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  1. #1
    Gold Member
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    Barossa Valley, South Australia
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    Duetz DX3.70, Fendt 305LSA,260S & 205P,Kubota B6100E & 3 x B5100E & RTV900, Caterpillar 428C

    Default LED polarity important?

    I recently bought two dashboard warning lights to replace a blown alternator warning light on ny Kub B5100 (second as a spare). They happened to be LEDs and I figured that was probably a good thing, for longevity.

    Fitted the first - didn't work, fitted the second - didn't work.

    Played around with both on the bench, connecting them to a charger 12V supply.

    Eventually discovered that Pos & Neg have to be one way and one way only - then they worked!

    That was news to me.

    Fitted it back on the tractor with correct polarity and it worked, but then stayed on even when the alternator was charging.

    Disconnected the spades to the warning light and connected them to a test light. Discovered that the negative disappears when the alternator is charging and so, the light should go out.

    Re-connected the spades to the light and then, with the alternator charging, disconnected just the negative lead - then the light does go out despite the fact there is no negative in the lead.

    It's got me baffled. Finally reverted to a larger incandescent globe left dangling out below the dash.

  2. #2
    Veteran Member Treemonkey1000's Avatar
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    Jan 2008
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    1,894
    Location
    Renton, Washington
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    Kubota L3750

    Default Re: LED polarity important?

    Quote Originally Posted by allenr View Post
    I recently bought two dashboard warning lights to replace a blown alternator warning light on ny Kub B5100 (second as a spare). They happened to be LEDs and I figured that was probably a good thing, for longevity.

    Fitted the first - didn't work, fitted the second - didn't work.

    Played around with both on the bench, connecting them to a charger 12V supply.

    Eventually discovered that Pos & Neg have to be one way and one way only - then they worked!

    That was news to me.

    Fitted it back on the tractor with correct polarity and it worked, but then stayed on even when the alternator was charging.

    Disconnected the spades to the warning light and connected them to a test light. Discovered that the negative disappears when the alternator is charging and so, the light should go out.

    Re-connected the spades to the light and then, with the alternator charging, disconnected just the negative lead - then the light does go out despite the fact there is no negative in the lead.

    It's got me baffled. Finally reverted to a larger incandescent globe left dangling out below the dash.
    Yes all LED's are polarity devices. So polarity makes a difference. Most LED's are about 1.7 volts to turn them on. There are tri-state LED's that turn red or green by voltage level. When you get a LED that is for 12 volt applications a resistor is generally in the circuit to bring down the voltage.
    I am guessing your LED is staying on because it doesn't draw as much current as the Lamp that you are trying to replace. I think the Alternator warning circuit works by current difference from when the alternator is charging (working right) versus when it is dead and running off the battery.
    I am sure other guys can elaborate
    1st Peter 1:6-9

  3. #3
    Platinum Member
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    Feb 2004
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    937
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    15 mi. N. of Winchester VA
    Tractor
    CK30HST

    Default Re: LED polarity important?

    4 or 5 leds in series will do 12 volts, try that

    mike

  4. #4
    Veteran Member
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    Aug 2007
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    Wayne County Pa.
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    Massey Ferguson model 85, Allis-Chalmers WD-45

    Default Re: LED polarity important?

    An LED is a Light Emitting Diode. They are a diode, flowing current one way but not the other.
    Knowing is not enough, you must apply.
    Willing is not enough, you must do.
    Bruce Lee

  5. #5
    Gold Member
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    Barossa Valley, South Australia
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    Duetz DX3.70, Fendt 305LSA,260S & 205P,Kubota B6100E & 3 x B5100E & RTV900, Caterpillar 428C

    Default Re: LED polarity important?

    Thanks guys, for the information.

    Forever grateful for the wealth of knowledge out there and those who share it willingly.

  6. #6
    Gold Member RWolf's Avatar
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    Apr 2008
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    Central Texas
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    Current, Power King (antique), Soon to have JD 5103

    Default Re: LED polarity important?

    One other little tidbit. On some charging systems the light is in series with the one of the alternator leads (don't remember which one) and as the level of current drops because the battery is charged up the light will fade out. In this case the light is an intragal part of the charging system and replaceing it with an LED is not an option.
    2008 5103, FEL, 6ft. HD Box Blade, 6ft. Rotory cutter. And many more to come.

  7. #7
    Epic Contributor Soundguy's Avatar
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    Central florida
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    ym1700, NH7610S, Ford 8N, 2N, NAA, 660, 850 x2, 541, 950, 941D, 951, 2000, 3000, 4000, 4600, 5000, 740, IH 'C' 'H', CUB, John Deere 'B', allis 'G', case VAC

    Default Re: LED polarity important?

    Quote Originally Posted by RWolf View Post
    One other little tidbit. On some charging systems the light is in series with the one of the alternator leads (don't remember which one) and as the level of current drops because the battery is charged up the light will fade out. In this case the light is an intragal part of the charging system and replaceing it with an LED is not an option.
    If you are perhaps refering to the Delco 10SI, 12SI, or CS-130, and the other similar variants, then you are close.. but no cigar.

    The poular idiot lamp scheme on the '3-wire' delco style alternators places a lamp in line with the #1 terminal on the alternators plug. the #2 terminal is then run right to the battery positive as a voltage drop sense line.. and the charge stud is hooked into the electrical system wherever the engineers put it.. like on the start solenoid.. or on an isolation strip with various solenoids to feed power to various parts of the vehicle.

    As power usage increases, and votlage drop int he car's wire harnes goes up, the alternator senses the difference in it's output and the voltage at the battery via Line #2, and outputs more voltage.. usually up to a max of 14.7v.. though 14.4 is more common.. regulator depending.. etc.

    The #1 line has the lamp inline with switched power.. when you turn the key on, power is flowing thru the lamp tot he internl regulator circuit.. the lamp is lit.. as the alternator excites and turns on and makes power, the votlage on the regulator comes up to output voltage / battery voltage.. because the same potential voltage is on either side o fthe lamp.. no current flows.. because no current flows.. the lamp goes out.

    A diode inline with the #1 wire works perfectly fine.. many antique tractors use this method of automagic alternator excitation, vs a clumsy dash lamp. And 1a/50piv will work.. but 3a/200piv is better... a 1N5408 diode works great. Radio shack part number 276-1661 and 276-1141 both work fine. An led would work if you found one to meet the specs.. etc.

    ( The CS-130 can use the lamp / led method.. but requires 15-45 ohms of resistance in it's excitation line.. cs-130 also has multiple wires in it's plug pin out that can be used for various applications.. but the principle is the same.. etc.. )

    ( to the original poster... as the LED is a diode.. yes.. polarity is important. i believe you will find that in a negative ground environment, that you will need to place the cathode of the LED towards the alternator, thus making the anode towards the switched power.. The cathode is demarked by a line perpendicular to the wire on the diagram, and the anode is an arrow shape pointing to the line.. ----|<------- kinda like that.. )

    Actually.. her's a better diagram of a plain diode.. led will have a circle and an arrow pointing out.. or just an arrow pointing out, in addation to the diode pictoral.

    Soundguy
    Attached Thumbnails Attached Thumbnails -diode-jpg  

  8. #8
    Elite Member DieselPower's Avatar
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    Fairfield, PA
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    JD 3020, JD 4230, JD 7410, JD 2440, MF 750, NH LS170

    Default Re: LED polarity important?

    I have installed probably in the thousands of LED lights on medium and heavy duty trucks over the years. Some are polarity sensitive while others are not. Seems to depend on the manufacturer. The LED itself is polarity sensitive but depending on the manufacturer of the light some wire them to be non polarity sensitive.

  9. #9
    Epic Contributor Soundguy's Avatar
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    ym1700, NH7610S, Ford 8N, 2N, NAA, 660, 850 x2, 541, 950, 941D, 951, 2000, 3000, 4000, 4600, 5000, 740, IH 'C' 'H', CUB, John Deere 'B', allis 'G', case VAC

    Default Re: LED polarity important?

    If they are in a setup where they are fed by a fwb.. then they won't care..

    soundguy

  10. #10
    New Member
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    Feb 2002
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    Location
    Elko, MN
    Tractor
    Kubota B2910

    Default Re: LED polarity important?

    LED's definitely must be wired with the correct polarity. LED modules may have diodes or rectifiers in them to allow connections without regard to polarity.

    Also, the resistor used with LED's that some mentioned limits current, not voltage.

    LED center Why do I need a resistor with an LED?

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