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[on:]

I'm about to send of a design for some nice flexible I2C backpack PCB's and just need to clear up issue over the backlight connections.

I've found and seen lots of conflicting information on this.

1) A post on here saying there are two 15mA LED's fed via a 10R, with Vcc intended at 3.3 v to provide 30mA total power.
2) Some SPI/I2C schematics showing a 68R in the LED feed circuit, controlled in various ways from PNP/NPN transistors or a P-Fet.

I can't find anything definitive in any datasheets on these generic modules, with respect to the backlight LED info.

On the modules I have, there is a 100R. The VD across the LED when lit is 3.0V. This all ties nicely with a measurement of 21.85mA total consumption of the LCD module @ 5V. The LED being (5-3)/100 = 20mA.

So, this suggests no external reistor is needed.

The mjkdz I2C modules I have use a PNP transistor and have no LED resistor.

Just after opinions before I send the Gerbers off for a batch. At the moment I have no backlight resistor on the design, after removing it.

[Reply #1 on:]

Incorportate it in the circuit, use a jumper or 1 ohm resistor if it is not needed.

[Reply #2 on:]

You always need a current limiting resistor when you drive an LED from a conventional fixed-voltage power supply. 

My own personal opinion is that you should make provisions for a resistor on your PC board if you have the room.  You can always add a jumper , a very low value resistor, or a zero ohm resistor if the display already has one. 

Another alternative is to provide the resistor pads but connect them with a thin trace on the PC board.  If you need to use the resistor pads you can cut the trace.

Don

[Reply #3 on:]

Great idea on adding the pads but providing a thin trace. I may go for the option of adding a 68R on one side of the board and a trace on the other to short it out. If it gets used with a module with no onboard resistor then the trace can be cut to bring the resistance into circuit.

Or, even a 2 pin shorting jumper next to the resistor with jumper in the off position by default.

I quite like that idea actually as, on a module with a 100R already present, the jumper off would make backlight dimmer to save a little power. It gives additional options which is the kind of flexibility the board has.

At the moment I have it so it can have upto 2 x 8574 to provide both LCD and Keypad support. Addresses are set by jumpers. It can be used as LCD or Keypad or Both, depending on which components and headers are installed. Backlight can be set by a jumper as I2C or direct pin. Contrast can be set by a jumper as onboard trimmer or direct pin. The 6 pin I2C/Backlight Option/Contrast Option header can be set at either end of the PCB or both for an I2C pass through to another device. All on a 50mm x 23mm board, so only a few mm large than some of the LCD backpacks available.

Thanks for the idea Don and Larry. :-)

[Reply #4 on:]

At the moment I have no backlight resistor on the design,

Most designs are like that: it provides the broadest means for the user to control the brightness of the display.

[Reply #5 on:]

Another alternative is to provide the resistor pads but connect them with a thin trace on the PC board.  If you need to use the resistor pads you can cut the trace.[/color]

Don

Yes, through hole resistor pads would be great.

Another alternative would be to reverse it.
Use a resistor but have a parallel trace with an open solder jumper on the board.
That way the LED is never overdriven but if the backlight is too dim because the users LCD
already has a resistor on its module, the user could solder closed the jumper pad to connect
the traces to bypass the resistor.

At the moment I have no backlight resistor on the design,

Most designs are like that: it provides the broadest means for the user to control the brightness of the display.

How is no resistor providing the broadest means?
I've got an i2c lcd backpack with no resistor between the transistor and the Anode
and to add one is a real pain.


--- bill

[Reply #6 on:]

I'm going to use a surface mount resistor but I'll have a 2 pin jumper in parallel. The jumper can be off by default, leaving the resistor in circuit. This will make sure there is always a resistor in series with the LED, regardless of the module design.

If the backlight is dim then the backpack resistor can be shorted wit the jumper. OR, even if there is a resistor on the LCD module, the jumper can stay off to have a dimmer backlight using less current. The user just has an additional option.

[Reply #7 on:]

How is no resistor providing the broadest means?

Bill - you seem to forget that he feels that you don't need the series current limiting resistors at all.

Don