Beginners LED Project, Need Schematics and Electrical Math Help

Discussion in 'The Projects Forum' started by dogpokerpainting, Oct 8, 2012.

  1. dogpokerpainting

    Thread Starter New Member

    Oct 7, 2012
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    0
    I am attempting to build my first LED light string... yes yes I know it can be very simple, but everyone has to start somewhere! :D

    Ok here we go!

    Power Source: old laptop ac adapter rated output of 19v 4.74A

    LED's: http://www.mouser.com/ProductDetail...=sGAEpiMZZMu4Prknbu83y1kHJl487Qqnh7qFW6eM39A=

    I will be running the LED's at 500mA instead of the 1A it says on the datasheet for longer life and lower heat.

    Schematic: http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/?ALLSTEPS#step7

    Questions:
    1. Im having trouble finding out how to calculate R1 in this schematic. I am somewhat familiar with ohms law but im having trouble visualizing how to apply it to get my R variable. The poster of the schematic lists a 1/4 watt 100k ohm resistor, and I'm not sure if this needs to be adjusted as I will be using more led's then the poster, but he seems fairly confident that his schematic only needs the addition of the zener diode or r2 to beef it up.

    2. Im not sure what the advantage of using the zener diode over R2 is

    3. How can I calculate the optimal amount of 500mA LED's I can put onto this circuit? I know my adapter is capable of pushing 4.74A so theoretically I should be able to get 9 on to the circuit, but I've read some posts about reducing the load to half of the maximum output to avoid burning out the adapter. Is this something I should be concerned about?

    4. It looks like R3 would be exactly 1 ohm and .25 watts to set the current at 500mA. Should I leave headroom in this calculation?

    5. Should I upgrade Q2's voltage capacity? He says the schematic should handle up to 60V with the zener diode or r2 added but this is a grey area for me.

    Thanks! :) :) :)
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,027
    That's excellent. Be aware though that these things can be REALLY bright - blinding. Be careful about your eyes. Also, that much power in a small spot means heat dissipation will be important. The LEDs are expensive and you don't want to burn them up while playing with them. Plan for handling the heat.
    It's a bit arbitrary. All you want is a big voltage drop to appear across R1 when/if current is flowing through Q1 (or the zener). The smallest value of R1 could be calculated from the highest allowable current through Q1 or the zener, whichever is limiting, and the highest voltage that might appear on top R1. But 100k ensures a tiny maximum current and thus very little heat. You DO need enough current that the zener can function at its control point. The datasheet will state this.
    A typical MOSFET will be damaged by a gate voltage (over source) exceeding 15V. The zener clamps the gate voltage to prevent that. The zener opens, current flows through R1, and the voltage at the gate drops. A resistor would do much the same; any current drops voltage across R1. The zener allows the gate voltage to get higher though, ensuring it stays more fully "on" when it needs to.
    Yes, I wouldn't plan to use more than 75% or so of the adapter's power rating. You could probably use 100% and get away with it, especially if you provide some cooling with a fan, but it's just asking for trouble to run ANY device near it's rating.
    Q1 will start to conduct and turn off the MOSFET when its base voltage hits about 0.6-0.7. So I think it needs to be a bit higher value to limit current to 0.5A. The power dissipated by R3 is I^2*R. But you should use a resistor rated for at least double that amount, again to avoid an unpleasant surprise.
    With the 20V supply and the zener or R2, you should be fine. The gate is protected and the drain only sees the voltage after the drops of all the LEDs in series.
     
  3. dogpokerpainting

    Thread Starter New Member

    Oct 7, 2012
    2
    0
    Hello wayneh. Thank you for your contributions :)

    Well the highest top voltage would be 19v from the adapter, but im not sure if I should be looking at Q1, the zener, or both. Im unclear on the data sheet for the zener 1N4732A http://www.datasheetcatalog.org/datasheet/fairchild/1N4732A.pdf which value is the required current control point. Is it the 200mA If listed near the bottom of page1?

    Im also curious how the 100k works in to an equation to determine current. I know this may be simple but im trying to understand which variables and what function to use.

    By double that you are referring to the wattage it can dissipate not the ohms right?

    The forward voltage drop of the LED is 2.5V, by limiting the current by 50% does the voltage drop change?

    Thanks again for your help :) :) :)
     
    Last edited: Oct 8, 2012
  4. wayneh

    Expert

    Sep 9, 2010
    12,089
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    Don't you just love datasheets that don't define all those terms they use?

    Anyway, the 200mA is the max current rating. The minimum current to maintain a voltage near the target is probably that 1mA. Below that, the voltage can be a bit different.

    The voltage drop across R1 is estimated at simply the voltage on the high side minus the zener drop. If the zener is conducting in the specified current range (roughly 0.5mA to 200mA), it has its rated voltage across itself. It's like a voltage regulator. Ohms law gives the current across R1.

    Yes and (effectively) no to the other two questions, respectively. LED voltage drop is a weak function of the current, and may move up half a volt or less from dim to fully bright.

    [update]I'm of the opinion that 100k doesn't allow enough current for the zener to work reliably, potentially exposing the gate to more voltage than is safe for it. However, decreasing R1 seems counterintuitive if your concern is the gate over-voltage. I'd like someone else to chime in here.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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  6. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    Wikipedia agrees with Bill. Insufficient current will cause ∆V to be less than the rated voltage, not more. So 100k for R1 is fine.

    Yay! we learned something.
     
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