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As well all know, using an LED without some sort of current control is a bad idea. One of the most basic types is a simple Ballast Resistor, paired with Ohm's Law. Based on Vin, Vf, and the desired Current, we can determine the resistor needed.

But in broad strokes, what is the best scenario for choosing a resistor? Given an LED with 20mA at 3.3V Forward Voltage, with a very close (Variable) source voltage, how much headroom does the resistor need for optimal control?

If we provide too much headroom (Vin of 12V) for a single 3.3V LED, we are just wasting energy and require a higher wattage resistor. If we provide little head room (Vin of 3.4V), we give the LED only 0.1V, is that enough? Changes in Vf and If due to temperature look like it would cause a looping cause and effect situation.

So the question is, for optimal control, how much headroom in terms of voltage, should the resistor need?

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  • \$\begingroup\$ related: Correct formula for LED current-limiting resistor? \$\endgroup\$ – Nick Alexeev Oct 5 '14 at 23:53
  • \$\begingroup\$ @NickAlexeev It's related, but only in the sense that it's about LEDs and resistors. This question touches a different aspect of the choices to make. \$\endgroup\$ – Asmyldof Oct 5 '14 at 23:59
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    \$\begingroup\$ @nick yes I am aware of that one and the hundred other led/resistor questions, but none address the min/max voltage a ballast resistors needs to be effecient. Like, how closely tailored should my Vs/Vled be when using a constant voltage source and resistor for led, instead of other constant current sources. \$\endgroup\$ – Passerby Oct 6 '14 at 0:23
  • \$\begingroup\$ If you wish to do this "open loop" with no 'cut & try' or select-on-test aspects then establish worst case results and decide if they are acceptable. eg an LED may be spe at 3.1 - 3.6 V at 20 mA. Resistor may be +/-0.1%. Power supply may be +/- 5%. You may want current to be 20 mA +- 1 mA max. From these and data sheet curves (to get trens between specified points) you can establish the minimum R and minimum V supply liable to give an acceptable result. \$\endgroup\$ – Russell McMahon Oct 6 '14 at 6:39
  • \$\begingroup\$ You need to define OPTIMAL, and I suspect this will have very much to do with how bright you need your LED to be. The absurd example, of course, is that if you don't need the LED to light up, an open circuit is the best solution to minimize power across the resistor (and LED for that matter). If you absolutely need long battery life, your LED can be very dim \$\endgroup\$ – Scott Seidman Oct 6 '14 at 13:44
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This is something that is dependant on the LED and power source, as well as the operating conditions. If you want a cover-all rule of thumb, assume at least 0.5V, or go if you need absolute reliability for 1V.

Of course, if you want absolute assurance you are not wasting too much energy and still not damaging the LED, you can:

  1. Get all the datasheets together, inspect the LED's power curves at all the possible temperatures, then compare that to whatever voltage regulation you intend to apply, choose the worst-case scenario and model your resistor for that, accepting that in the best case the current may drop noticably.

  2. Design a switching current-mode driver. There are a million and one LED driver chips in the selections of Linear Technology and it's companion companies (Probably TI and AD have some as well, for example) that drive a LED through switching a coil while monitoring the LED current with a small sense resistor, often only "wasting" 100mV or even less.

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How much will it need? Very little. A typical Vf/If curve for an LED is quite sharp around the Vf point, so only a tiny amount of headroom is needed, if any at all.

Yes, the higher the input voltage the more power will be wasted through the resistor - that's a fact of life. However, at the typical small LED currents that power is minimal anyway.

Yes, temperature can cause issues, and self heating of the resistor with higher current LEDs is a problem. For that reason it is much preferred to use a constant current source or sink with higher powered LEDs instead of a simple resistor.

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  • \$\begingroup\$ LEDs with curves like the one in fig5 on page 5 of the vlmo1300 datasheet are not as uncommon in the Red/Orage/Yellow range as one may think. Take that LED at a very small voltage (I infer here 0.1V) over the resistor with a 2.1V power supply, making about 20mA, if that supply has an accuracy of 5%, it can go to 2.2V, causing 0.18V over the resistor, increasing the current dramatically. As a rule of thumb, cover all measure, very low will not keep all LEDs safe. \$\endgroup\$ – Asmyldof Oct 5 '14 at 23:53
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I'm not an LED expert. But I'd assume that the forward voltage of leds drops by ~2mV per degree K (near room temperature.) (I found some graphs on line that confirm this. Here's one) But different color led's will no doubt be a bit different.

So if you want to to operate over (say) 100 C then at least ~200mV of head room would be prudent. (I'd go for at least double that...)

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