LCD backlight causing problems

[MrMikey83]

I have a large 20x2 LCD display from one of the cash registers at work. It has an LED backlight but when I turn on my circuit with the backlight plugged in, it gives me some strange values on my sensor outputs. I have Gyro and a radio control input and when the backlight is connected, the values are not stable. When I disconnect the backlight, all is normal.
Do I need to isolate the backlight somehow? What would cause this to happen?
~Mike

 

[Nigel Goodwin]

Is the extra current for the backlight causing the problem?, it will probably draw much more current than the rest of your circuit put together!.

 

[MrMikey83]

You're right Nigel. With the backlight off, it draws 4.4mA but when connected, the entire circuit draws 547mA. The wall adapter is only rated for 200mA.

 

[Nigel Goodwin]

You're right Nigel. With the backlight off, it draws 4.4mA but when connected, the entire circuit draws 547mA. The wall adapter is only rated for 200mA.

That seems particularly excessive!.

Perhaps you should feed the backlight via the usual series resistor?, I strongly suspect you're not doing so!.

 

[MrMikey83]

You're right Nigel. For some reason, I was thinking that the display might have had a built in resistor but I guess not. I tried a 100Ohm and the backlight was very dim. 10 Ohm takes it to about half brightness and the backlight itself is drawing .1Amp. Does this sound right? What size resistor would you suggest?

 

[Nigel Goodwin]

You're right Nigel. For some reason, I was thinking that the display might have had a built in resistor but I guess not. I tried a 100Ohm and the backlight was very dim. 10 ohm takes it to about half brightness and the backlight itself is drawing .1Amp. Does this sound right? What size resistor would you suggest?

I would suggest you increase the resistor value until it's just bright enough, that way will minimise current consumption - without having the spec on the display it's hard to know what it might be capable off.

You might try looking up specs on backlights in other LCD displays, that would give you some idea of their capabilities.

 

[MrMikey83]

There is no spec sheet that I can find for this particular LCD display. I did look up a similar one and it consumes 130mA for its LED backlight. So, I played with resistor values till I got a bright enough display and measured the current at 130mA. I used two 10 Ohm's in parallel (5 Ohms).
~Mike