The datasheet specifies it pulls about 60mA with the back light on. If I hook up my multi-meter it appears to be pulling 120mA, near double the datasheet. Is this common for the datasheet to be so far off or is it possible my board is defective. Whats weird is if I set my multimeter to 10A is shows as using up .12 amps or 120mA however if I set it to my 200mA mode it says the display is only pulling 80mA, very weird?
In addition its weird because the current draw is such that I am not getting the full 5V out of my Arduino, I am using the 5V supply pin not an I/O port. It is dropping to about 4.6 volts. This is weird because I have read you can pull up to 200mA from that 5V supply pin safely. Any thoughts?
In addition on my oscilloscope, the 4.6V is extremely unstable. I put a few caps on and it helped a little, but it still is jumping around a good bit and from time to time you can see the display get dimmer then brighter again. Any thoughts?
It sounds like the back light hasn't got a series resistor in place. However as it is a custom built system maybe the wrong value has been fitted. Can you identify it and check.
Yes I looked at the schematic and it is rather useless, it doesn't show the LCD at all.
The back light limiting resistor should be in the order of 10 ohms. Don't confuse it with the contrast pot that is not going to give you a problem.
The difference in your current measurements on different ranges implies an LED back light problem because the slight change in resistance of the meter's shunt is affecting the current reading so much.
I wouldn't rule out the multimeter being the issues, as it is not of the highest quality. However considering all the other power issues as well I also wouldn't be surprised if some resistor was the wrong value, I will play around some more and see if I can find any resistor on the board in the order of 10 ohms.
Any idea as to why the 5V pin on the Arduino is dropping to 4.6V with only a load of 80-120mA?
OK, I probed all the resistors I could see on the board and the only ones close the value you specified (there are two) are each around 3 ohms.
I think I might try and use a pot to control resistance on a straight short to ground from the 5V pin and see how much current I can get out of my 5V pin before it starts dropping below 5V; safe method?. Maybe there is a problem internally in my arduino 5V pin.
If you read the SerLCD v2.5 Datasheet document (Page 2, Section 3.1), it looks like the LCD backlight can be turned on with a software command, and is PWM-able. I don't even see any A-K connections in the Sparkfun schematic.
Grumpy_Mike:
It sounds like the back light hasn't got a series resistor in place. However as it is a custom built system maybe the wrong value has been fitted. Can you identify it and check.
That module has a BJT controlled by the add-on microcontroller (for supplying the backlight). It's PWM adjustable. I suspect the OP hasn't set the backlight level.
Correct, I set the backlight to 100% via a serial command, which works. I have full power and full communications, but it is just drawing quite a bit more current then the datasheet specifies.
