555 PWM with 24V through LEDs

Discussion in 'The Projects Forum' started by BG79, Aug 7, 2011.

  1. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Hi all,

    I made this circuit http://forum.allaboutcircuits.com/showpost.php?p=353757&postcount=10 using NE555. It works fine and now I´d like to upgrade it.

    I want to control 7 high power leds with 24V Vin (500mAh), but I don´t think it´s a great idea to burn about 4,5W using LM7815. Is there a way to use a battery (9 or 12V) to feed the 555 and use the 24V directly to transistor?

    If not, what would be an efficient way to do that?

    Already thanks.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yes, disconnect the load from the 6V power supply and connect it to 24V, no problem. has to be the same ground of course.

    Question: instead of 500mAh you meant 500mA, right? Is this the total current? Or each LED draws 500mA?

    If it's total 500mA then the BD139 is ok. If not use a Power-MOSFET or a more powerful npn-transistor.
     
  3. simo_x

    Member

    Dec 23, 2010
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    555 IC has a maximum voltage power supply around 15V & 18V (just check datasheets).

    I don't think using a 9V or 12V battery is a good idea for all this current.. If you have a normal regulated power supply with output voltage greater than 24V, the circuit would be easier.. If not, you have to build your own power supply, with voltage regulator with external transistors for high current source.

    Each LED consumes about 500mA? 555 have a maximum current source around 200mA around 15V of power supply, then you should be able to drive external transistors (maybe Darlington).
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A simple transistor driver would do it. How much current are you wanting for the LEDs? I'm guessing 700ma with 7 in series.

    The 555 drives the transistor, and uses 12V. The transistor can use a different voltage than the 555.
     
    Last edited: Aug 8, 2011
  5. BG79

    Thread Starter New Member

    Apr 19, 2011
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    500mA is the total current.

    My doubt is exactly with the ground. When you say the same ground, you mean to connect 6V and 24V ground?

    Thanks for answer
     
  6. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Total load of 500mA with 7 leds in series.
     
  7. simo_x

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    Dec 23, 2010
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    So just choose the properly transistor for power dissipation.
     
  8. Wendy

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  9. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Pretty nice articles.

    My problem is about using 24V with the 555. I believe BD139 will handle it fine, but I don´t want to use a converter to feed the 555. Looking for a simple way to send 24V only to the leds, I tought about using a small auxiliar battery (9V or 12V). How could I do that?
     
  10. Wendy

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    Mar 24, 2008
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    Drop a 12V zener diode in line with the power supply feeding the 555. It doesn't have to be complex. Here is another circuit with a similar goal I designed for someone else on this site...

    [​IMG]

    This particular oscillator is meant to provide rail to rail performance at a reasonably high frequency. The transistors have no protections, abuse them and they become good fuses. I've never actually built it, but I'm confident it will work.

    ZR2 and ZR3 are there to prevent shoot through, a condition where both transistors are on during switching. The idea is when the voltage is mid range both transistors are off.

    For an LED network all you need is one transistor. MOSFETs have much better characteristics than BJTs for high current applications.
     
    Last edited: Aug 8, 2011
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  11. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Great to know that. I´ll try with the zener. About the MOSFET, do you indicate any for this use?
     
  12. iONic

    AAC Fanatic!

    Nov 16, 2007
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    If your main supply is 24V then using a 7812 or 7815 is more than capable of operating the circuit or even a 12V zener as Bill suggested. The 24V would be connected to a power transistor where the heavy current would be consumed by the LED's.

    I think the LM78L12 or LM78L15 T0-92 pkg might be enough for the circuit also.
     
    Last edited: Aug 8, 2011
  13. BG79

    Thread Starter New Member

    Apr 19, 2011
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    I don´t want to use LM78XX because of the energy loss. As I understand, 24V-15V (LM7815)= 9V X 500mA= 4,5A turned in heat.
     
  14. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Then as Bill suggested, a 12V Zener. A 9V battery could be used, but It wouldn't last very long. I don't know what application your circuit is intended for or where it will be located, but replacing 9V batteries may cost you more than the lost power via a LM7L15 T0-92.

    The 500mA should not be the operating current of the PWM circuit. The 12V/15V voltage regulator would not be powering the 500mA LED's just turning the power transistor on/off.
     
  15. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    4,5 W you mean. And you don't have the 500mA through the zener nor the 7815, because the load is connected to the 24V power supply (before the zener/7815).

    Total current through the 7815 will be the base current for your driver transistor and the supply current for the oscillator.

    A zener (bringing supply voltage of the 555 down to 15V) or a 7815 doesn't make a difference, the current through both will still be the same and the voltage drop over the zener x current through it will be dissipated in it.

    Just iOnic said in his last sentence.
     
    Last edited: Aug 8, 2011
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  16. Wendy

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    Mar 24, 2008
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    You want a simple current regulator use something like this...

    [​IMG]

    This would need major modifications to power high power LEDs, but it would render the power supply regulation moot, since it is a constant current regulator.
     
  17. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Just to close this post. It´s another question about the same circuit.

    How can I control the maximum current of this circuit? With 6V, the maximum is ~500mA, with 12V it goes to ~700mA. Which component is responsale for that?
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    The current limiting resistor for the LEDs. If you build a constant current source this stabilizes to a set current.
     
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