Voltage reduction calculation.

Discussion in 'General Electronics Chat' started by electronis whiz, Sep 27, 2014.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    I was given a really old video conference computer. From like early 1990s. I tried booting it, it starts, but the CMOS battery has failed. It's some odd ball battery says TL 5186 on it. I thought ok 3 volt battery, i'll just pull it and insert input from a 2 AAA battery pack. It doesn't help. Some reason even changing it and then it reloading while still on clears the bios, so i can't boot from CD due to defaults being wrong. I noticed that the battery is actually odd, and it's 3.6V. I don't want to buy a new one, so thought maybe feed in 4.5 V from a 3 cell pack. But what would i need for a resistor to make ~3.6 V?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  3. subtech

    Senior Member

    Nov 21, 2006
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    If you check brand new AA's or AAA's, you'll find that they often measure 1.60 to 1.65 Volts each.
    I'd connect two in series for 3.2 - 3.3 volts and give that a try. It won't cost much, and it isn't likely to damage anything.
     
  4. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    I tried a pair of triple AAA in holder, i got similar error, but also another said CMOS battery failed in addition to normal check sum error. So either the board id shot, or 3V is not enough to do it.

    I see the picture with the diodes what kind should i try for that, or does it matter? I have some that look like GE diodes that are clear. Some others like you see used in rectification that are black with a white band.
     
  5. MikeML

    AAC Fanatic!

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    The schematic above requires three AA or AAA and two Si diodes, like 1N914s or 1N4001-7
     
  6. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    Ok found a few diodes, got down to about 3.650V errors went away, but some reason even exiting bios, and then verifying like before it is reverting to defaults. Based on age (about 20 yrs) of this thing, and shape it was in when i got it i think i'm going to say something is probably damaged on it. I think better just part it out, and try to see if maybe an older pIII or P4 board fits. It was some sort of camera control system and hooked into up to 8 old 56 K dial up lines.
     
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