DC Dimmer Switch

[anmango35]

Please help,

I want to build a circuit for dimmer switches for 4 LEDs (Red-Green-Blue

White) color. The voltage drop on each of LED needs to be around 3.9v and

current is 1000mA (1A). I have the LED driver:

AC input = 110-220V

DC output = 24V

I (current) output = 1000mA (1A).

Thanks,

Anmango35

 

[HarveyH42]

Well, at least there are some numbers...

What kind of dimming circuit do you have in mind? Do you you want to automatically fade, manually set the brightness of each LED? Is a microcontroller an option? PWM? Simple variable resister?

What is your application? If this photographic lighting, then PWM probably wouldn't be a first choice. 5 watt LEDs? I haven't played with them yet, a little pricey and need some serious heat management...

 

[anmango35]

I want to build a demox box dimmers are controlled by potentiometers

(by manually).

 

[anmango35]

I have 5W Red Green Blue LEDs. I want to use current to control the color

manually by Potentiometers then I have a lot of different color by changing

the current for each LED.

Thanks

 

[mneary]

So you want 1000mA for each of 3 LEDs from a single 1000mA supply? Of course the power is there (because you have 24V) but you'll need a switching controller, probably with inductors, to transform 24V @ 1000mA to 3.9V @ 3000mA.

 

[anmango35]

Hi Mneary,

Yes, I have three seperate drivers (constant current) for 3 LEDs because 3

LEDs are connected in parallel.

 

[HarveyH42]

I haven't messed with LEDs in this range (yet), but have done some 70ma with a LM317 wired as a constant current supply. Problem though, is its 1 amp max, and uses about 1.25 volts. So for a 3.9 volt LED, a 6 volt supply. 3 in parallel would require a supply greater than 3 amps, or it will get hot.
I think this could work for you, as you most likely do want or actually need to drive your LEDs at max 1000ma.

Couldn't find my notes for calculating the resistor, but the equation is in the data sheet for LM317, and its simple.

 

[anmango35]

Thanks a lot for your information. The hard thing is the LEDs current is high

I am working on it and I'll let you know what going on.

Thanks

 

[audioguru]

You should not connect LEDs in parallel. Their voltages are slightly different and the one with the highest voltage will take most of the current and burn out. Then the others will also quickly burn out.

24VDC is much too high for a 3.9V LED. Connect the LEDs in series then less current is used and much less power is wasted.

 

[anmango35]

You're right. I should not connect LEDs in parrallel because I(total)=I1+I2...

I have different driver for each LED.

 

[audioguru]

Are your LM317 regulators connected as current regulators or as voltage regulators?
They will get very hot since your supply voltage is so high. If the LED voltage is 3.9V, the current regulating resistor drop is 1.25V and the minimum voltage across the LM317 is 2.5V then a supply voltage of only 8V is needed and the LM317 will dissipate only 2.85W at 1A.

Since your supply is 24V then the LM317 will dissipate 18.85W and will need a huge heatsink and/or a high velocity fan.

 

[anmango35]

I have LM1117T as a voltage regulator. It's not too hot.

Thanks for your advice.

 

[audioguru]

The max current for an LM1117T is only 800mA. It might or might not give 1A.
LEDs operate from a current, not a voltage. The regulator can be made into a current source, or a resistor can be added in series with the LED to convert voltage to current. If your input is 24V then the resistor will become very hot at 1A, it will dissipate about 19W.

 

[anmango35]

I connect 25W resister in series with LED. I think i'ts safe for LED.

 

[Hero999]

Using a linear regulator for an efficient LED seems a bit wasteful, a switching regulator will probably work out cheaper because it doesn't require a huge heatsink.

You could build three 1A constant current switching regulators or a 4.0V 4A switching regulator and use cheap 1W - 2W resistors for each LED.

Note that if you go for this option you don't need any expensive control ICs. Providing the supply voltage is reasonably stable the switching supply can be an open loop design and no smoothing capacitors on the output are required. The formulae for calculating the currents and duty cycle can be found on Wikipedia.
https://en.wikipedia.org/wiki/Buck_converter
**broken link removed**


Another idea would be to build a 1A constant current regulator (whether it be a switching design or not) and short circuit the LEDs in a PWM fashion.

 

[anmango35]

My circuit is Dimmer is not switching circuit. Do you have any idea for

Dimmer circuit? Please help.

Thanks

 

[Hero999]

You're missing my point, a dimmer is a type of switching circuit. The idea is if you switch the LEDs on and off fast enough your eyes will think they are continiously on and if you vary the duty cycle (on vs off ratio) then you can vary the brightness. Please Google for PWM.

There is no point in building a 1A regulator for each doide when you can just use one regulator, connect them all in series and short circuit each diode to turn it off.

If you really want good efficiency then use a switching constant current regulator. Please check out the link to Wikipedia in my previous post.