Can I prevent a 9V LED flickering when dimmed to around 30%?

Question

I bought an overly bright 9 volt, 3 watt LED to light up a bird box which has a webcam in it. The LED module is this one: https://www.amazon.co.uk/gp/product/B07CWSRF6N

However when I dim it using a simple 12V dimmer to 50% or less, it starts to make the video feed flicker due to the strobing LED's interaction with the camera frame-rate. It's being driven by a spare 9V DC power supply previous used for a broadband router.

Can I fit a capacitor across the LED to help smooth it? If so, what kind of value would I need? Or am I better just to fit a big load resistor in front and remove the dimmer?

Thanks!

EDIT: Here's a pic of the dimmer innards:

Answer 1

There is no inductor on the dimmer PCB so it is almost certainly driving the LED directly with the PWM signal.

If you attempt to filter the output just with a capacitor it will probably cause the LED to run at almost full intensity even if adjusted for a lower value. The capacitor will charge during the on period and then power the LED during the off time. It will also cause very high peak currents in the dimmer that could potentially damage it.

To avoid flicker you would need to provide the LED with DC. Your big resistor suggestion would do just that.

Answer 2

You don't need a dimmer if you don't intend to modify the luminousity frequently. What you need is to set the right voltage so that it will draw the right amount of current. If you put 9V to it you will draw the maximum 300mA. If you want 50% less, you must draw 150mA. Idealy you could use a constant current power supply giving 150mA and 1.5W (between 1 to 3W) (or half than 3W). If you can't find such supply, try with a 6V supply. Ideally an adjustable power suply so that you can regulate manually. but also, can be hard to find at this voltage. It's not sure which voltage will give you half than 300mA, but I can say it will be much more than half the indicated voltage for 300mA. Can be 7 or even 8 volts. Only way to know is to try.

Answer 3

Smoothing capacitors can be calculated using the following formula:

C = I⋅Δt/ΔU

where

C = capacity of the capacitor in μF

I = Charge current in mA

Δt = half-period in ms

ΔU = ripple voltage in V

However, in this case adding a smoothing cap might actually make the problem worse. It is important to note that the intensity of an LED is dependent on current and time it is switched on, but not voltage (so long as it's above the forward voltage).

Adding a cap might smooth the voltage of the PWM to a level below the minimum forward voltage, and the LED will no longer light up. If is is not running hot at full power, I recommend you try to reduce the brightness using a mechanical shutter such as a translucent film to block some of the light.

Answer 4

If the LED is strobing when you reduce the duty cycle, it suggests that the dimmer in question is using some sort of (poorly) low-pass filtered PWM, or the LED's drawn current is higher than the dimmer's rated output current. As you already suggested, adding a capacitor will reduce the cut-off frequency and hold the output voltage a bit longer until the next cycle.

As long as you add any capacitor which is rated to the required voltage and ripple current, it should improve the behavior. If you want to select a capacitor based on the PWM frequency, you would need to open the dimmer and measure it, and add an additional capacitance which leads to a cut-off frequency of alround 10 times smaller than the PWM's frequency.