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Will it be safe power more than one LED with this circuit (555 timer,Highpower LED)

Discussion in 'LEDs and Optoelectronics' started by The_Sinpy, Aug 17, 2014.

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  1. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    I'm using a 555 timer to flash some high power LEDs that I got off ebay. They are 3watt blue LED 3.4~3.8volts 700mA. I like to know will it be safe to power more than one of these LEDs with this circuit shown down below

    *I like to power total of six LEDs. The 1st schematic showen below is the circuit as it is right now. The other file beside it is what Im wanting to do.


    My power supply is a car battery. Light is going be used in car. Only when running so guess I'd have about a 14 volt power supply. The whole circuit will be fused by the plug.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Welcome aboard.

    Your car's battery has 12V (nom.) Assuming a total loss of 1V across the current source (MOSFET, 0.82Ω) you have 1V for the LEDs. At 3.8V per LED this makes for max. 2 LEDs (or 3 LEDs at 3.65V per LED) in series. Add more LEDs in series and they won't light up.

    You can use one 555 to power 2 current sources and put a string of 3 LEDs into each current path.

    Do not conect LEds in parallel, see also our ressource section "got a question about driving LEDs?"
     
  3. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    How dose the 555 timer have two current sources? Far as I know the only place you have is pin 3

    Also I've have looked at the "got a question about driving LEDs?"
    The (figure 3.5) on that post about driving leds.


    --------------------------------------------
    I have powered three leds with 12volts and the 555 timer. I just not sure if that would be to much for it. I dont want burn my mosfet or 555 up
     

    Attached Files:

    Last edited: Aug 17, 2014
  4. Gryd3

    Gryd3

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    Note that you can drive a mosfet or transistor with the output from the 555. What was recommended above was using the output from the 555 to drive 2 mosfets or transistors. Each mosfet would then power a seperate string of 3 LEDs.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Hi and welcome to Electronics Point :)

    Before I answer your question, here are three comments about the design.

    1. You should always use a decoupling capacitor on the 555. That is a 0.1 µF ceramic capacitor, or similar, connected between pin 8 and pin 1 as close as possible to the chip. Soldering it directly between the IC pins on the underside of the circuit board is a good idea. This ensures that the 555 will operate (reasonably) reliably even with all the electrical interference you get from the automotive circuitry when the engine is running.

    2. Switching R2 between 10k and 330Ω will change the flash rate, but it will also change the duty cycle (the ratio between ON and OFF times) very noticeably, because 330Ω is less than R1. If you want the duty cycle to remain round 50% you should either use much higher values for R2, e.g. 100k and 3.3k, or switch C1.

    That could be done pretty easily by making C1 3.3 µF (which will give you the fast flash rate) and connecting a 100 µF capacitor through a switch so that when you close the switch, the 100 µF capacitor is connected across (in parallel with) the 3.3 µF capacitor. When the switch is closed, the LEDs will flash slowly; when the switch is open, they'll flash quickly.

    3. Sensitive electronic circuitry can be damaged by high-voltage noise from automotive supplies, and especially by "load dump" (http://en.wikipedia.org/wiki/Load_dump). You can add circuitry to protect it, or in the meantime, make sure you unplug it from the cigarette lighter before you turn the ignition off.


    The circuitry at the right side is called a "constant current sink". It regulates the current through the LEDs to be about 0.8A when the 555's output is high.

    If each LED has a forward voltage that could be as high as 3.8V, and you connect three of them in series, you need (3 × 3.8) = 11.4V. There will also be nearly 1V dropped across the MOSFET and R5, so this can't be guaranteed to work properly using a supply voltage less than about 12.4V. Even if the LED forward voltage is only 3.6V you would still be cutting it very fine.

    So you can only connect up to two of these LEDs in series. This means you would need a second constant current sink circuit for each circuit, to drive the third LED. Just repeat the circuit consisting of R3, Q2, Q1 and R5, connecting the new R3 to pin 3 of the 555 along with the existing R3. BTW, R4 isn't needed.
     
  6. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    Thanks for all the replies. A lot of good information there. Been reading these things over and over again kinda understand it, I'm more better at seeing photos than reading things.

    What would worse thing be if I tried power all these LEDs like I showed in files in my first post.
     
    Last edited: Aug 20, 2014
  7. Gryd3

    Gryd3

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    Because the required voltage for the 3 LEDs is soo close to the supplied voltage, you can expect very large swing in the current through these devices as the voltage varies (slight variations in voltage).
    Without the use of a constant current source, or using only 2 LEDs to allow for a more stable current you run the risk of very quickly killing your LEDs. (You can help compensate by using a higher than ideal resistor value, but your LEDs will be much dimmer during normal operation to compensate for the likelyhood that the source may reach over 14.4V)
     
    KrisBlueNZ likes this.
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    If you build the second circuit from your first post, the LED brightness will drop when the supply voltage drops below about 12.5V. That circuit also has no protection against high supply voltages caused by spikes, surges, and load dump, which could damage or destroy the 555, the MOSFET, and the LEDs.
     
  9. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    What can I do about the protection for the circuit? Also I have a question about the schematic off the "Got a question about driving leds?" post. I followed the schematic part but it makes the LED dim. Guess only way show what I mean is show a photos of it.

    Here link to "Got a question about driving leds"
    https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/ (figure 3.5)

    Side note the LED used in these photos not the one I've been talking about
     

    Attached Files:

  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I don't see your ground connection in pic3. One end of the 2 ohm resistor should be connected to it.

    Your modification, if the rest was wired correctly would cause the LED to be off and the 555 to (probably) be destroyed.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Oh, if you have a P channel MOSFET and no ground connection, I would imagine the LED would behave like that.

    What is the part number for that MOSFET?
     
  12. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    Well I feel dumb but I kinda got it fixed. I had a voltage regulator that looked same as the mosfet in place of it. I cant read the part number on the mosfet all I can say it came from in store at radio shack. The main mosfet I have that Im not using for testing is this one http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=fqpf13n06l The MOSFET are N-Channel the transistor 2n2222

    The kinda fixed is the led lights up. But it dim light for a half second then turns bright and shuts off and turns back on same way. I replaced the 555 timer, the transistor and put the "mosfet" in right place along with that ground wire. I've checked my wiring and made sure nothing was loose.
     
    Last edited: Aug 26, 2014
  13. The_Sinpy

    The_Sinpy

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    Aug 17, 2014
    Here is a video of it
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Try reducing R2 to 1k. Does that make it switch on faster?

    Alternatively, I have seen LEDs behave strangely when they're damaged. Do you have another LED?
     
  15. The_Sinpy

    The_Sinpy

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    Aug 17, 2014

    Attached Files:

    Last edited: Aug 27, 2014
  16. Gryd3

    Gryd3

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    Have you attempted to light the LED with any other methods?
    Do you have a multi-meter you could monitor the voltage to the LED?
    The fade effect seems slow enough to watch happen on a meter if you probe around. Sadly I do not know enough FETs to me of much help.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I was referring to the latter. My reply was to post #9 in this thread where you show a short to ground that turns the LED on. The R2 I'm talking about is the gate resistor for the mosfet that I specified as 47k
     
  18. The_Sinpy

    The_Sinpy

    16
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    Aug 17, 2014
    I didn't have to change it, the circuit is working fine now. I think the problem was the few 555 timers I was using that got messed up by the voltage regulator. Or the thing you said in Post #10

    The only question I have left is the thing said in Post #8 About protecting against high supply voltages caused by spikes, surges, and load dump. What could I do to help keep my circuit safe?
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sorry, I thought you were still having a problem with the LED being slow to turn on.

    The easiest protection from high voltage spikes would be a 16V zener diode across the connection to the 12V with a low value resistor (enough to drop about 1V under normal load) in series with the power supply before it. (if your load is 200mA, make it 4.7 ohms).
     
  20. The_Sinpy

    The_Sinpy

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    1
    Aug 17, 2014
    Ill give it a try, thanks

    I really appreciate all the help from you all
     
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