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I am starting a project where a 12V LED Strip should slowly fade in and out. With a Lab Bench Power Supply I figured out that the fading can be done by changing the voltage between 9V and 12V but also by changing the current between 0A and 200mA.

Which approach is better and what would be a basic circuit to accomplish this? I have most common components available (MosFets, 555, Capacitors ...)

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    \$\begingroup\$ What kind of LED strip? Just LEDs with a series resistance or something more fancy (some LED strips have a data input that you can use to dim them). In 999 out of 1000 cases the easiest way to dim a LED is to use PWM. Look up what PWM is. You can make a PWM generator with an NE555. Use that PWM signal to switch on/off a MOSFET. Use MOSFET as a a switch for the LED strip. \$\endgroup\$ Jan 30, 2020 at 13:14
  • \$\begingroup\$ It is just a plain strip with a 12V input and a ground. I cant spot any resistor or other component on it. \$\endgroup\$
    – clamp
    Jan 30, 2020 at 14:37
  • \$\begingroup\$ Of course it will have additional components. If you can't see them, it doesn't mean they are there. Maybe the current regulators are incorporated into the housing of the LEDs. \$\endgroup\$
    – Ariser
    Feb 10, 2020 at 10:21

2 Answers 2

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If you change the voltage the current will also change automatically. Decreasing the voltage will also decreade the current, although this is not a linear relationship.

I recommend to dim the LED strips by using a mosfet and a 555 as pwm source. Depending on the current going through the mosfet, you might need to attach a heatsink.

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  • \$\begingroup\$ Since the 3S+R has a current limiting R above 9V that is much greater than the 15*3=45 ohm LED string value which is nonlinear. , the voltage control is now quite linear. so any method will work. \$\endgroup\$ Jan 30, 2020 at 14:32
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i will gives you the most important thing about PWM with 555. the voltage flow like a rapid on / off from switch. which is different from normal DC. normal 12VDC : the terminal - will be 0 and terminal + will be 12V (straight 12V) PWM : terminal - 0, terminal + will showing 12V and 0 back to back in rapid change in a sec.

so considering the rapid change will kill your led faster than droping your voltage.

lower voltage = lower brightness = lower temperature of led.

correct me if I'm wrong about this

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    \$\begingroup\$ You are wrong about this. Heat is conducted more slowly than electricity can be switched on an off. What matters, either with dc or PWM, is the average power dissipated in the components. \$\endgroup\$ Jan 30, 2020 at 16:53
  • \$\begingroup\$ as i said lower voltage means lower temperature, since the PWM on average /sec will be lower than it peak voltage. I'm just stating the rapid change will kill led faster, same with any bulbs \$\endgroup\$ Jan 30, 2020 at 16:57
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    \$\begingroup\$ No, LEDs are not "any bulbs". The behavior of incandescent bulbs, for example, is much, much different than that of LEDs. And you are conflating the behavior of an assembled bulb with the behavior of individual LEDs. How do you think dimmable LED bulbs work internally? \$\endgroup\$ Jan 30, 2020 at 17:24

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