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Technical Alternator idiot light - will LED work?

Discussion in 'The Hokey Ass Message Board' started by Redrodguy, Feb 10, 2020.

  1. Redrodguy
    Joined: Nov 18, 2016
    Posts: 115

    Redrodguy
    Member

    Will be using Powermaster 150 amp 12SI alternator with 3 wire setup. Will a LED idiot light work or do I have to use an incandescent lamp for the light? Does the lamp have to have a certain amount of resistance - if so, how much?
    Thanks.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    Try it, see what happens.

    I think what you might end up needing to do, is to put a 10 ohm (5 watt) resistor in parallel with the LED. But that's just a slightly educated guess.
     
  3. lonejacklarry
    Joined: Sep 11, 2013
    Posts: 1,498

    lonejacklarry
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    I think squirrel is right. I had to do that to make the back up lights on my late model OT truck work. I did it for the turn signals because of the resistance required for the flasher but still had to do it for the back up circuit even though there was no flasher.

    I'm pretty good at wiring house lamps with plugs, however.
     
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  4. I did it this way in my El Camino. The light excites the alternator and illuminates. When the alternator starts to charge the light goes out.

    Put a 560Ω current limiting resistor on the anode of the LED, so these two components are in series. Place a 1N4004 diode backwards across the led to protect it from the reverse voltages - these two components are in parallel. This circuit will replace the light.

    Tricky part is determining which way to put it in - it won't work backwards. Take the old light out. Switch on the ignition without starting (not "acc", but "on"). Measure in the lamp socket to ground on each side. One side will be +12V, the other near 0V. The resistor hooks to the +12V, the cathode of the LED (and anode of the 1N4004) hooks to 0V.

    To test it, take the belt off so the alternator doesn't turn when you start it. Start it, verify the light came on, measure about +12V on the battery. Stop, put the belt on, start again, light should be out. Measure about +14V to verify the alternator is charging the battery.
     
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  5. brading
    Joined: Sep 9, 2019
    Posts: 704

    brading
    Member

    Here you go
     

    Attached Files:

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  6. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    if a 100 ohm resistor will work, then use that.
     
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  7. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,079

    scrap metal 48
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    I have used a LED dash light but wired in and incandescent light in series and hid it under the dash.. Both lights come on with ignition but both go out once it's started....
     
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  8. nochop
    Joined: Nov 13, 2005
    Posts: 3,818

    nochop
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    from norcal

  9. My head hurts
     
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  10. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,079

    scrap metal 48
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    Because a LED light doesn't have enough resistance on it's own to turn the light out...
     
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  11. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    the big question is why you'd want to use an LED in the first place....
     
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  12. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,079

    scrap metal 48
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    They are a very small dash lite that is almost unnoticeable...
     
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  13. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
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    Agree^^.
     
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  14. 5window
    Joined: Jan 29, 2005
    Posts: 9,514

    5window
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    There are places LED lights are fantastic. OT daily drivers-fluorescent replacement shop lights-lights in areas that are difficult to access-places where energy conservation is important. Places where LED lights are not appropriate- where you need the heat as well as light from an incandescent bulb like a dog kennel and TRADITIONAL HOT RODS.
     
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  15. Redrodguy
    Joined: Nov 18, 2016
    Posts: 115

    Redrodguy
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    Thanks for all the replies. Yes - the reason for wanting to use a LED is because they are very small. Not a lot of room on a '32 dash, but I guess I'll stick with the traditional incandescent bulb to keep from upsetting the apple cart.
     
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  16. fastcar1953
    Joined: Oct 23, 2009
    Posts: 3,567

    fastcar1953
    Member

    Put a big ugly light right in front of you. lol. I would do led . maybe at bottom of dash ,less noticeable. 50's and 60's cars are in the dash and can get by with old style.
     
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  18. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
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    I'm using a #194 indicator light for the 10SI in my '32, but placed under the dash on the firewall. An occasional quick look tells me if things are OK. A small led will someday be wired parallel and mounted in the dash, like on the '40.
     
  19. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    An ammeter is traditional. An idiot light isn't something that belongs on a traditional rod, whether it's an LED or a bulb.

    But we've outlawed ammeters, so don't worry about it.
     
  20. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,250

    Ebbsspeed
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    I assume you meant you wired them parallel to each other, not in series........

    .......because if they are in series you have actually increased the resistance of the circuit.


    Ammeters haven't been outlawed in my shop, and never will be. I know there's a big scaredy-cat population on the HAMB and elsewhere about the supposed fire hazard of using an ammeter. Do these same people put the ignition and light switches somewhere in a fireproof enclosure?
     
  21. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,079

    scrap metal 48
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    Correct, I wanted to increase resistance so the LED light would go out...
     
  22. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    I think you want to decrease resistance? a typical LED with a series current limiting resistor has more resistance than an incandescent bulb, assuming the same light output. To get something to trigger that works with a bulb, but not an LED, you would need to decrease resistance (less Ohms). so you'd put a resistor in parallel with the LED.

    But I'm not sure I understand the scenario completely.
     
  23. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,589

    Blues4U
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    from So Cal

    Hmmmmm, won't the diode in parallel with the resistor just bypass the resistor? They should either be in series, or the resistor on the anode of the LED.
     
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  24. Years back I used a resistor wire to excite the 10SI in my car. The resistor wire came out of GM pickup harness, that had the optional voltmeter in the gauge package(no idiot light). Think the harness was mid-70's. Could this type resistor wire be used to run through the LED light, to excite the alternator?
    Pretty sure the wire was single strand. Hey, just a thought.
     
  25. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    The important thing about a resistor is its resistance, measured in ohms. Whether it's a long wire, or a small plastic or ceramic package, doesn't really matter (although the watt rating could matter). Gm did use a long resistance wire in series with a light bulb for the alternator idiot light, in some applications.

    Sent from my Trimline
     
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  26. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,079

    scrap metal 48
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    I could never get the small LED light to go out unless I wired an incandesent bulb inline with the LED light.. I assume that was wired in series.... I wired a lot of my model A builds that way and it always worked.. May not have been right but it worked for me.....
     
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  27. PhilA
    Joined: Sep 6, 2018
    Posts: 2,061

    PhilA
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    1. Hydro Tech

    There's a good reason for the LED by itself never going out, particularly on the 10si and 12si series alternators (and with alternators that have similar regulator packs).
    It's all down to the amount of current that the LED can pass. A regular 2 Watt light bulb will, at 12V forward voltage pass 0.16 Amps (160mA). An LED is rated in the region of 20-30mA (0.03 Amp) at about 2.5 Volts. A "12V" LED that fits in a socket has an integral resistor inside to reduce the amount of current that's going to pass through the LED at 12-14 Volts from a regular battery (or heck, even 6 Volts) else it turns into a little plastic fuse very brightly and quickly.

    So, when you switch on the ignition, with an incandescent bulb you are connecting 12V from the battery through the bulb to the regulator pack, via a transistor that has a Zener diode (electronic pressure relief valve, if you will, that conducts once a certain voltage is across it) connected to the "sense" wire that the regulator pack uses to determine if the battery voltage has reached fully charged.
    With the battery voltage greater than the output of the alternator, this transistor can sink current through the lamp and so the bulb has a path to ground and illuminates.
    Once the rotor is spinning, the base current to the rotor windings is determined by the current that is flowing through the bulb into the regulator circuit through a comparator transistor. Both these paths require about 60mA (0.06 Amp) to trigger the transistors into conducting. That's why revving the engine up will eventually trigger the comparator because the output from the stator rises above that of the battery with the base rotor current. At that point the output of the alternator meets that of the battery so the bulb no longer has zero volts on the other side and no current flows- the light goes out.
    Also, a side effect is that you have a very dead, crummy battery and rev the engine hard the output of the alternator is high enough against that of the battery and the gen light will glow- current passes the other way through the bulb. An LED will not do that, by nature of being a diode.

    In short, the LED will always glow because inadequate current can pass through it to trigger the regulator pack into correct operation. Placing a resistor shunt across the diode allows enough current to pass yet not a complete short circuit that inadequate current flows through the LED to make it illuminate.

    This LED plus resistor method is what's used in modern cars, and it does work satisfactorily.

    So, if you MUST use an LED, that's the proper way to do it- wire a 100 Ohm 1 Watt resistor across the back of your idiot light terminals. (That's also why wiring a bulb in parallel with it makes it work).

    --Phil
     
  28. Redrodguy
    Joined: Nov 18, 2016
    Posts: 115

    Redrodguy
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    Thank you @PhilA for the reply and explaining the technical side of my question - I appreciate you taking the time to respond.
    -Don
     
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  29. PhilA
    Joined: Sep 6, 2018
    Posts: 2,061

    PhilA
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    1. Hydro Tech

    No problem. I didn't know how my alternator worked, so I took it apart and looked at the schematic (Thank you Delco for publishing the service manual!) to determine why it was broken.
    Taught myself a little about the operation- regulator pack varies the current flowing through the rotor windings, to vary the magnetic field generated- the greater the magnetic field, the greater flux and therefore then the larger the output of the stator windings (which connect back to the battery).

    It's backwards from a dynamo, which has the stator windings permanently energized from the battery and the rotor windings having the current induced in them from the magnetic field of the stator- thus unregulated internally and requiring an external box to regulate the output, which is lower due to the smaller number of windings on the rotor compared to an alternator (1 on a dynamo versus 3 on an alternator).

    Interesting stuff, if that's your bag.

    --Phil
     
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  30. Redrodguy
    Joined: Nov 18, 2016
    Posts: 115

    Redrodguy
    Member

    @brading showed a diagram with a 1 amp 200v diode (post #5) before the LED and resistor. This may be a question for him, but what purpose does it serve?
     

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