LCD backlight overheats 7805

[RYTECH]

hi i have my lcd backlight plugged into the 5v rail via a 11ohm resistor... it seems to cause the 7805 regulator to over heat. Why is this.. what value resistor should i be using with my 16x1 lcd display backlight?...

[keithdoxey]

What does the LCD datasheet give as the forward voltage for the LED ?

How much current is flowing through the backlight with your 11 ohm resistor?

What else is being powered from the 5V rail.

What is the current rating of your 7805 ? 100mA 1A 2A

Does the 7805 have a heatsink an what is its input voltage ?

Has the smoke escaped yet because when it does it will stop working !!!

[RYTECH]

The lcd datasheet doesnt say much about anything.

i metered the lcd backlight, it seems to only pull .05 amps or 50 milliamps via the 11 ohm resistor at 4.9v from the 7805.

The only other things powered off the 7805 is the LCD itself, a pic 16F84a.

The current rating is 1amp on the 7805, no heatsink is attached.

Input voltage is between 12 and 18v. When the LCD backlight is unattached, the 7805 remains cool, the circuit and lcd function normally.

When the backlight is attached to +5 via the 11ohm resistor, the 7805 heats up quickly and i shut down the circuit as soon as it's too hot to touch.

no smoke yet... but i realized the 7805 was overheating when it smoked an electrolitic cap on teh 5v output. (the cap was almost touching the 7805)

lemme know what u think.

[keithdoxey]

The lcd datasheet doesnt say much about anything.

i metered the lcd backlight - 50 milliamps

The only other things powered off the 7805 is the LCD itself, a pic 16F84a.

The current rating is 1amp on the 7805, no heatsink is attached.

Input voltage is between 12 and 18v. When the LCD backlight is unattached, the 7805 remains cool, the circuit and lcd function normally.

I think you need to add a heatsink. Your regulator is dropping upto 13V with worst case supply voltage. At 50mA the LCD will be causing it to disipate an additional 0.65W with only the tab to get rid of the heat.

The 7805 datasheet I have just looked at says that maximum power dissipation is "Internally Limited" so it *shouldnt* self destruct but best not risk it

What is the TOTAL current you are drawing from the supply.

[RYTECH]

LCD datasheet

[mister_e]

just use a heat sink could help... but i would check what's happen at the 7805 input first with a scope. Is this really clean? is this really between the voltage you mentionned OR it have also up to 20 volts peak...

My best suggestion will be to change your PIC to have a PWM out and use it. Take the output, send it to a transistor. The LCD voltage will be taken at the input of the voltage regulator. This way, you will reduce heat.

a 16F628 is just perfect, cheaper and also give few free i/o if the internal OSC is enough. 1 more if you don't need the MCLR pin.