High Power LED Strobe

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I am attempting to drive 3 High-power LED's in parallel and wish to strobe them. The issue I'm running into is in parallel, the LED's require 3v and 8.4A, however, the 555 timer requires higher input voltages and doesn't output enough amps. Secondly, I'm researching the use of an MCU but am very new to all this.

Question 1: Can I overdrive the 555 and it operate, or will it burn up?

Question 2: Will an MCU be the appropriate solution utilizing only a battery source of 3.7v and 12A? I only need it to strobe for about 10 seconds and then the circuit shut down.

Thank you

Best Answer

The 555 is not designed to drive large loads such as these, it will certainly "burn up."

MCU's are complex, but versatile. There is a steep learning curve...

It sounds like what you want to do is use the 555 just as a "pulse source", then have some other circuit element do all the power switching. Typically that is done with MOSFETs or other power-switching devices.

For your LED's, the important value is their pulsed current (amps) rating. If you don't know exactly what this value is, google the LED model number to find it's datasheet, the value should be in there. So say it is 2.8A. Another useful value is the Vf or "forward voltage drop" of the LED. Say this is 3.0v @2A. What we really want to know is what the drop is at 2.8A, often this is plotted in a chart.

So lets say we have an LED that should drop 3.2v at 2.8A. (Speculating numbers, replace with measured values.) But we have a 3.7v supply, so we can't just hook up a 3.2v LED to it directly because "overdriving it" will push far more than 2.8A through it and probably burn the LED up. So we need to drop 3.7v - 3.2v = 0.5v. So put a resistor in series with the LED. Ohm's Law: E=I*R. Solving for R, R=E/I. So 0.5v / 2.8A = 0.179 ohms. Round that to 0.180 (180 milli-Ohms.) That is the resistance needed.

Now resistors get hot, and that needs to be considered. Power (watts) = Volts * Amps. So for that resistor, it's dropping 0.5v * 2.8A = 1.4W. Now these are pulses, not a continuous duty cycle, so this can be de-rated. If the pulse duty is 50% (half the time on, half off) then the resistor would dissipate 0.7W. Now by convention, power-rating of resistors is always >2x the actual power. So for this 0.7W need, we'd spec a 2W resistor (minimum.) A 5W or even 10W will work just as well (and get less hot.)

Ok so you have a resistor for each LED. Then use a MOSFET to "turn on" each LED as shown in the above link. What kind of MOSFET? One that ideally has a logic-level input, so it can operate from the 555 output. And one "big" enough to switch at least 2x 2.8A. The lower it's "RdsON" value, the less heat it will dissipate, but there's a gotcha here. Most MOSFETS with very low RdsON values, have large gate capacitances, so when the 555 sends the signal to turn on, their response time is slowed due to this large capacitance. So it would be fun (and educational) to try several different ones.

Related Solutions

Electronic – Timers and internal clock generator on Freescale MCU

Apparently, as the timer frequency is very high, to set it up we would require a large Prescaler Setting(i maxed out at 128). After having had a talk with my colleague, he says that i need to generate a Periodic "Tick" with the flash period for each LED set accordingly. So as to make sure that LED(ON or OFF) corresponds to the TICK rather than the Bus Clock Frequency.Ya, this works but i am still confused.

If you are able to use the datasheet to understand the various register functions, then why, setting the frequency of the timer as per requirement, doesnt work?
Moreover, isnt this "Tick" the same as the Delay funciton??

here is the code for the same(by the way this code was suggested to me at the freescale forum, but it might also help someone else; i take no credit for this code) :

// Assumed bus frequency: 8.0 MHz
// TPM prescale divisor:  4



// Global variables:
byte tic_cnt;             // Timer tick counter
byte ch_nbr;              // LED channel number



// LED mask values:
#define mLED0    0x01
#define mLED1    0x02
#define mLED2    0x04
#define mLED3    0x08
#define mLED4    0x10
#define mLED5    0x20
#define mLED6    0x40
#define mLED7    0x80



#define TIC_INCR 20000    // 10 ms tick period, with prescale 4


//***************************************************************

// LED blink polling function

void LED_poll( void)

{

   if (!tic_cnt)

  {        // Timeout occurred

      tic_cnt = 50;       // Start new 500ms timing interval


      ch_nbr++;

      if (ch_nbr == 8)  ch_nbr = 0;

      if (ch_nbr == 0)  PTFD = mLED0;  // Turn LED0 on, other LEDs off
      if (ch_nbr == 1)  PTFD = mLED1;  // Turn LED1 on, other LEDs off
      if (ch_nbr == 2)  PTFD = mLED2;  // Turn LED2 on, other LEDs off
      if (ch_nbr == 3)  PTFD = mLED3;  // Turn LED3 on, other LEDs off
      if (ch_nbr == 4)  PTFD = mLED4;  // Turn LED4 on, other LEDs off
      if (ch_nbr == 5)  PTFD = mLED5;  // Turn LED5 on, other LEDs off
      if (ch_nbr == 6)  PTFD = mLED6;  // Turn LED6 on, other LEDs off
      if (ch_nbr == 7)  PTFD = mLED7;  // Turn LED7 on, other LEDs off
   }
}

__interrupt VectorNumber_Vtpm1ch0 void ISR_TPM1_ch0( void)

{

   TPM1C0_CH0F = 0;       // Clear TPM channel flag

   TPM1C0V += TIC_INCR;   // Set next software compare

   if (tic_cnt)  tic_cnt--;
}

Looking for the right LED driver, roughly 4V, 1.5W or 3W, 10-20Hz

I would use a LM555; it's a very flexible, cheap, and easy-to-find timer chip. And you can find lots of examples on how to use it. Here's one to start with. The 555 will source around 125mA, which isn't quite enough for your application. If want to drive all of the strings at once, you can do it with an NPN transitor switch hooked to the output of the 555. I would probably start with a TIP120; that's hefty enough to handle the current you need to switch. You will need a way to power all of the strings at once. If you are using batteries, I would use 3 D cells to drive all of the strings. If you are okay with a plug-in solution, you can find lots of small wall wart power supplies that put out 5V at 1Amp; they are used to charge cell phones and other kinds of electronigs

Best Answer

Related Solutions

Electronic – Timers and internal clock generator on Freescale MCU

Looking for the right LED driver, roughly 4V, 1.5W or 3W, 10-20Hz

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