current limiting a 4 by 20 lcd display

[jonisonvespaa]

hi
im using the above display, just wondering if i need to limit the current to the display, looks like there is already a current limiting r 68 ohms on the backlight already going from the + to the led

i cant really trust the data in the sheet says it runs at 220ma, but i only measure 20ma.

thanks

[floresta]

im using the above display

That doesn't really pin things down.  If you provide a link to a specific datasheet then we may be able to clear up your problem.

Don

[jonisonvespaa]

hi don

sorry your right,  was just asking a general question really, if i see a resistor going to the positive of the backlight led of the display can i assume its a current limiting resistor and therefore i dont need to protect it
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http://www.ebay.co.uk/itm/350560713502?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

ive got other displays that also have a 68 ohm resistor to the anode, i soppose it  must be as it limits the back light to 75ma, and the data sheet says absolute max is 220ma

thanks

[floresta]

i cant really trust the data in the sheet says it runs at 220ma, but i only measure 20ma.

No.  It says that if you run it at 220 mA then the forward voltage will be about 4.1 volts.

According to section 6 of the data sheet you can apply +5 V directly to pin 15 which implies that there is a current limiting resistor on the pc board.  You have indicated that you found a 68 ohm resistor on the board so everything looks ok so far.

According to section 5 of the data sheet the LED will typically drop about  4.1 volts if it is drawing 220 mA.  With a 5 volt supply this condition would occur with a 4 ohm current limiting resistor.  Although LEDs are nowhere near linear the fact that your resistance is more than 10x this value and your current is about 1/10 this value looks ok as well.

The bottom line is that if your backlight is bright enough and its current is less than 220 mA you are safe.  Don't forget that you are probably powering all of this from your Arduino and its available current is really quite limited so the less you use for the backlight the better off you are.

Don

[jonisonvespaa]

the display is sitting in a pcb powered via  a seperate 5v powersupply

thanks very much for your help don