High-Power LED Flasher

Discussion in 'The Projects Forum' started by turnertj, Nov 4, 2009.

  1. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    All...

    By reading some of the posts here I know there's a bunch of people who could help me out. And I knoiw some of what I'm asking may be a repeat, but with my limited electronic background I'm having a tough time figuring out what I can translate and what I can't. I'm an ME by training, and circuits always baffled me.

    Anyway...I'm trying to flash 2 LEDs, alternatively. I want to use 2 Luxeon IIIs, and run them at 700mA, and have them flash at 5-10Hz. Eventually I will fix the flash rate, but it might be nice to have it vary at first. The most probable power source would be CR123 batteries or maybe even a 9V battery. Either one would work. I've been given a few circuits to look at, and I'm just plain confused.

    I was thinking of just a simple flip-flop, but I'm not sure how to drive at the required current. And then I was told I'd need a driver like a buck-puck and to use a 555 timer chip in conjunction with each LED driver.

    Any suggestions?

    Thanks!
     
    chloride likes this.
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This part isn't too hard. Look at Bill_Marsden's blogs.

    For maybe a few minutes, if you're lucky. 700mA current is far too much to expect from CR123 or 9v "transistor" batteries. You will need a much more capable power source.

    A couple of buck-pucks driven by a 555 timer circuit would likely be the easiest.

    700mA is a good bit of current. Using a traditional linear regulator would be very inefficient. Building a switching regulator may be somewhat bewildering for a novice.
     
  3. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    Thanks. It doesn't have to run too long. Even a 10 minute run time before switching the batteries would be fine.

    How do I go about hooking up buck-pucks to a 555 timer? I've read Bill's posts, and I'm afraid I'm still a little lost. I've seen the 555 timer circuit with two LEDs attached. But doesn't the 555 only output 200mA or so? Is there a way to use the timer to turn on/off the buckpucks drawing directly from the power source?

    Thanks!

    Tj
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You would use a 555 timer to turn on and off MOSFETs that sink current from the buck pucks.

    It seems like a couple of PowerPuck #2008-700mA would be just what you need. They're around $18-$24 USD depending on where you get them.

    You'd connect one lead of the puck to your battery + supply, the other lead to the drain terminal of a power MOSFET, the gate of the MOSFET to the 555 output using a resistor (about 50 Ohms), and the drain of the MOSFET to ground.

    You'll probably be better off using a logic level power MOSFET. An IRLD014 might work well for you; it can sink up to 1.7A current, and is in a DIP package.
    Jameco.com, Mouser.com, and Digikey.com all sell them.

    You'll also need a way to invert the output of the 555 to drive a 2nd MOSFET. An NPN transistor and a couple of resistors could take care of that. I don't have any tools here to draw a schematic, though.
     
  5. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    I do have a tactical flashlight that has a strobing mode, similar to what I want. It is very bright, 120 Lumens, so that LED must be pulling about 1amp or so. The batteries on that thing last about 2 hours. How do they make their power supply last so long?

    Thanks for the reply.

    Tj
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Interesting, I looked up the spec sheet for this battery.

    http://www.duracell.com/oem/Primary/Lithium/123.asp

    I assume you are wanting to use one of these batteries. Have you picked out a puck buck? If so, what is it? I'm after the spec sheet.

    The other thing is the LEDs themselves, we'll need some specs on them too.

    The actual 555 oscillator is easy, interfacing not as much, but very doable. Wookie is very good at this too, for what it's worth.

    I haven't had a lot of experience with the puck bucks, so I'll be exploring new ground.

    Wookie, I'm thinking MOSFETs are the way to go, but 3.6V is low even for digital models. Is there a way around this? Regular transistors will work, but I don't like the losses I'm seeing using them.

    As for inverting the output of the 555, just get a 7556 and use the second 555 to do the job. Simple, and parts free.
     
  7. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    I wouldn't mind using two of the 123 batteries, that would be okay too. Or if it's better, even a 9V would be fine.

    For the LEDs, I was thinking of using Luxeon IIIs (one in red, one in green): http://ledsupply.com/docs/Luxeon-StarIII.pdf

    I'm not sure about the buck puck. But I have one 700mA already...this one: http://ledsupply.com/02008b-700.php So I was thinking of just getting another. Someone did suggest that I try this puck though: http://www.taskled.com/ccxw.html

    Do you guys know which is better to go with?

    Thanks for the help!

    Tj
     
  8. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    Is a 7556 just like two 555 chips? And if so, would that make the two LEDs alternate, or both flash independently? Alternating would be best...
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The ICM7556 is a CMOS dual 555 timer IC.
    Datasheet: http://www.intersil.com/data/fn/fn2867.pdf
    It operates using less current than a bjt 555 (like an LM555).

    CMOS 55x timers have less current source/sink capability than bjt (bipolar junction transistors) 55x timers do. This isn't much of an issue if you're driving logic-level MOSFETs though; driving BJTs becomes a big issue.

    MOSFETs are turned on/off by charging/discharging the gate. Once the gate is charged or discharged, it requires virtually no current to keep them at their charged/discharged state.

    Bjts' are current controlled devices, particularly when used as a saturated switch. If you want to sink 1A from the collector, you must supply 100mA to the base. This can be a large extra power drain when running on batteries.

    I don't have any schematic drawing software installed on this computer; I've been out of town for several days and probably won't be home until tomorrow evening.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I'll draw it up, but what MOSFETs will work?
     
  11. turnertj

    Thread Starter Member

    Nov 4, 2009
    78
    1
    Thanks guys!
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, been doing some research on them. I want to use power pucks, but there are a couple of issues (not insurmountable, but I've not dealt with these devices before). The biggest is they seem to have a minimum 5V or 6V input voltage requirement. The second is the output is independent of the input, no common pin seems to exist.

    Here are links to the two I've looked up so far...

    http://www.ledsupply.com/02008b-700.php

    http://www.leddynamics.com/LuxDrive/datasheets/3021-BuckPuck.pdf

    What I'll do is draw an alternate to using a puck, and see what other people come up with. I had planned on using CMOS 555 / 556 because the power supply voltage requirement goes down to 2V as opposed to a standard 555 4.5V. I'll go back to the standard 555, it can always be substituted later.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    An IRLD014 might do the trick. Logic level power MOSFETS in a 4-pin DIP package, Id max=1.7A.
     
  14. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, these are NOT working schematics, I'm putting them out to get some other ideas. My problem is I don't know if the PuckBuck will work using the same power supply as the 555s, otherwise I'd put it across the transistors and LEDs and eliminate R6.

    [​IMG]

    One of the reasons the PuckBuck is such a good idea is that it is efficient, which doesn't describe the above. It will use every bit of the power out of the batteries to power the LEDs. The resistor R6 would disappate around 2½ Watts. Not good, very bad.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Don't have time to review this; have a long drive ahead of me. I'll try to look at it sometime late this evening or tomorrow.
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, my confidence in figure 1 is very high, I think it would work quite well. You could use a coin cell with the 7556 and it would outlast several sets of the CR123 batteries, the CMOS 555s are a lot more efficient. For the MOSFETs to switch well you may need to use qty 2 3V coin cells to power the 7556's, but the chip doesn't care.

    [​IMG]

    My confidence in figure 2 is very low, it is more of a case of wishful thinking.

    Without knowing what is in a PuckBuck I'm flying blind. Wish I could be of more help, but at the moment this is the best I can do.

    I may try to come up with something equivalent to a puck buck myself. It wouldn't be as small, and probably not as efficient, but I would know the circuit.
    .
    .
     
    Last edited: Nov 6, 2009
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The IRLD014 really needs to have nearly 5v Vgs to turn on fully. When it's fully ON, Rds is about 0.2 Ohms. If Vgs (voltage on the gate respective to the source) is 4v, Rds (resistance from the drain to the source) goes up to 0.28 Ohms, which increases power dissipation in the MOSFET.

    The maximum limits for Vgs is +/-10v.

    I'm a bit concerned about what might happen if both MOSFETS were off simultaneously; the output voltage from the PowerPuck may climb quite high.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the maximum Vf of those Luxeon LEDs?

    One possible remedy with the concern of excessive voltage from the PowerPuck when both MOSFETs are off, a TVS/MOV could be used across the PowerPuck output to limit the maximum voltage; something like this one:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=478-2487-1-ND

    The thing is, you'd need to know the maximum expected Vf of the Luxeon LEDs before you could select an appropriate TVS/MOV.

    A Zener might be used, but it would need to have a large power rating. Here's a 6.8v 5W Zener:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N5342BRLGOSCT-ND
     
  19. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The only way both MOSFETs will both be off (baring my incompetance on MOSFETS) is if there is a wiring error or major fault, such as a dead battery. It is an either/or circuit. You could also use BJTs in emitter follower mode with this, with the PuckBuck as current limiter.

    I suspected the gate volt would be a problem, which is why I suggested the 2 coin cells.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, the IRLD014's have a turn-on time of 9.3nS and a rise time of 110nS.
    The turn-off time is 17nS with a fall time of 26nS.

    There is a window, albiet a small one, where both of them could be off simultaneously. If this is not considered, there will be unexpected "zapping" of MOSFETs and/or LEDs. I do not know if the PowerPucks have any kind of internal protection against such a "no load" event.

    Your "Fig 2" schematic would work if there were a Schottky diode between S1 and C4 to prevent the PowerPuck from draining the charge from C3/C4. There would need to be some kind of low voltage indicator circuit; if the voltage on C3/C4 fell below around 4v, the MOSFET power dissipation would increase. With Vgs=4v, Rds(on) would be 0.28 Ohms, and power dissipation in the MOSFETs would be roughly 137mW; about 2/19th their max rated power dissipation @25°C
     
Loading...