | G | |
.. | | | | | |
.. | | \ ------- | VNC |
.. '-----|-------------|ARBITER|--+----------------'
.. | / ------- |
.. | | | |
.. | VN1 | G |
.. '-----------+---D S----'
..
.. N-CH1
..
..
..
..
..
.. ARBITER
..
.. D1
.. VPC>---|>|---.
.. |
.. .---+--------+-----+---+---+-----+--------+---.
.. | | | | | | | |
.. | | | | | | | |
.. | | | | | | | |
.. [680] | | [27k] [27k] | | [680]
.. | | | | | | | |
.. | | | | | | | |
.. | - | | | | - |
.. | ~~V D2 |IC1:A| |IC1:B| D3 V~~ |
.. | - |/| | | |\| - |
.. >| | /-|-----------+---|+\ | |<
.. 2N3906 |-+-[2 K]-< | | | | >-[2 K]-+-| 2N3906
.. /| \+|---+-----------|-/ |\
.. | |\| | | |/| |
.. G<--+ | +-------|-----|-----[2.7M]-+-->G
.. N-CH2 | | | | | | N-CH 1
.. +-[2.7M]-----|-----|-------+ | |
.. | | | | | |
.. | | [56k] [56k] | |
.. | | | | | |
.. [10k] '---------+---------' [10k]
.. | | | | |
.. | .-----' | '-----. |
.. | | | | |
.. VN2>--|------------+-----|<|-+-|>|-----+------------|--<VN1
.. | D4 D5 |
.. | |
.. '----------------------+----------------------'
.. |
.. VNC
..
..
.. D1,3,4,5,6,7 1N914
..
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..
..
..
..
..
..
..

20. ### Jim ThompsonGuest

Yep, Sorry, That does work for "loads". My approach is for
charge/discharge isolation, as in LiIon battery controls.

...Jim Thompson