LED and Driver questions. A couple of basic ones.

Discussion in 'The Projects Forum' started by Tom Kay, Feb 16, 2009.

  1. Tom Kay

    Thread Starter Active Member

    Feb 10, 2009
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    Hi;

    I am still researching the concept of using LEDs in my home. Some would have to be dimmable to be useful, so that means I've been reading about dimmers/drivers and PWM circuits.

    Also, I don't really know how a constant current source typically works, although it sounds self explanatory.

    Does this mean that even if you apply a load that would only normally draw 100 mA, that the supply will shove its full output rated current through the load? Typically lights, motors, etc., will draw what they require. Do LEDs behave differently, and therefore require the current-limiting resistor to protect them?

    Thanks, Tom.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Leds are like diodes.
    If there is no current limiting resistor, the current will be "infinate" and the led will blow up.
    On this page there are several curves for voltage vs current, current vs ilumination etc.

    http://www.oksolar.com/led/led_color_chart.htm

    Greetings,
    Bertus
     
  3. Tom Kay

    Thread Starter Active Member

    Feb 10, 2009
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    Hi Bertus;

    That's helpful, thanks. So, even with a constant current source, does that mean the right value of resistor in series with the LED will limit the current flow so that the LED survives?

    I guess it's the "constant current" concept that's throwing me off a bit.

    Let's take an example. If I buy or make a driver with a constant current of 5 amps, this still does NOT mean that all 5 amps would be pushed through a single LED, if I had the right resistor connected to it, right??

    Thanks, and I'm getting there, albeit slowly.
    Tom.
     
  4. bertus

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  5. leftyretro

    Active Member

    Nov 25, 2008
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    When using a constant current source the amount of current supplied will be fixed and forced to the value determined by the constant current circuit alone, and it can deliver this fixed current even into a short circuit.

    So that is why one can drive a LED with a constant current source and not need a current limiting resistor. The value of the constant current must be no more then the rated maxium current specification for the LED used. Also several LEDs can be wired in series such that the constant current will flow though all of them as long as the constant current source has a high enough input voltage to overcome the combined voltage drop of all the LEDs wired in series.

    So one can drive LEDs with a constant voltage if a current limiting resistor is in series with the LED or one can drive a LED with a constant current source where a resistor isn't needed. Does that make better sense?

    Now the dimming function is an independent function and can be done in several methods. Either by turning fully on or off the current or voltage to the LEDs using a variable duty cycle determined by the PWM duty cycle, or by switching something in the constant current or voltage regulator such that the constant current or voltage generated is a function of the PWM duty cycle signal.

    Lefty
     
  6. Tom Kay

    Thread Starter Active Member

    Feb 10, 2009
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    Yes I did read Bill's info album, although I started to lose it part way through. The reality is that my electronics hobbiest skills are very rusty, and designing circuits was never my forte. However, I will keep going through Bill's work and try to grasp it over time.

    Just to show you how rusty I am at getting this, it took me a while to figure out how he calculated the right resistance for his first drawing with the single LED. Then it dawned on me (9V-2.5V=6.5V. Take that value, divide it by the desired current of .020 amps, and voila, 325 ohms).

    Each little example will be a hurdle. However, I'll keep reading.

    Thanks again, Tom.
     
  7. bertus

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  8. Tom Kay

    Thread Starter Active Member

    Feb 10, 2009
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    Hi Lefty;

    Thanks for your input too. Most of that does make perfect sense, especially the part about using a constant current source without a resisitor. Why would you need a resistor when the current is preset to, say, 20 mA. Answer, you wouldn't ! I get it. Well done !

    Of course, the question that pops to mind is, where would you get a source that's preset at .020 amps? Or, are a variety of constant current sources available that are adjustable, even down to 20 milliamps?

    Let's try an example. Let's say I want to light my home, starting with one room. I have a string of 10 LEDs, all rated at forward voltage of 3.4 volts, and a current of 20 mA.

    So I use a power supply that supplies 20mA at 34 volts DC. I feed this into a PWM circuit, such as a 555 timer circuit. I might have to supply slightly more than 34 volts, if the PWM requires some voltage of its own. The PWM will change the duty cycle (on/off ratio) of each wave that is supplied to the LEDS, hence dimming the LEDs and still supplying constant current.

    If I need more light, use several banks of 10 LEDs, all hooked in parallel. Ah, a concern or two: First, the current will have to be increased, since I am connecting more banks in parallel. The current will then be shared by each of the separate parallel banks, roughly equally. As Bill said in his large volume (hereinafter known as the Forum LED Bible) if one string of 10 LEDs blows, then its current will still be shared by each of the other surviving strings of 10 LEDs, thereby stressing them with higher current. And that's a problem.

    So maybe it makes sense to go with a constant voltage source, and a current limiting resistor on each of my 10 LED strings. If one string blows, all the other separate parallel strings are still protected by their own current limiting resistor, so the current doesn't spike. Voltage simply gets split in parallel fashion, so that each string gets its own voltage supply, again, roughly equal.

    I'd do this: Have a power source, preset at a desired voltage. Make it a supply that can provide a few amps, somewhat higher than my total current draw requirement. Then, feed the output into a few individual PWM dimmers, which in turn, feed however many parallel strings of (for example) 10 LEDs I need to light the room. Each PWM dimmer would control one circuit, of 4 or 6 "bulbs", each bulb made up of X number of 10 LED strings. Blow one string, and the rest of the bulb still works. So does the whole circuit, and the single, common power supply.

    This is my attempt at coming full circle. This all started because I bought some GU-10 110VAC bulbs from EBay that didn't dim, despite the promise that they would with a basic dimmer. I think I now understand why they won't dim right. They are made up of 2 parallel banks of 36 series-connected LEDs. If the rectifier and smoothing cap were removed from the inside of each bulb, they'd actually have a better chance at dimming, provided that we started with 110V DC, not AC, and if the dimmer could still supply shorter wave modulated pulses.

    So, as I have claimed before, this is SLOWLY starting to sink into my electronically challenged mind, and I appreciate the help.

    Since a major part of the goal is to save money, and never replace another light bulb, I will ask at some point about the most efficient little DC power supply that I could use to supply all the LED bulbs that I will install. Save that one for another day.

    Any corrections to my basic understanding of the way LEDs are supplied?
    Thanks a lot. Tom.
     
  9. bertus

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    Apr 5, 2008
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    Hello,

    When you want to drive 10 leds (34 Volts), I would recommend to have at least 2 Volts across the current limiting resistor.
    The power supply should be at least 34 + 2 = 36 Volts.
    You can make a PWM circuit with a 555 or like this one from SgtWookie.
    http://forum.allaboutcircuits.com/showthread.php?t=9016
    The 4093 must be at 10 to 16 Volts, the FET can have more (almost 100 Volts).

    Greetings,
    Bertus
     
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