Waffle about the lamps... Back to the index page.
There are currently quite a few LED caving lamps advertised commercially and no small number giving details on 'amateur' websites. There is no shortage of innovation either but almost exclusively, a switching regulator is employed to control energy conversion. This takes the form of an IC, either purpose made or a microcontroller carrying out the function. The Luxeon LEDs are popular but there are designs for Cree and Osram LEDs as well. Housings are generally custom built but the Oldham type is quite common.
One advantage of the use of a switching regulator is the wider range of battery voltage and battery type that can be used whilst maintaining a respectable efficiency level. Some allow battery voltages above and below the LED voltage although most either step up or step down. This ensures that the output - the LED brightness - remains constant as the battery discharges. This wrings almost every last drop of energy out of the battery. Some include a 'battery low' indicator - whereby the battery commences energy saving light output levels as to warn the user of the impending loss of available energy.
Most designs use white LEDs although one advocates the use of cyan exploiting the eye's apparent increased sensitivity to light at this wavelength.
Lamps tend to operate in the 1W area although there are notable exceptions where multiple Luxeon 1W emitters are employed. Typical efficiencies of 85 - 90% are claimed and with the added flexibility of disparate battery configurations, these are clearly desirable.
Linear regulators are unpopular in variable battery voltage/type applications as efficiency can suffer badly where excess energy is dissipated as heat. The simple resistor current limit is also inefficient at higher power levels and where single LEDs are employed.
Lamps I've made have utilised both single 1W and 3W single emitters and also multiple 'distributed power' arrays using smaller 5mm LEDs. I have, however, restricted my creations to operating from 3.6/3.7v from either 3 x AA NiMH cells or single Lithium Ion cells. Whilst this is restrictive in terms of battery type and configuration that can be used, in practical terms, the batteries I use are part of a system - the sole purpose of which is compatibility and simplicity.
The choice of 3.6/3.7v is not accidental - it was chosen purposely as white LEDs operate with forward voltages in the range 3v to 3.5v but generally in the range 3.1 to 3.3v. Li-Ion batteries have a practical range of 3.6 to 4v and NiMH a range of 3.5 to 3.9v. Consequently, the difference in LED voltage and battery voltage is small - of the order of 0.5 to 0.7v.
Consequently, a linear regulator operating a 1W Luxeon LXHL-NWE8 emitter plus optic at 340mA driven from a NiMH battery might have the following characteristics…
Vbatt = 3.9v
VLED = 3.15v
I = 340mA.
Power in = 3.9v x 340mA = 1.326W
Power out = 3.15 x 340mA = 1.071W
Efficiency = power out / power in = 80.76%
As the battery depletes, the efficiency increases…
Vbatt = 3.7v
VLED = 3.15v
I = 340mA.
Power in = 3.7v x 340mA = 1.258W
Power out = 3.15 x 340mA = 1.071W
Efficiency = power out / power in = 85.14%
At the lower voltage point…
Vbatt = 3.5v
VLED = 3.15v
I = 340mA.
Power in = 3.5v x 340mA = 1.19W
Power out = 3.15 x 340mA = 1.071W
Efficiency = power out / power in = 90%
These figures are not unacceptable and as the battery depletes, the regulation is lost and the lamp dims which reduces the current drain - consequently, the user gets some warning of the impending depletion of battery energy.
Before the 1W and larger single LEDs came along, the 5mm LED with its inherent 'beam' forming casing had been tried. At the time, LED technology was immature and light output from the LEDs comparatively puny. My first attempt with white LED used 12 LEDs with luminous intensities of around 3000mCd. Later these rose to 8000mCd and then to 13000mCd - quite an increase - and bright enough for a viable lamp. However, currently, 5mm white LEDs are available with a claimed light output of 50,000mCd. 12 of these make a very respectable luminaire.
Feeding 5mm LEDs provides a bit of a dilemma. Any series chain requires a step-up converter and running them in parallel risks unequal current sharing due to variations in VF. There is, however, a way of overcoming the problem! Back to basics - add a series resistor to each LED and feed them in parallel. This has the added advantage of increased reliability as the circuit number increases with the number of LEDs. Efficiency is also maintained…
V(diode) = 3.4v @ 25mA
Resistor = (3.7v - 3.4v ) ¸ 25mA = 12?
Power in resistor = 0.025 2 x 12 = 7.5mW.
Power in = 3.7v x 0.025 = 92.5mW
Power out = 3.4 x 0.025 = 85mW
Efficiency = Po/Pi = 91.9%
Multiply this up by the number of LEDs you choose to use in the lamp.
This implies that a lamp with good efficiency can be built without the complication/cost of a switching regulator.
I've built two lamps like this - one with 12 LEDs running about 1W (Pete Jurd expressed an interest and bought it!) and others with 24 LEDs running about 2W.
A common feature of the switching regulator lamps is variable brightness. This is easily achieved by reducing the 'on' time, bringing the average voltage/current down but pulse width variation is not solely the domain of the switching regulator. The linear regulator or, in this case, simple resistive current limiter, can be switched on and off at a frequency above the response time of the human eye. Varying the mark-space ratio of the supply created a 0 to 100% brightness variation to which LEDs respond well. Thus, I have applied this technique to the 2W lamp allowing a reduction in brightness and hence energy consumption when the full 2W output is unnecessary.
Thus, I've designed two LED lamps for caving - both use linear regulation and one has a brightness control. (In fact it wouldn't be difficult to add PWM to the 1W lamp - I've just tried to keep them simple).
The lamps are rugged - built into Oldham type housings. These are quite large but are proven in their suitability for use underground. I've not included the 'Through The Headset' charging connection, as the battery to lamp interface is a connector - important, as one of the main objectives in the lamps' design is the ability to change a depleted battery for a spare underground. If a battery headset combination were desired where the battery was permanently connected to the headset, 'through the headset' charging would be comparatively easy to include.