LCD backlight overheats 7805

[keithdoxey]

So lets say I move the lcd backlightpower to the source before my 7805 regulator which is powered by a 12v wall wart adapter. Do i need a voltage divider before feeding the power to the LCD backlight?

You just need a bigger resistor instead of the 11 ohm you currently have.

With your 11 ohm resistor you have 50mA flowing through the backlight.

11 * 0.05 = 0.55V dropped across the resistor so your LED backlight has a forward voltage of 4.45 volts.

For a 12v supply you need the resistor to drop 7.55v

7.55/0.05 = 151 so 150 ohm 0.5 watt resistor for 12V supply @ 50mA

For the 18v supply you need the resistor to drop 13.55v

13.55 / 0.05 = 271 so 270 ohm 1 watt resistor 18V supply @ 50mA

[RYTECH]

thx for the info

[DynamoBen]

Just for the record, I ran into the same situation recently. I end up tying the backlight to 12V. Works well and keeps the 7805 nice a cool.

[mvs_sarma]

Hi
i saw another application where 47E is used instead of 11E in series to backlit LEDs circuit
However perhaps it is better Rytech cold analyze the current breakup-- may be the current is draining thro another circuit path

[mvs_sarma]

You just need a bigger resistor instead of the 11 ohm you currently have.

With your 11 ohm resistor you have 50mA flowing through the backlight.

11 * 0.05 = 0.55V dropped across the resistor so your LED backlight has a forward voltage of 4.45 volts.

For a 12v supply you need the resistor to drop 7.55v

7.55/0.05 = 151 so 150 ohm 0.5 watt resistor for 12V supply @ 50mA

For the 18v supply you need the resistor to drop 13.55v

13.55 / 0.05 = 271 so 270 ohm 1 watt resistor 18V supply @ 50mA

having regard to RYTECH, i wonder whther 78L05 was used and if so proper termination adopted or Not?
afterall for 50mA with 12V input, the device need not be hot. let RYTECH measure the current at input 12V point and it will really speak.

[Acetronics2]

Hi, Sarma

I just have MEASURED 3v9 @ 50 mA on one of my LCD Backlight ...

so, ...

( 12v - 3.9v ) x .05 = 0.4 W to be dissipated ...


now, as you do, pluggeg in the 5v with 11 Ohms

( 5 - 3.9 ) /11 = 100 mA !!!

so, your 7805 is then overcharged and is close to it's maximun dissipated power before shutdown ( 130°C internal ? ) !!!

Try a 22 Ohm resistor and everything might be Ok !!!

Alain

Ohm's law will still make ink flow ...

[mister_e]

[Acetronics2]

really satanic !!!

[mvs_sarma]

Hi,

Acetronics is right in recommending 22ohms and i saw other designs (IK3OIL's frequency counter using LCD display) indicating backlit LCDs draw 60 mA and recommend 7805 but NOT 78L05. No heatsink is noticed.

Hi DynamoBen,

did you use 12V ,7805 and 11ohm resistor for Backlit? Still find No heat?

[DynamoBen]

I have since moved the backlight to my 12V source. That seems to work fine.

[Archangel]

Thanks mister_e