How to tell if your flashlight uses PWM or not

[ron.ck722]

circuits that provide steady power to the led must resist some of that power to in order to dim it,
this creates some heat in the resistance circuit and eats power that otherwise could go to producing light

PWM reduces or eliminates dimming completely, so nearly all power goes to lighting.
PWM fools the eye into thinking it is steady light and can dim it easily just by keeping it off more often

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A very good point

Like older switching power supplies, older PWM LED Drivers switched at low frequencies, some even in the audio range. Low Frequency PWMs can show the shower raindrop or other visual artifacts when moved in front of another pulsating or strobed light source. So early PWM's rightfully irritated many users.

That shouldn't be today's case.

Modern PM Controllers (typically Buck, Boost, Buck-Boost, Cuk, SEPIC or a Hybrid Combination) switch between 200 kiloHertz and 2 MegaHertz, well above the audio range. Brightness modulation is imperceptible. High switching frequencies facilitate capacitive and inductive filtering of the regulated current output. Light Output Ripple can easily be less than 5% peak-to-peak. Less than 1% peak-to-peak isn't difficult to achieve.

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At MAX Output, A typically 46% Efficient Linear Regulator, driving a 3V, 10W, LED at 3.3A with a fresh pair of CR-123s, dissipates 11.9 watts (6.6V - 3V)(3.3A) of heat, in addition to the LED's 10 Watts. The Total Power Input is 21.9 Watts.

An 80% efficient PWM driving the same LED at the same current, requires (10Watts/0.8) or 12.5 Watts Input, 10 Watts still goes to the LED,
but only 2.5 Watts is dissipated in the Regulator.

I did, however, evaluate one PWM Buck-Boost Regulator in 2013 and found its Efficiency less than 40% across most of the 18650 battery range. It can happen with any poor design. Gave it a scathing "Do Not Buy" review. Needless to say, the distributor wouldn't publish it.

The Linear will poop out when the battery drops to near the LED's 3V. The PWM may work down to below 2V. Remember, High Heat = Poor Use of Battery Charge. A good PWM will run until less than 20% of the battery's charge remains. Battery life or useful charge time should virtually double.

The way to tell if your flashlight is PWM - this should work - I haven't tried it yet, but I will - set it next to an AM radio and tune across the 530 to 1710 kiloHertz band, a Lower Frequency Band or a Low Frequency Short Wave Band. Since it doesn't make a lot of sense to tightly shield or even partially shield a PWM flashlight, it should interfere with weak stations. A linear won't, unless it's oscillating - if it's properly designed and stable, it shouldn't emit any AM interference.

 

[inetdog]

I accidentally found an interesting way to detect PWM is to shine the light in a low power mode on some models of solar garden light at night. If the light does not turn off, it may be because the PWM output of the solar panel does not properly switch the light sensor.