LCD backlight causing problems - Electronic Circuits Projects Diagrams Free

MrMikey83 · 5th February 2005, 06:12 AM

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** · 5th February 2005, 09:02 AM

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 · 5th February 2005, 02:56 PM

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** · 5th February 2005, 03:00 PM

Quote:

Originally Posted by 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.

That seems particularly excessive!.

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

MrMikey83 · 6th February 2005, 07:16 AM

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** · 6th February 2005, 09:45 AM

Quote:

Originally Posted by 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?

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 · 6th February 2005, 11:12 PM

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

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

5th February 2005, 06:12 AM	(permalink)
MrMikey83	LCD backlight causing problems 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 __________________ All Electronic components run on smoke. Let the smoke out and it no longer works. ~Tim Baker (Electronics Instructor at John A. Logan College)

5th February 2005, 09:02 AM	(permalink)
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!. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

5th February 2005, 02:56 PM	(permalink)
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. __________________ All Electronic components run on smoke. Let the smoke out and it no longer works. ~Tim Baker (Electronics Instructor at John A. Logan College)

6th February 2005, 07:16 AM	(permalink)
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? __________________ All Electronic components run on smoke. Let the smoke out and it no longer works. ~Tim Baker (Electronics Instructor at John A. Logan College)

6th February 2005, 11:12 PM	(permalink)
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 __________________ All Electronic components run on smoke. Let the smoke out and it no longer works. ~Tim Baker (Electronics Instructor at John A. Logan College)