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Replacing original LCD using CCFL backlight to LED backlight display

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rfranzk

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Hello All,
I am working on an air quality instrument where the original LCD display was damaged by water intrusion. The original CCFL backlight displays are either obsolete discontinued or only available in lots of 100 plus.

I have located displays matching the original except for the LED backlight. The original has an external DC to AC inverter that is supplied with 15 VDC. There is room on the board to replace the inverter with a small footprint circuit to power the LED. The LED backlight calls for 5 VDC supply and typical current of 120 milliamps. Do I need a simple resistor divider, zener regulator, regulator ic, or dc-dc converter to power the new backlight. The instrument runs 24/7 but the backlight is only used when performing maintenance on the instrument. The backlight might run continuously for an hour or so and then is not used until the next maintenance interval which is approximately once a week.

I just don't know the most efficient and effective circuit to use and any recommendations would be helpful.

Thanks.

rfranzk
 
There is probably a good source of 5 V some where in you instrument.
I do not know for certain but I think you can steal 120 milliamps from the 5V.
 
I have also had to do this..... A backlight may spec 120mA, however! Running at 60mA will be fine... The conversion I did was even lower... I ran my backlight at 24mA when it was spec'd at 120mA to save battery power... It was readable and the customer was happy... I had to remove the old AC generator from the board first... I also had to mount a transistor as the old system was a "switched" backlight... But 24mA from a rail that didn't expect it, wasn't an issue...
 
Thanks for the replies,

I was hoping to utilize the 15 volts that powers the original CCFL driver as the instrument has a sleep mode where if no commands sent to the instrument the display backlight shuts down until another command is sent.

Spec, that prostack thing looks pretty interesting. I was hoping to avoid any excessive heat from whatever performs the voltage conversion.

Ian, did you just use a dropping resistor off another rail to provide the needed current? I suppose I could do that with the 15 volt supply and experiment with currents to get what I need. I am guessing 60 milliamps will be sufficient. The footprint I have to work with is 26 mm x 20 mm.
 
Thanks for the replies,

I was hoping to utilize the 15 volts that powers the original CCFL driver as the instrument has a sleep mode where if no commands sent to the instrument the display backlight shuts down until another command is sent.

Spec, that prostack thing looks pretty interesting. I was hoping to avoid any excessive heat from whatever performs the voltage conversion.

Ian, did you just use a dropping resistor off another rail to provide the needed current? I suppose I could do that with the 15 volt supply and experiment with currents to get what I need. I am guessing 60 milliamps will be sufficient. The footprint I have to work with is 26 mm x 20 mm.

Hy rfranzk,

One of the 'three terminal' switch mode regulators, especially the Murata product, would be ideal if you were prepared to pay the price. But all the others would be equally good for your application.

By the way, the current drain from the 15V supply would be around 50mA with a switch mode approach.

If you let us know your location we would probably be able to find you a supplier for the best product for the switch mode power supply.

Rather than dropping the voltage from the 15V supply to 5V by a resistor you would get far better performance by using a linear regulator. At 120mA LED current, the dissipation would be 1.2W, exactly the same as for a resistor, which should be manageable. Even a dirt cheap LM7805 three terminal regulator would do the job well. They are also dead easy to use.

If you wanted to adjust the LED current you could probably use an LM317 adjustable regulator which are also dirt cheap, but would require two resistors to set the output voltage.

120ma at 5V sounds like a lot of power for what I imagine is a small LCD display. Is the current really that high?

spec
 
Last edited:
Thanks Spec,

I haven't measured the actual current, just going off the datasheet for the original display. Its a 240 x 64 graphical display.

I am located in the USA and typically deal with Digikey, Mouser, Newark, Allied, etc.

I haven't pulled the inverter off the board yet but I like the switching regulator idea and am pretty sure there is enough room on the board to add it in place of the inverter. They are a little pricey but if they are robust and its a quality permanent fix I'm good with that.

I am essentially looking for recommendations on the most efficient and reliable approach I can have. I believe if I just supply a clean 5 volts to the new display the backlight is covered. I will have to take a closer look at the datasheet to be sure.

Thanks.

rfranzk.
 
Thanks Spec,

I haven't measured the actual current, just going off the datasheet for the original display. Its a 240 x 64 graphical display.

I am located in the USA and typically deal with Digikey, Mouser, Newark, Allied, etc.

I haven't pulled the inverter off the board yet but I like the switching regulator idea and am pretty sure there is enough room on the board to add it in place of the inverter. They are a little pricey but if they are robust and its a quality permanent fix I'm good with that.

I am essentially looking for recommendations on the most efficient and reliable approach I can have. I believe if I just supply a clean 5 volts to the new display the backlight is covered. I will have to take a closer look at the datasheet to be sure.

Thanks.

rfranzk.


Thanks for info rfranzk

As you are in the States component supply will not be a problem. Which State- I am being nosey.

Would you be able to post the data sheet for the display, especially the LED backlight information. I would be very surprised if the current were as high as 120mA for that type of display but I am only guessing.

Both the linear and switching regulators would be reliable, but the linear regulator would get quite hot unless you could mount it on a heatsink.

Murata stuff is always top rate, so subject to further analysis, that companies products would be my recommendation.

spec
 

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  • ERM24064-1_Series_Datasheet.pdf
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Hi Spec,
Thanks for your assistance and guidance with this. Here is a link to the display I am thinking of using. I am basically repairing this display as a spare for future needs so need to be budget minded to some extent. https://www.ebay.com/itm/301017374539?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

And here is a datasheet which shows led current at 160 milliamps.

I am located in Northern Colorado by the way.

Thanks.
rfranzk.

Hy rfranzk

No probs about assistance- I now know something about small switchers due to your inquiry.:)

It looks like the typical current for the LED back light is 120mA but the maximum is 160mA.

The Murata module (post #9) would be an excellent choice.

If you wanted to save money you could consider a board from eBay, but while that would do the job adequately, it is not in the same class as the Murata module in terms of quality and reliability and would probably be electrically noisier which would not be a good thing.

You are from Northern Colarado.:cool: I see you may be a separate State one day!

spec
 
Last edited:
Hello All,

Thanks for the input and as soon as I get the components here and installed I will report on the success.

Thanks again.

rfranzk.


Spec, About the separate State someday, there has been discussion but it is Highly unlikely.
 
Hello All,
I first started with using a couple AAA batteries to power the back-light and confirm the display worked with the instrument. Today I had time to swap out the high voltage inverter with the 5 volt switching regulator. Simple simple and the operation was a success.

Thanks for everyone's help.

Rfranzk.
 
Hello All,
I first started with using a couple AAA batteries to power the back-light and confirm the display worked with the instrument. Today I had time to swap out the high voltage inverter with the 5 volt switching regulator. Simple simple and the operation was a success.

Thanks for everyone's help.

Rfranzk.

Good news

spec
 
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