24v DC to 12v DC converter for LED backlight

[PJALM]

Hi everyone.

I am hoping someone may be able to help me with a project I am working on.

I have 3 26" LCD monitors that were given to me free because the backlights died so I opened them up and and removed all the CCFL tubes and replaced them with LED strips (the ones you get on the reels). Problem is that the output to the inverter was 24v and the LED strips use 12v. I tried to just use a simple 12v regulator but because these monitors are backlit not side lit, I had to use almost 2 full reels of LEDs just for one monitor. The only way I found to power them was to use an external 12v AC adapter that was 4.6 Amp. Using the external AC adapter also has the side effect of staying on when I turn off the monitor.

My goal is to make a 24v to 12v circuit that can output between 3.8 Amp or higher, make use of the BL enable from the monitor circuits and also possible use the dimming.

The 24v output from the monitors power supply is 3.6 Amp so would converting it to 12v give me enough power? I was thinking of using a low pin count PIC to do the conversion of the backlight dimming sing the monitor outputs a analog voltage, I could use a ADC to read that and output a PWM signal to drive the LED backlight.

Would all this be possible? Any help would be greatly appreciated.

Thanks

[Heckler]

LED's are a current driven (dependant) device. In other words don't be concerned about the voltage... pay attention to the current.
Do your LED strips include current limiting resistors? If so you should be able to include an external resistor to further limit the current and still be able to use the existing 24V supply.

Another thought would be to series two of the LED strips together and therefore make a 24V LED strip.

You can drive the same LED with 3, 12, 51 or 120 Volts by just changing the current limit resistor to a value that allows only the proper current to flow.
If you google "LED resistor calculator", or some similar phrase, you will find numerous examples of how to calculate the risistor value for a given LED and voltage.

hope this helps

[PJALM]

Thanks for the help. The LED strips are the waterproof type unfortunately so I can't change the resistors on them. Every 3 LEDs has a resistor and each strip has 66 LEDs. I did try to use 2 strips in series but didn't work so well.


Here is an Idea I am working with so far, let me know if this looks feasible or not. I am not all that sure about the VRs, can you use multiple in parallel to achieve higher amperage?

Thanks.

[PJALM]

The LED strips are arranged as follows and are fixed so I can't change them. They come on a 5M reel and have 12v rated current limiting resistors.

6321

[Heckler]

I did try to use 2 strips in series but didn't work so well.
Why did that not work?? It seems like it should have. You also should be able to supply an external Power resistor to drop half the supply voltage (which would be wasted as heat, not so good).

One thing you might consider would be to drive the ULN2003's using a PWM (pulse width modulated) signal at 50% duty cycle (or less for less brightness) and still use the native 24V supply.

Using the LMxxx regulators you are also going to be wasting half the consumed power as heat, dropping the 24v to 12v. A switching regulator (something I'm not vary familiar with) is the more efficient way to go.

Again, I would go back and find out why the parallel LED strips arent working. You would probably need to make sure each of the LED strips have the same number of LED's in each strip. Then just feed into one strip, cross over to the second strip and then to ground.

PS what is your source of LED strips?? I have been looking for some for a project.

[PJALM]

These are the LED strips (http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=110792791120&ssPageName=ADME:X:RTQ:US:1123#ht_3846wt_1165) I used. I wanted the Non-Waterproof but mis-ordered :(

[Heckler]

Again, I would go back and find out why the parallel LED strips arent workingOops, I should have said "series LED strips".

I checked your link to the Ebay LED strips... then clicked on over to their store. I may try the RGB led strips. Thanks!!

and good luck with your project.

[PJALM]

Would THIS (http://www.ebay.ca/itm/DC-DC-Converter-24V-Step-down-12V-5A-60W-Regulator-/370499716872?pt=LH_DefaultDomain_0&hash=item5643810708#ht_3050wt_1165) work for what I want?

[Heckler]

the dc/dc converter you pointed to SHOULD work BUT may generate some(a lot??) of RFI. You may want to get one and test it.

I'm curious... why were you not satisfied with two LED strips in series?? Was it too dim? or too dificult to wire up that way?
From my perspecitive that really seems the very simplest way to solve your problem without the need for any more external components.

I would also breadboard up the circut and try the PWM via the ULN2003's and use the native 24vdc. (they can easily handle that voltage) If you keep the ON-duty cycle very short you should be ok.

Anyway... just trying to help you find the simplest solution.

[PJALM]

If I try connecting 2 LED strips in series the internal power supply just clicks and turns off and on. Maybe this whole time there was nothing wrong with the original backlights and was the internal power supplies at fault the whole time lol. Too late now, I allready converted them all to LED backlights.

[PJALM]

Well the power supplies seem to be fine so I am going to continue with this project. Until I manage to get the 24v-12v DC/DC converters I am going to use the external 12v PS and start building the PWM control board and see if I can start writing the code.