overrule PWM?
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Wim Hertog
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No, a capacitor in parallel with the LED acts as a lowpass filter and makes the PWM useless. This will not work!
Are you sure of that Wim? AFAIK a capacitor blocks DC (i.e. steady voltages) and lets current through when the voltage is changing (i.e. charges and discharges). This is the opposite of a low pass filter. The effect of a capacitor across the LED would be to smooth out the fluctuations in the voltage, which should reduce the flickering noticed.
You need big capacitor in parallel in order to remove the flickering. That's why they don't put it there. But then it may be constantly at 90%...
Another way is to add inductor but it is also too big for small flashlights.
Adding both (cap + inductor) is what makes a regular switching power supply. Inductor tends to limit current changes.
This will be the best choice but not practical for small flashlights.
(I use step-down switching power supply in my rc heli.)
tspoon: Capacitor blocks DC only when in series, in parallel it doesn't block DC.
Another way is to add inductor but it is also too big for small flashlights.
Adding both (cap + inductor) is what makes a regular switching power supply. Inductor tends to limit current changes.
This will be the best choice but not practical for small flashlights.
(I use step-down switching power supply in my rc heli.)
tspoon: Capacitor blocks DC only when in series, in parallel it doesn't block DC.
winny
Flashlight Enthusiast
tspoon said:
You are both right, but Wim is using it in parallel and you are using it in series. BIG difference!
Curious_character
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You don't suppose that the engineers who designed the lights thought of it, do you?yellow said:
Depending on the circuitry involved, the capacitor could ruin the efficiency. But assuming this isn't the case, let's see about how much capacitance would be required. An approximation to the capacitance required is:
C = I * t / V
where I is the current, t the time, and V the voltage drop during the interval t. This assumes constant current which isn't entirely true here, but good for a ballpark figure. The L0D CE, for example, switches at a 100 Hz rate, so t = 1/100 = 0.01 second. To keep flicker low you'd have to keep the LED voltage constant within very roughly 0.1 volt. (If you begin at the 100 mA level, decreasing the LED voltage by 0.1 volt decreases the current and light output by about a factor of two.) So at a low light level where the current is, say, 100 mA, you'd need
C = 0.1 * 0.01 / 0.1 = 0.01 farad, or 10,000 microfarads.
Higher currents would require proportionately larger capacitance. I don't think a 10,000 uF 5 volt capacitor with reasonable ESR (internal resistance) would fit very well inside my L0D CE. There are better ways to eliminate flicker for the people who find it unacceptable.
c_c
Wim Hertog
Enlightened
A capacitor will smooth out the pulses, true. It basically creates a variable DC voltage across the LED. As you vary the PWM from, let's say, 100 to 50% duty cycle the voltage across the LED will drop to 50% of the original value. As you probably know the voltage-current characteristics of a diode (=LED) is non linear.
PWM applies 100% of the supply voltage for a period of time. Our eyes integrate the "flickering" signal over a period of time and we see a variable light output.
Inductors may work if you pick a correct value, but a better method would be increasing the PWM freq. to >500Hz.
PWM applies 100% of the supply voltage for a period of time. Our eyes integrate the "flickering" signal over a period of time and we see a variable light output.
Inductors may work if you pick a correct value, but a better method would be increasing the PWM freq. to >500Hz.
Wim Hertog
Enlightened
Curious_character said:
Correct!
Curious_character
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I suspect the low frequency commonly used is chosen to minimize audible noise from the switching circuit. Switching at above the audible range seems feasible but might involve significantly more power loss.Wim Hertog said:
c_c
Right you hit the point, but it is not as simple as adding a capacitor.
In low level, LED is driven by pulses which separated from each other much wider than in mid level. Every pulse is about 3.4V ( just for example ), and when pulse comes, the LED strobe once. Of cause it will flick by this way of driving.
If you add a capacitor to the circuit, it can smooth the pulse, so SEEMS LED won't flick. But there is a serious problem, that is while the capacitor smoothing the pulse, it also dicreases the output voltage. In low level, the pulse may only take 10% time of the total working cycle, so after that capacitor, the output voltage may fall down to 1.xV or even lower ( there are many formulas can give the exactly value ), in that case the LED won't work probably.
In low level, LED is driven by pulses which separated from each other much wider than in mid level. Every pulse is about 3.4V ( just for example ), and when pulse comes, the LED strobe once. Of cause it will flick by this way of driving.
If you add a capacitor to the circuit, it can smooth the pulse, so SEEMS LED won't flick. But there is a serious problem, that is while the capacitor smoothing the pulse, it also dicreases the output voltage. In low level, the pulse may only take 10% time of the total working cycle, so after that capacitor, the output voltage may fall down to 1.xV or even lower ( there are many formulas can give the exactly value ), in that case the LED won't work probably.
