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I need to have two electrically isolated power sources.

Here are the details of the application: http://revolectrix.fmadirect.com/support_docs/item_1377.pdf page 26:

enter image description here

I need to make this work with a car 12V DC installation as the only power source. This should work with a current up to 70 A (max of my car alternator).

Is there something I can attach to the car's 12V installation to make two isolated power sources out of it, to power these two FMA Cellpro Powerlab 8 units?


Clarification: the isolation of the power supplies is required because the two battery-charging devices connected to them are connected to each other in series, so the (+) terminal of the first one is connected to the (-) terminal of the second one. If the power supplies would not be isolated, then they would have common (-) and effectively the (+) and (-) terminals of the first device would be shorted.

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  • \$\begingroup\$ Is there any chance you could use the configuration H (page 23-24 in the pdf)? Maybe switching to an other type of battery pack is the easier option here. \$\endgroup\$ – 0x6d64 Dec 3 '11 at 11:02
  • \$\begingroup\$ @ox6d64: unfortunately no, because the battery pack as a whole needs at the same time to supply power to other devices, so separating the battery pack in two would make it not usable during charging, which is unacceptable here. \$\endgroup\$ – miernik Dec 3 '11 at 11:21
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    \$\begingroup\$ This strikes me as going about things the wrong way; it's certainly possible but quite cumbersome. Likely the best solution is to back up, re-think approaches to the original problem this equipment is trying to solve, and choose more suitable modules or a different configuration. \$\endgroup\$ – Chris Stratton Dec 3 '11 at 14:19
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    \$\begingroup\$ No, I'm not going to dig thru the stuff you linked to. Links should be for background only, not for critical information related to the question. Directly tell us the relevant parts. \$\endgroup\$ – Olin Lathrop Dec 3 '11 at 14:56
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There's no way I'm reading all that PDF to find out what you need the power supplies to do. Please summarize it for those of us that don't need to know all the details of your project just to advise you on one part of it.

However, that said, here's my immediate thoughts:

  1. The only way to properly isolate a power output from its source (and hence from any other power output on the same source) is with a transformer.

  2. The only way to use a transformer is with AC.

So, if you can generate an AC wave form with enough current capacity to do what you require, you can use that to drive a transformer, which you can then rectify/smooth/etc the output of. This could be a multi-tap transformer to give you multiple outputs, or it could be a two single tap ones.

Of course, if you don't require 100% isolation, then there are other options. Two switched-mode power circuits running off one feed will give a certain level of isolation between the two circuits. It all depends if you require complete isolation, or just enough such that over-current on one output can't cause a voltage drop on the other output.

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  • \$\begingroup\$ Well, you don't need to read the whole PDF, or read anything, just go to page 26 and see the diagram, its easier to see the schematics then to explain. Anyway I added some clarification. \$\endgroup\$ – miernik Dec 3 '11 at 10:28
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    \$\begingroup\$ We have a policy that no question or answer should rely on any external website / data if at all possible. I have added a screen-shot of the relevant diagram from the data sheet to your question. \$\endgroup\$ – Majenko Dec 3 '11 at 11:11
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My first idea would be to directly connect the first PowerLab (PL1) directly to the car battery, and establish the galvanic isolation between PowerLab nr. 2 (PL2) and PL1. To achieve this, you could generate AC, feed this AC into a transformer (which does the actual isolation) and then do a AC-DC conversion to have 24V DC for PL2.

This whole setup obviously would have a lot of power losses along the several conversions and would require quite beefy converters which can handle the currents (if I understand you right, we talk about 35A for PL2).

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The quick and dirty answer is that that you can use an inverter, feeding into two isolation transformers that in turn feed to two (now isolated) AC/DC supplies. Be careful how the computer hooks up as there will be shared grounds and loops -> so you will need isolation there too.

If you are looking for an elegant solution, you can look at boost DC-DC convertors that use transformers (Perhaps SEPIC) and in some cases you can rewind the transformer to be an isolating type. Of course in these they provide feedback for the output voltage through a opto-coupler (no direct pin feedback for proper isolation) although it is possible to get load and voltage feedback from additional windings on the transformer. We used to build these out of transistors and wind our own transformers on ferrite cores (Pot cores).

Here is an example of an isolated convertor. Picture from that link too. enter image description here

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