connecting two 12V power supplies together

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>--|------------+-----||-----+------------|--

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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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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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.

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