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Thread: Fischer isolation

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    RBW Member panthalassa is an unknown quantity at this point panthalassa's Avatar
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    Fischer isolation

    Howdy-

    This may already be answered somewhere, but if it is my googling has so far been unsuccessful.

    Can anyone explain the isolation circuitry on the Narked-at-90 fischer cable? (specifically the one that comes with this: http://www.narkedat90.com/3_cell_bac...evo_p/0008.htm). Or what it means for a backup fischer cable to be isolated? Is it to prevent current from flowing back into the primary electronics and screwing up the cell readings? Most of the (limited) threads I've seen discuss this in a way that assumes the reader already knows the answer to these questions.

    What does it consist of? What is it ultimately meant to do? I have seen posts that say NOT to put all three of your cells on backup. What about the isolation circuitry makes this a bad idea?

    I have a decent understanding of electronics but I'm not an engineer (so temper your explanations accordingly, if possible). I've emailed narked but haven't heard back so I figured I'd ask the community.

    Thanks!

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    RBW Member Doomanic is an unknown quantity at this point Doomanic's Avatar
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    Re: Fischer isolation

    I'm interested too, so if you can dumb it down I'd be grateful.

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    Supporting Member Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36's Avatar
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    Re: Fischer isolation

    I will try to answer but other with more functioning brains cells may explain it better.

    The isolation is merely a safety factor that prevents a shorted cell or cable from effecting the reading of the other cells. It is accomplished by the use of a 1K resistor in the positive side of the cell output cable.

    Some po2 monitors (such as the rEvo dream) already have high enough impedence that a resistor is not needed.

    Please correct me if I am wrong. It has been a long time since I was actively researching and implementing these resistors for isolation on my rig.
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    RBW Member sailor is on a distinguished road sailor is on a distinguished road sailor's Avatar
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    Re: Fischer isolation

    Hi,

    I don't know how N@90 is doing it in paticular but I can expalin how most homebuilders I'm aware of will do it.
    The isolation resistors prevent a cell or the cells to suffer from a short circuit when the backup cable or backup fisher connection fails for some reason (e.g. cut wire or open fischer connection).
    A short circuit without isolation resistors will drop the cell voltage to a meaningless low value as the cells are not designed to provide a higher current drawn by a short circuit, making the cells output useless for the main system.
    With the isolation resistors installed on the backup system it is technically not possible to draw a higher current than a healthy cell can handle.
    This will save the cells output for the main system in case of a short circuit on the backup system.
    The resistors will not influence the backup system significantly as the internal restance of the backup computer is 10 times higher in general.

    Hope I explained it in an understandable way as a non-english native speaker and as a non electronic engineer, Reiner
    Last edited by sailor; 26th March 2017 at 22:58.

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    RBW Member panthalassa is an unknown quantity at this point panthalassa's Avatar
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    Re: Fischer isolation

    That makes perfect sense. Thanks.

    So, what then is the concern with placing all the cells on backup? Or is it just an extra margin of conservatism?

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    RBW Member sailor is on a distinguished road sailor is on a distinguished road sailor's Avatar
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    Re: Fischer isolation

    Reliability?
    If a dual (or possibly triple?) cell failure happen, then it will influence both systems in the same way.
    Last edited by sailor; 27th March 2017 at 07:39.

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    RBW Member panthalassa is an unknown quantity at this point panthalassa's Avatar
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    Re: Fischer isolation

    Yeah but cell failure is a different thing. I'm talking about some failure (short, broken wire, flooded connection, etc.) of the backup specifically. Which the isolation circuitry is presumably meant to handle.

    I'm just interested to know if it's a particularly bad thing to have your backup monitoring all three of your primary cells, and why? Is there another failure mode that the isolation isn't designed or equipped to handle? (and I'm not talking catastrophic failure, I just mean some failure on the backup side: from the fischer cable onward)

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    RBW Member panthalassa is an unknown quantity at this point panthalassa's Avatar
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    Re: Fischer isolation

    Anyone? Is there a reason why it's a Bad Idea to stick your backup monitor (e.g. Shearwater) on all three of your (Inpso) cells? I am using a presumably isolated fischer cable.

    What other failure modes (aside from basic cell failure) can the isolation circuitry not handle?

  9. #9
    RBW Member JohnnyC is an unknown quantity at this point JohnnyC's Avatar
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    Re: Fischer isolation

    Quote Originally Posted by panthalassa  View Original Post
    Anyone? Is there a reason why it's a Bad Idea to stick your backup monitor (e.g. Shearwater) on all three of your (Inpso) cells? I am using a presumably isolated fischer cable.

    What other failure modes (aside from basic cell failure) can the isolation circuitry not handle?
    Meg Fischer backups read all 3 cells. They run an isolation board. No issues. There's a cool video of Leon cutting cables and counterlungs and still flying the unit off the HUD.

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