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I am in need of powering an LED (need to chose which) without an appropriate power source. I have a 1.8V/3.3V voltage source with very low current capability and I want to use it to power an LED with 10/100 mA sink.

Any ideas? I thought of a cap but the design is SMT and Farad-rated caps do not suit very well.

EDIT:

Sorry for being vague, my fault.

Here are some numbers: my DC source can provide 7 mW maximum continuously, the design is SMT and the PCB has to be very small (a circle of about 3-4 cm radius) .

For the LED: it is a white LED that would act in a "flashy" way, but the longer it's on, the better. Moreover, it should have, at least for a short amount of time (in the order of 10 msec), a CONSTANT current flowing. Let's assume the LED needs 10mA for that amount of time.

JJJ
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2 Answers2

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You have 3 main energy storage options for circuits:

  1. Capacitor for very short term storage.
  2. Super-cap for in medium term storage.
  3. Rechargeable battery for long term storage.

Without more information as the comments have asked for, one can't get much more specific than this.

horta
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  • But what about the constant current requirement for e.g. 10 msec? I don't think a cap would mantain a constant current flowing, not even for a short time given its discharge curve. – JJJ Jun 12 '14 at 07:12
  • @JJJ Determine the voltage drop of the LED at the current level you're after. Red LED's are usually ~1.8V; other colors are usually higher. Take your voltage of 3.3-1.8=1.5 and add in a resistor that allows you to achieve your ~constant current. 1.5V/0.01A = 150 ohms. If your capacitor is large enough or you get a super-cap, you'll have enough energy in it to have minimal effect on the voltage drop. – horta Jun 12 '14 at 14:13
  • ok, 150 ohms and 2 V voltage drops for example. My cap is charged to 3.3V, then it gets discharged but the voltage drops very quickly. According to K's laws, V=RI which leads to a constant current flowing not even for a nanosecond. Right? – JJJ Jun 12 '14 at 19:35
  • @JJJ With a resistor in place, your voltage and current will drop relatively linearly. With just an LED in place, your current will start out very high and will drop exponentially. A resistor is far from perfect, but it's a lot better than nothing at all. If you have a super-cap, your voltage will likely hardly move at all depending on the size of it. – horta Jun 12 '14 at 19:51
  • yes, but the problem is the drop itself. It can drop exponentially or linearly, my need is that for a given amount of time (10msec for example), there is no drop. The requirement comes from the need of a stable LED color temperature for that amount of time. – JJJ Jun 12 '14 at 20:30
  • @JJJ Please see: http://electronics.stackexchange.com/questions/102751/highly-accurate-constant-current-source/102785#102785 for information about constant lumen output from an LED. Constant current won't cut it. – horta Jun 12 '14 at 20:34
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So, you need some energy storage. I think you have only 2 options:

  1. Big capacitor
  2. Rechargeable battery

If you are worried about reflow soldering, not SMT in general - use PCB mount battery in some kind of socket or just use battery with wire and connector.

Kamil
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