Holding 5V Signal

[joe_1]

Hi:

I am trying to connect Hi signal (5V) from port A of a PIC to the V+ of the LED of LCD.
The signal coming from port A is 5V, but the minute I connect it to the V+ of the LED of LCD, it drops to 3.7V.
Is there away to hold it steady at 5V after the connection is made?

Thanks.

 

[vinke]

Why do you need to keep it steady? The LCD isnt working?
Surely the voltage will drop because of the loading effect, you can use a driver to get the 5V you want

 

[joe_1]

Why do you need to keep it steady? The LCD isnt working?
Surely the voltage will drop because of the loading effect, you can use a driver to get the 5V you want

The LCD is working, but with only 3.7V, it is very dim.
Also, how do I use a driver?

 

[blueroomelectronics]

I take it you're driving the LCD backlight directly from a PIC pin? Nope won't work needs at least 65ma for a modern LCD backlight and 150ma for the old green yellow ones. You need a driving transistor.
**broken link removed**

 

[vinke]

A very simple driver would be to use a general purpose NPN transistor as a switch.from your PIC output connect a 1Kohm resistor in series with the base of the transistor.Connect 5V to the transistor's collector and connect a 330 ohm resistor between the emitter and your LCD.You will have more power to your LCD.

 

[audioguru]

A very simple driver would be to use a general purpose NPN transistor as a switch.from your PIC output connect a 1Kohm resistor in series with the base of the transistor.Connect 5V to the transistor's collector and connect a 330 ohm resistor between the emitter and your LCD.You will have more power to your LCD.

Your transistor is an emitter-follower with a voltage loss of about 0.8V. The LEDs are about 3.5V for white ones so the total voltage lost is 4.3V.
So only 0.7v is across the 330 ohm current-limiting resistor which limits the current to only 2mA. Way too low.

 

[vinke]

Your transistor is an emitter-follower with a voltage loss of about 0.8V. The LEDs are about 3.5V for white ones so the total voltage lost is 4.3V.
So only 0.7v is across the 330 ohm current-limiting resistor which limits the current to only 2mA. Way too low.

Put a 10 ohm resistor then,you will get 70mA.

 

[audioguru]

I think it is safer if the transistor is a common-emitter. Then the resistor is 20 ohms for 70mA when the LEDs are 3.5V. But if the LEDs are only 3.0V in your circuit they will have a current of 120mA and will burn out. In my circuit the new current will be 95mA which is safer.

 

[vinke]

I think that joe_1 was talking about LCD and not a LED. 120mA wont burn a LCD,well it depends on which LCD he is using.Neways the common-emmitter is surely safer

 

[blueroomelectronics]

V+ of the LED of LCD.

Pretty sure it's the backlight LED of the LCD, and yes it needs a driver as a PIC can supply nowhere near enough current.

 

[audioguru]

The max continuous output current allowed from an output on a PIC is only 25mA. There is another max allowed total current if a few outputs have a load.

I think the current used by an LCD display is nearly nothing. My LCD watch runs for two years from a tiny button cell. It doesn't turn off the LCD at night.