It's been suggested that power wasted in well designed power converter electronics is comparable to the power lost in a resistor, but this simply is not the case, even in low brightness modes.
I agree with above remarks that finger-sized lights will never throw. Putting a 20mm optic on one requires entry into another size class. Let's talk power.
Assume 3.6Vbatt and reasonable Vfs for the LED. My experience is that even at 1-2 amps, QTCs do not heat up noticeably. I am not sure why. Some people tell me that it must somehow not have a voltage drop to dissipate heat, but I don't believe in magic. Let's calculate the heat dissipated assuming that it is a conventional variable resistor. Since it is sandwiched between two metal plates it can't heat significantly at a half watt for a few minutes' runtime.
Now. Why in the world are QTCs used in small lights?
1. Efficiency at low current, and operation at high current.
Current, Vf. (Vbatt-Vf)*current dissipated, (Vf*current used), (Pdissipated/Ptotal)% efficient.
1A, 3.15v Vf. (0.45W dissipated, 3.15W used, 87.5% efficient). This is a bit less than high-power, 1-cell driver circuits. I could do this trial with an XP-E and get better results through Vf matching, or use a lower-voltage Li-Ion chemistry for the same.
0.5A, 2.95v Vf. (0.325W dissipated, 1.475W used, 82% efficient). About what a cheap driver manages.
0.1A, 2.75v Vf. (0.085W dissipated, 0.275W used, 76% efficient). Not good performance for specialty low-performance drivers
0.01A, 2.5v Vf*. (0.011W dissipated, .025W used, 69% efficient). The 'overhead' of many drivers is 1 mA at 3.6v, or three times what is used here.
0.001A, 2.4 Vf*. (0.0012W dissipated, 0.0024W used, 67% efficient). I do not know of microcontrollers able to run at 400 microamps, nor many MOSFETs rated to 1A that run at this current.
This shows the operation range for the QTC. The efficiency levels out at about 67%, the ratio of the minimum expected Vf* of the LED and the nominal battery voltage.
*At very low currents, the Vf becomes extremely variable.
2. Size. Show me a microcontroller that can be mounted in this light to give a similarly convenient operation.
3. Cost.
4. Reliability. Yes, it fails. In that eventuality, remove or replace the QTC sticker and you get full functionality or a one-mode light. I can't do this with a conventional driver, even if I could build one to spec and size and cost!
