I got around to fixing an ATOM headlamp, and upon opening it up realized it was only an LED that had apparently burned up (brown coloring around LED and wires from battery). I suspect that the absence of a resistor led to more current than the LED could handle. While the lamp still turned on, it was not as bright as before. The lamp is powered by 3 AAA batteries in series.

enter image description here

Now, I replaced the LED (from what I could tell online it's an SMD LED) with an LED of the solar light variety, but was not sure if a resistor was necessary. Do you think adding a resistor would be worthwhile since it wasn't included in the circuit in the first place? enter image description here

  • \$\begingroup\$ You're replacing one LED with a completely different LED with completely different specifications. That's not going to work. \$\endgroup\$
    – user39382
    May 31, 2018 at 22:56
  • 1
    \$\begingroup\$ That "brown coloring" is flux residues, not burnt up anything. \$\endgroup\$ May 31, 2018 at 22:59
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    \$\begingroup\$ The fact that the LED still turned on, means that the LED was not the issue. Until you desoldered it that is. \$\endgroup\$ May 31, 2018 at 23:00
  • \$\begingroup\$ Similar to Why should I use a resistor for a 3.3v-consuming led?. \$\endgroup\$
    – Transistor
    May 31, 2018 at 23:02
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    \$\begingroup\$ Possible duplicate of Why should I use a resistor for a 3.3v-consuming led? \$\endgroup\$
    – winny
    Jun 1, 2018 at 6:11

2 Answers 2


That's a Cree XP LED. The best replacement would be a 2 Amp (max) Cree XP-G3. This is the highest efficacy (167 lm/W) LED that can generate sufficiently more light (≈70%) than heat. There is no brighter LED available than the XP-G3. Over 150 lumens at 350 mA.

The Cree XP-G3 will be A LOT brighter than the clear LED you used.

The problem is what value resistor to use. An AAA lifespan voltage ranges from 1.2v to 0.8v.

To rely on the discharge rate of a battery is not good when the batteries are replaced with a different brand or chemistry.

With 3 AAA the efficiency with a resistor ranges from 76% at 3.6V and improves to near 95% as the battery discharges.

So yes, a resistor is highly recommended. I would try a 3Ω or 4Ω resistor and see how hot it gets. The Cree XP LEDs can only take about 250 ma without a heatsink so a 3Ω (300 mA) may be insufficient and get too hot. A 4Ω (200 mA) may work better. Depends on heat transfer characteristics, i.e. how the star board is attached to the head lamp housing.

In an experiment I found the temperatures without a heatsink to be:

mA => temp °C
200 => 55-60
250 => 65-70
300 => 100

Based on experience with the Cree XP LEDs, I would expect the forward voltage to be about 2.75V.

To solder put some 360°F 63/37 solder on the pads place the LED on the pads. Preheat the oven to 500°F and put it in making sure the LED stays in place. Turn off oven if temp is over 450° after you close the oven. Bake for about 90 seconds, no more than 2 minutes.

With a 5° spot lens you can get a very bright small spot out of a Cree XP LED.
Example: LEDiL FCA12077_IRIS

  • \$\begingroup\$ Gee thanks! This is the most complete answer I've ever seen. Do you think the Cree XLamp XP-E2 White High Power LED Star would be suitable as a replacement? ledsupply.com/leds/… \$\endgroup\$
    – st4rgut
    Jun 3, 2018 at 15:58
  • \$\begingroup\$ It will work fine, all the XP LEDs have the same PCB footprint. The E2 is older and the G3 is 44% brighter and more robust (2 Amp vs. 1 Amp max). \$\endgroup\$ Jun 4, 2018 at 22:01

You do need a resistor in series with any bare LED to limit the current. (but there are some LEDs that include a current limiting circuit.)

Many inexpensive flashlights depend on the battery's internal resistance to limit the current - in that case, replacing the LED with one requiring much less current will likely destroy the smaller LED.

  • \$\begingroup\$ smaller LEDs also have higher internal resistance, I've measured about 50 ohms on 20 mA LEDs \$\endgroup\$ Jun 1, 2018 at 21:36
  • \$\begingroup\$ @Jasen: resistance measurements of LEDs (or any diode) are fairly meaningless, as the forward voltage drop of a LED depends on its colour, and that voltage drop will affect the apparent resistance as measured by an ohmmeter. \$\endgroup\$ Jun 1, 2018 at 23:37
  • \$\begingroup\$ I was measuring the slope of th V-I curve. \$\endgroup\$ Jun 1, 2018 at 23:58

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