I'm looking for ideas on how to make an LED flash so brightly at a low duty cycle that it's reasonably bright--maybe even close to what it would be if it were on DC.
I'm building a project where I need to flash white LEDs very brightly
30-50 times a second at about a 0.4% duty cycle. (I'm strobing a spinning disk, and want to freeze images near the LED--too high a duty cycle, and the image blurs.) 0.4% is not much time for an LED to be on. I've heard that you can drive LEDs to up to 10x their normal forward current without damage (though I guess lifetime is shortened) if you keep duty cycle to
you could probably use a switched constant current source and connect more than one led in series for more light. If you need a lower duty cycle you can interleave leds.
Hah! Now I must defend my honor! (Though I must admit to being scum in other contexts.)
Actually, I'm actually doing this for a dozen LEDs at once. A rotating transparent disk has [A-Z0-9 ] printed just inside the circumference around the circle. A motor spins the disk at 20-50 RPM (controlled by the MCU with PWM). An IR photo-Q watches for a black mark on the bottom of the disk, and through a Schmitt trigger, interrupts the processor once per rev. The 12 LEDs are arranged radially below the row of letters flashing by, and since the MCU knows where the disk is at all times, it can flash to display a specific character at a specific position. Shifting the character positions lets you scroll a character at a time, slight delay or advance on the clock gives smooth scrolling, alternating between one character and another provides dissolve, etc. Plus another photo-Q will receive text messages from a tiny IR keyboard (Targus). I may also network-enable it so it can show RSS feeds.
Kind of like a POV toy, but strobing spinning characters instead of drawing the characters with finely-spaced LEDs.
Actually, it's the circuit to pump current through those LEDs that I need. My code works already. It works, but the light is feeble. The asm source wouldn't do me much good, since I'm using AVR. (Now, there, I'm going to get called scum again!)
Can someone explain to me why a MOSFET would be better here than a bipolar? Seems I should be getting enough current through the LED with my 2n2222 darlington arrangement, but my transistor design skills have always been modest. And when I learned it, MOSFET drivers were still exotic, believe it or not. We did only bipolars.
Those are nice, but they're kind of expensive. I'm actually doing a dozen of these LEDs. And the .5W ones are *really* bright at 100mA. If I can't get the 0.5 W ones to work, I'm going to look into tiny xenon strobes.
On a sunny day (Mon, 4 May 2009 08:28:19 -0700 (PDT)) it happened Tim Williams wrote in :
Well, yes, timing reasons and low part count. Actually I looked up the little board, only one transistor.. and the PIC 12Fsomething.. just put it away again. Indeed I wanted to see for myself if I could get better brightness from a white LED with short very high current pulses, then with just DC. It is not better, I wanted the pulses to be at about mains frequency, or at least faster then when you would see flashes.
555 timer could do it, but I have a load of those 12Fsomething PICs, and no 555s, so why not use the PIC? In my view the times of analog MVBs is over, big capacitors are more expensive then chips (PICs), and you need to keep them in stock. Sure you can do the same with 2 or 3 transistors... But not with the same stability.
I do not see what people have against PICs, programmable logic, you see people here make huge complex circuits with 7400 series or even CD400 series, or whatever, as solutions to some question. Was it you who did a z80 setup to just display some 7 segment data while a PIC
On a sunny day (Mon, 4 May 2009 08:54:02 -0700 (PDT)) it happened mj wrote in :
Hi mark, I looked it up, it is very simple, a BCsomething switches the LED from
12V via a resistor to ground,. Probably not advanced enough for when you want high efficiency. The PIC drives the transistor via a base resistor, it was only one transistor I see now. The PIC gets 5V via a 7805...
On a sunny day (Mon, 4 May 2009 08:57:05 -0700 (PDT)) it happened mj wrote in :
MOSFETs (special ones with low gate threshold voltage) interface nicely to micros without drawing much current from the output pin. At least for the low speeds that you are using. MOSFET input capacitance can be as big as a few nF, so at higher speeds switching get compromised (bad rise and fall times), say above a few kHz for a pin that can source / sink 10 mA. C.U = i.t
Doesn't anyone teach you googlies about the other side of google? I just now put "circuit to pump current through an LED", without quotes, into google's web search window, and got "about 227,000" hits.
Some thoughts about the 2N2222. They probably can handle the pulses of 1.5A for 200us and .4% duty (I am assuming, for now, that your
20-50 RPM is really 20 RPS = 200us/0.4%.) However, the Rc, Re, and Rb plus the 26mV*(1+ln(1+I/Is)) are going to suggest something on the order of 1.9 to 2.0 volts at the base. (I see Re=0.2, Rc=0.3, and Rb=10 for one model I have, with a blind Is=1e-14, and I don't think you can expect to get better than beta=30 here.)
Since this is a darlington, I'm assuming something like this:
| about 2.7V | e | | |/c Q1 about 2V --> '------| 2N2222 |>e | | gnd
That's Q1, I'm talking about. The Vbe of Q2 is about 0.7V and, if Q2 is considered saturated at about Vce=0.2V, I'd expect to see a Vce on Q1 of 2.2V or so. [Note that Q2's base is about 0.5V above Q2's collector, so that is going to conduct a little (.2V/60mV, about 3 orders, or maybe 0.1% of the base current -- nothing to get excited about.)]
With a 5V source above, that leaves about 2.8V for your LED. Not the
3.6V you were talking about at 150mA. And you want 1500mA, not 150mA! So your circuit probably won't get there.
Here's the same circuit with 150mA as the estimate:
| about 1.45V | e | | |/c Q1 about 0.85V --> '------| 2N2222 |>e | | gnd
This provides about 4V compliance (not sure if you are using a resistor on the collector leg) for your LED. So at 150mA, it probably has just about enough for your 3.6V requirement.
....
All this suggests to me that you aren't going to see a lot more than
150mA. I don't have the LED model, but since it is white the LED is blue with some phosphor used for the white appearances. Guessing a simple model has me taking about 2.8V as the minimum on-voltage (which I get from (700nm/400nm)*1.6V, extrapolating from a red led) and a model where R=(3.6-2.8)/150mA or in the area of 5 ohms. Actually, that seems too high to me, so that 2.8V estimate is probably wrong. Let's assume it is closer to about 2 ohms or so (3.3V minimum.) At
1.5A, that's still 3V all by itself. Adding that to the 3.3V figure gives you well more than 5V. So again, a problem even assuming your darlington arrangement could support a Q1 Vce of 0V, which it cannot.
Your darlington arrangement creates one problem. Q1's Vce will be on the order of 2V at those high currents and you don't have that kind of headroom to spare. Your LED itself creates another. It's not likely to allow 1.5A with only 5V of drive, under any circumstance. These are guesses, admittedly. But what this seems to say is that you probably need another voltage rail, at a minimum, if you want to get up to 1.5A on your LED.
If you can live with less than that but still want a lot more than
150mA, the perhaps your LED will allow ... hmm ... say (4.4V-3.3V)/2 ... or about 500mA, let's call it, assuming you can limit your BJT switch to a Vce of 0.6V or less. And no more than (4.8V-3.3V)/2 or about 700mA. I think that's the best you can hope for. But I'd plan no more than 500mA, after some testing first. In this case, you can keep your rail, maybe, but you have to lose the darlington.
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