Tohuwabohu
Newly Enlightened
I was preparing a comparison measurement of current-draw and current-pulseform of some AAA-powered lights when I took the following scope shots of my Fenix L0D Q4
running on 3.6V LiIon.
My test setup with all the lights I wanted to compare:
The 10 milliohm shunt used as current sensor is beneath the circuit board.
In medium (primary) mode with no visible flickering:
10ms/div, blue: current, red battery voltage.
A periodic row of identical looking current pulses and very short spikes.
The battery voltage drops from 3.75V to below 3.5V during each current pulse.
The thick lines during the current pulse indicate that there is a boost converter running at a very high frequency.
In high mode (no PWM) at higher time resolution (20 microseconds/div):
The boost converter is constantly working. The battery voltage is well below the forward voltage of the LED.
In low mode with the light showing a distracting flickering:
There are small irregularities at the beginning of some of the pulses.
Zooming in to have a closer look at the pulses 6 to 9 in the above scope shot:
Pulse 6 has nearly the same shape as when the light is running on a NiMH battery.
Pulse 7 has a remarkable leading edge. It looks as if the LED is direcly driven for a short period before the boost converter starts working.
Pulse 8 again has 'normal' shape.
Pulse 9 looks like pulse 7, only the initial part where the LED is directly driven is longer.
Setting the scope to a higher time resolution I took some more shots of the leading part of some pulses:
Nearly normal
Short direct drive period
More than 60 microseconds of direct drive
If it is really an irregular period of direct drive that is causing the flickering then the flickering should be dependent on the Vf of the LED, the temperature of the LED, the impedance and the charge state of the battery.
This would explain why the flickering is somtimes only visible in low mode, at other times in medium mode or in both modes and why it can appear or disappear after changing batteries.
running on 3.6V LiIon.
My test setup with all the lights I wanted to compare:
The 10 milliohm shunt used as current sensor is beneath the circuit board.
In medium (primary) mode with no visible flickering:
10ms/div, blue: current, red battery voltage.
A periodic row of identical looking current pulses and very short spikes.
The battery voltage drops from 3.75V to below 3.5V during each current pulse.
The thick lines during the current pulse indicate that there is a boost converter running at a very high frequency.
In high mode (no PWM) at higher time resolution (20 microseconds/div):
The boost converter is constantly working. The battery voltage is well below the forward voltage of the LED.
In low mode with the light showing a distracting flickering:
There are small irregularities at the beginning of some of the pulses.
Zooming in to have a closer look at the pulses 6 to 9 in the above scope shot:
Pulse 6 has nearly the same shape as when the light is running on a NiMH battery.
Pulse 7 has a remarkable leading edge. It looks as if the LED is direcly driven for a short period before the boost converter starts working.
Pulse 8 again has 'normal' shape.
Pulse 9 looks like pulse 7, only the initial part where the LED is directly driven is longer.
Setting the scope to a higher time resolution I took some more shots of the leading part of some pulses:
Nearly normal
Short direct drive period
More than 60 microseconds of direct drive
If it is really an irregular period of direct drive that is causing the flickering then the flickering should be dependent on the Vf of the LED, the temperature of the LED, the impedance and the charge state of the battery.
This would explain why the flickering is somtimes only visible in low mode, at other times in medium mode or in both modes and why it can appear or disappear after changing batteries.
