Need circuit for Alternater idoit light

Discussion in 'The Projects Forum' started by Redryder, Oct 13, 2010.

  1. Redryder

    Thread Starter New Member

    Oct 13, 2010
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    I have a motorcycle with no ampmeter or idiot light. I have small LED that changes color at 12v, >12v green, <12v red. Can anyone tell me how or post a circuit that will allow me to wire this in to indicate as to where or not the alternater is putting out.

    Thanks in advance
    Redryder
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You're not giving us much to go on.

    Keep in mind that your motorcycle battery isn't getting much of a charge unless the electrical system exceeds ~13.7v at 25°C/77°F.
     
  3. Redryder

    Thread Starter New Member

    Oct 13, 2010
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    The intent is to have a light to let me know when the alternater has failed or is not putting out. As it is now I have no indication(execpt diming lights) as to output of the alternater. Thanks again for any and all help.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    That's unusual. If true, perhaps you could just hook it up directly to the battery, or in parallel with anything else that's already always-on (headlight?).

    BUT FIRST, perhaps you could elaborate on that LED. Most would be instantly and explosively destroyed by hooking directly to a 12v battery.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    With a fully charged battery inplace, use a comparator circuit set to where it's just on the edge of changing state when the engine's not running.
     
  6. Redryder

    Thread Starter New Member

    Oct 13, 2010
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    The LED is sold by electriconnection.com and is intended as a low battery warning light.the indications are- >12 volts green, 11.9 to 11.5 volts yellow, <11.5 volts is red. I was wanting to use it on alternater output to indicate a failed alternater.

    marshallf3-I don't know much about electrical circuits. What did you by comparator circuit?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, OK - is it using this circuit? http://kd1jv.qrpradio.com/batmon/batmon.HTM

    If so, I'd re-adjust the circuit to indicate green if over 13v, yellow if 12.5v to 13v, and red below 12.5v.

    If it is in fact a single LED, then I suggest wiring it up in series with two 1N914/1N4148 or 1N400x series diodes, anodes towards +12v; each diode will drop roughly 0.75V-0.8V; the 1N400x diodes a bit less. This will raise the thresholds of where the LED changes colors.
     
    Last edited: Oct 18, 2010
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Both solutions SgtWookie proposes will work just fine.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    A little bit of an easier solution could be to hook a 13v zener between the alternator and a 12v NC relay. A 12v LED attached to the battery through the relay.

    When the alternator fails to output the zener voltage, the relay closes, lighting the LED from the battery.

    It can be done with 3 components.

    12v Automotive LEDs are easy to acquire at auto stores.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Retched,
    I'm afraid the relay/Zener approach wouldn't work too well.

    Relays usually actuate at around 70% of their rated coil voltage, and drop out at a much lower voltage; perhaps 20% to 40%. Too much hysteresis to be useful in this case.
     
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