Low Voltage Indicator

Discussion in 'The Projects Forum' started by IDriveaford, May 20, 2010.

  1. IDriveaford

    Thread Starter New Member

    May 20, 2010
    Hey guys, I was wondering if any of you know how to build a circuit similar of that in vehicles that measure voltage from the battery and alternator to trip the charge light.

    The circuit works by measuring voltage at the alternator; if voltage is less at the alt. then of that at the battery (such as vehicle off, ignition run) the light would come on. (12volts at battery, 0 at Alt)

    The light would then shut off when the voltage at the alt met or exceeded the voltage at the battery. (12volts at battery, 13.6 at alt)

    Anyone out there know how to wire something up like this?

    Thanks alot.

  2. retched

    AAC Fanatic!

    Dec 5, 2009
    A comparator using the car battery as the reference, and the alternator output as the comparator input, you can trigger an event when it is out of range.

    This event can be a simple LED being lit, or a relay being energized...the sky it the limit.

    Last edited: May 20, 2010
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    I doubt it is possible without modifying an alternator, other than piggybacking the existing charge lamp (which is dodgy).

    Most alternators have an internal bridge rectifier and the output of that is hard-wired directly to the battery positive.

    The alternator is regulated by controlling it's exitation current.

    The regulator circuit is fed from an auxiliary positive bridge, and in turn feeds the positive of the exitation winding.

    The charge lamp in the vehicle connects between the switched ignition power and the positive of the exitation winding. It puts a small current through the winding to 'bootstrap' the alternator so it can then produce it's own exitation (and main) power.

    If the lamp circuit is broken and there is not sufficient residual field in the alternator, it will not give any output.

    If you put too much current through that circuit, you are bypassing the regulator and the alternator will give too much output at low loads, likely damaging the battery.

    The only point you may be able to connect to safely is a Star Point output.

    Some alternators have an extra terminal connected at the star point of the main windings. That gives approximately half the main output voltage and is a direct measure that the alternator is working.

    On vehicles that had it, it was to actuate the automatic choke mechanism on the carburettor. As carburettors are almost obselete and it was only one of several automatic choke systems, it's quite a rare feature.
    It is possible that an alternator has an internal star point which has no external connection, but I doubt the effort and risk involved in connecting to it are worthwhile.

    Can't you just use an expanded scale voltmeter or a simple comparator that measures the battery being above about 14V? That should give an indication the alternator is actively charging the battery.

    Edit: Another thought - put a comparator on the alternator charge lamp terminal and look for the voltage rise when it self-exites and the regulator kicks in. There will be some voltage from the charge lamp feed, but that should increase once the alternator is actively generating power. It will vary with demand and engine revs, but should always be at least a bit higher than the none-running voltage.
    If the alternator brushes fail, it will give a high (= battery) voltage there with no output from the alternator, but even before the engine is started so it is a detectable fault.
    Last edited: May 21, 2010
  4. IDriveaford

    Thread Starter New Member

    May 20, 2010
    Thanks a lot RJenkins and Retched.

    RJenkins, you are very true about that. I could use a comparator. I find myself pretty dumbfounded however. The model engine I am working with is an 86 Mercruiser 3.0. Come to find out on their website, all of their alternators have an L2 plug then in parentheses they state that it is rarely used, but used for a low voltage/charge indicator. Just what I need without having to build a circuit for it. Neg. goes to the light, positive from the light to L2. EASY thank God.

    Appreciate the help. Now I need to start a new thread on how to wire up a comparator using a thermistor (heated up to 200 degrees F) to trip a High Temperature lamp. Oh boy.
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    Spare yourself the trouble and get a Klixon switch - http://sensata.biz/klixon/thermostat-precision-7bt2.htm