connecting two 12V power supplies together

panfilero · 2008-03-06T13:39:36+00:00

Hello,I'm trying to figure out a way to have two power signals coming into acircuit and the output remaining the same.... what I mean is that Ihave two inputs where I need to connect 28V DC, and I should have thesame result whether only one of my 28V inputs is plugged in or bothare plugged in... I'm going to try and feed these inputs into a 5 Voltregulator.... if I connect both the positive connections of these twoinputs into the regulator.... would that be ok? Like connecting tobattery or power sources in parallel? Something feels funny aboutthat to me... and I'm not sure if it makes sense to connect two powersources together like that.... my gut is saying no. But I'm not surewhy.Any thoughts on this is much appreciated,thanksJ.

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Fred Bloggs 18 years ago

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 . /| \\+|---+-----------|-/ |\\ . | |\\| | | |/| | . GG . N-CH2 | | | | | | N-CH 1 . +-[2.7M]-----|-----|-------+ | | . | | | | | | . | | [56k] [56k] | | . | | | | | | . [10k] '---------+---------' [10k] . | | | | | . | .-----' | '-----. | . | | | | | . VN2>--|------------+-----||-----+------------|--

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Jim Thompson 18 years ago

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

...Jim Thompson

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well, the loss in the diode isn't that much..... you'd have to really want every milliwatt to go to the complexity of a relay over the diode, but if you do, then you would.

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panfilero 18 years ago

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I'm sorry if this is obvious but I'm just trying to understand why this is a kind of involved subject... is the problem with switching between voltage sources and hooking them up in parallel that if they are not exactly equal sources... something bad could happen? And I'm thinking bad as in batteries trying to charge eachother or force eachother towards a certain voltage... but now I just said batteries... and that's because that's what comes to mind when I think of these DC Voltages in parallel... so I'm guessing the diode-OR is a way of keeping current from seeping back in towards a power supply when two are connected... I'm just tryign to get a good understanding of the issue and why we need Diode-OR and FETs to connecet power supplies like this....

thank you

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z 18 years ago

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actually, when i think about it, there are most likely diodes in the power supplies to act as rectifiers, so maybe there isn't anything external other than just connect them, as you say.

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ehsjr 18 years ago

The diodes prevent source A from forcing current into the output side of source B, or vice-versa. Pushing current into a source in the wrong direction can result in burned out parts, wasted power, overcharged batteries, whatever, depending on the sources involved. It's _not_ a good thing.

Ed

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