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  • July 15, 2008 6:37 a.m. petegossett Dork

    It's been 18-years since I've studied this stuff in school, so please go easy on me!

    I'm working on a custom gauge-cluster from a 63 Rambler & want to install LED lighting. One thing I remember from school, is that LED's need a current-limiting resistor in series with them, to prevent burning them out. However, I have a box full of tiny LED flashlights I'm planning on using for the LED's - but I noticed they don't use a resistor.

    So, here's my question: Has something changed in LED design in the last 18-years, that no longer requires a resistor in series? Or is there just simply not enough current available from a small battery to be an issue?

    Thanks! Pete

  • oldopelguy

    July 15, 2008 7:24 a.m. oldopelguy HalfDork

    Mostly the latter, just not enough current pushing power in a battery. They still need current limiting resistor unless they have them built in.

    To figure out how big your current limiting resistor(s) need to be you need to know breakover voltage for those diodes. Used to be you could just go with 2.1V for green and 1.7V for red, then more for the blue and white (@3.5V) but I haven't kept up well. Subtract off that voltage from your supply (supply =14V, for example, so 14V-2.1V=11.9V) then divide that by the amount of current to which you want to limit the resistor. (11.9V/30mA=396 ohm) You can wire several LEDs in series, say 5 green @2.1V=10.5V, 14V-10.5V=3.5V, 3.5V/30mA=120ohm resistor.

    As a general rule for single LEDs I always start with 330 ohms and adjust up or down from there.

    You probably knew all this, but it might help someone.

  • doitover

    July 15, 2008 9:48 a.m. doitover New Reader

    If there is no other circuitry in the flashlight either the LED's have an integral resistor or they have matched the voltage of the battery to the LED so that one isn't needed. If there is another chip in there then they may be using a constant current supply, which is the best way to drive LED's from a battery, it will give the most consistent light through the discharge cycle of the battery.

    For your car, unless you are going for ultimate efficiency a resistor is fine since the supply voltage is constant.

    If you are using LED's from cheap flashlights, or really any set of random LED's, you may want to put some effort into matching their light levels.

  • July 15, 2008 10:20 a.m. petegossett Dork

    Good point on matching their light levels...then again, it's a scratched, beat up, 45-year old plexiglass bezel, so a little light imbalance may just add to the character.

    Thanks for all the tips!

  • chknhwk

    July 19, 2008 8:37 a.m. chknhwk New Reader

    How about an electrical engineering question: Three 167kva single phase 13.8kv transformers hooked up in three phase on the secondary side, 120/240v. What is the amperage rating for this setup, assuming 350mcm cu secondaries.

  • oldopelguy

    July 19, 2008 10:00 a.m. oldopelguy HalfDork

    Those are huge transformers, 165kva. That's like 3-6 houses worth of transformer each.

    Are the secondaries of those transformers connected together in Delta or Wye configuration, and is it pure 3-phase or 3-phase with ground?

  • Jack

    July 19, 2008 10:14 a.m. Jack SuperDork

    oldopelguy wrote:

    Are the secondaries of those transformers connected together in Delta or Wye configuration, and is it pure 3-phase or 3-phase with ground?

      1. and what is the air speed of a laden swallow?

    Jack

  • chknhwk

    July 19, 2008 10:41 a.m. chknhwk New Reader

    Well what do you mean, an African or European?

  • chknhwk

    July 19, 2008 10:42 a.m. chknhwk New Reader

    Wye configuration with ground. Wait, don't you have to have a ground with a wye configuration anyway?

  • Jack

    July 20, 2008 1:19 a.m. Jack SuperDork

    I don't know!

    That's both the type of swallow and the ground issue, although I have dealt three, single phase transformers working off a single line of three phase power more than the swallows.

    Jack

  • chknhwk

    July 22, 2008 2:25 a.m. chknhwk New Reader

    LOL Well for reference we got about 600 amps off each main. Not sure how long it's going to last... I sent it to engineering to look at. May have to upgrade. Again.

  • Ian F

    July 22, 2008 10:30 a.m. Ian F New Reader

    chknhwk wrote:

    How about an electrical engineering question: Three 167kva single phase 13.8kv transformers hooked up in three phase on the secondary side, 120/240v. What is the amperage rating for this setup, assuming 350mcm cu secondaries.

    Hmm... ran a quick calc... depending on your demand load, you should be fine. I designed a nearly identical set-up for a supermarket years ago (was restricted to pole-mounted transformers in lieu of std pad-mounted due to potentional flood conditions), using an 800A I-Line panel for main distribution. Unfortunately, I forget what the MCB was... sorry... There was also a seperate 480V service fed with triple pole-mounted transformers since (3)167 @ 120/240 wasn't enough juice.

    I hope your running the parrallel feeders as 350's are only good for about 300 - 400 amps depending on which chart you're looking at.

  • chknhwk

    July 22, 2008 1:29 p.m. chknhwk New Reader

    Yeah, the feeders are ran in parallel, but that's still a lot of load... Residential. Sometimes working in the city sucks.

  • oldopelguy

    July 22, 2008 4:38 p.m. oldopelguy HalfDork

    Yeah, the # I got was closer to 800A as the absolute max depending on the ambient temps. With a Wye connected secondary the phase voltages are lower than line voltages and allow for a higher current on the secondaries vs. Delta configuration.

    As to the ground on the secondary side, it really doesn't need to be there for any 3-phase loads. None of the motors or transformers running off it will care. Back in the Canoe Club nothing was grounded, and even in the civilian power generation industry certain sides of certain transformers are run without grounds to isolate some equipment.

 
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