LCD powered by 9V battery need current limiting resistor

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amateur_24

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Hi
I've checked the spec sheets, it says the led bacjk light typically uses 240ma. So I used a 18ohm resistor to supply 5V. It also said the lcd itself only uses max of 3ma. Can I still use the same supply or would I have limit to 3ma?

Also is there a calculation I can make to see how long will my battery last?
 

hi,
Typically the LCD requires a 5V supply.
Whats the type of LCD and battery.??
 
Go to a battery manufacturer like Energizer or Duracell and look at the datasheet for a 9V alkaline battery. With a 250mA load its voltage drops to 7V in about 20 minutes so with 240mA it would last about 21 minutes.
When the battery voltage is as low as 7V then an ordinary 5V regulator will begin to have poor voltage regulation.
 

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My ST7066 d/s says +2.7v to +5.5v for the LCD Vcc

I guess you realise that the LED back light on a alpha numeric display is not essential for daylight viewing.???
 
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