Hi guys
How do i calculate time for a circuit or device from its Current?
Example:
If a 10mA LED connect to a 6V 4.5Ah battery with a 1K series resistor what is the maximam time of glowing LED ?

 

hi,
If you connect a standard LED, which has a forward voltage drop of 1.5V, then you will have 6-1.5= 4.5v across the 1K0.

So the current thru the 1K0 and LED will be 4.5/1000= 4.5mA..

To calculate the endurance of a fully charged 4.5Ah battery, in good condition, you divide the 4.5/.0045 =1000 hours.

This is an approximate calcultation as the current is so low, also the LED will continue to glow when the battery reaches it recommended discharged voltage of about 5.4V. The LED should be disconnected when the battery voltage fall to 5.4V for a lead acid battery.

EDITED:

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

same formula can use for other circuits like a timer,flasher or others?

 

Thats correct.
Its important to know the voltage drop across the LED, look at the LED specsheet, then subtract the LED voltage from the battery voltage.
Some of the higher powered LED have voltage drops in the order of 2 to 3Volts or more.

Does that help?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

I don't know of an LED with a forward voltage drop of less than 1.5V. See http://www.oksolar.com/led/led_color_chart.htm

__________________
Ron

 

hi Ron,
Of course you right!.

Calculation was based in error on bog standard diode!.

I'll edit accordingly.

Many thanks

T'is done!

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/