Thanks for the circuit. BTW, how did you make that display image saying "setup mode is active ..."? I'm documenting my LCD menu library and can use some decent display images like that.
Hi all,I recently did a small project that needed an LCD display with an LED backlight. I wanted to use a regulated current source so that the display wouldn't flicker with power supply voltage changes.Here's what I came up with, and it worked nicely.....The microcontroller circuit itself only used about 20 mA, so I just used a 22 ohm resistor for R and supplied the LCD backlight with a constant 230 millamps.Nice thing is, the LCD backlight current stays constant whether the input power is 9 volts, 12 volts or whatever. Of course, suitable heat sinking for the 7805 may be necessary, and Vin has to be at least 7 to 8 volts.Hope this helps someone......
How's that different from powering the led as a load to the regulator?
Your drawing show you grounding the center pin of the 7805. I don't think that is the proper way to wire up a 7805 as a constant current source. http://talkingelectronics.com/projects/30%20LED%20Projects/images/ConstantCurrent-2.gifLefty
Quote from: liudr on Nov 30, 2012, 06:34 pmThanks for the circuit. BTW, how did you make that display image saying "setup mode is active ..."? I'm documenting my LCD menu library and can use some decent display images like that.It's a Truetype font. I just went into my photo editor, made an "led green" background and then used the font to make the characters. I also made another image of all active pixels, then averaged them together to get the "faint shadow of a pixel" effect.The font is attached as a ZIP file. Hope you can make use of it.
Quote from: retrolefty on Nov 30, 2012, 06:57 pmYour drawing show you grounding the center pin of the 7805. I don't think that is the proper way to wire up a 7805 as a constant current source. http://talkingelectronics.com/projects/30%20LED%20Projects/images/ConstantCurrent-2.gifLeftyYes it is correct. It's a standard wiring for a voltage regulator. The 22 ohm resistor across the output causes a constant 230 milliamp draw from the input side... and since the 5 volts is regulated, the input current remains the same. Look at the circuit again.The point of my design is to BOTH provide a regulated current for the LED AND provide +5 regulated for the processor all from one device.
The regulator is setup as a constant current source. With 22 ohms across 5 volts, you have a constant 230 milliamps regardless of V-in.
Ok, perhaps it's because I don't see the actual pinout connections to and from the LCD module for both Vcc and the backlight led connections. If it works it works, that's for sure. Lefty
QuoteThe regulator is setup as a constant current source. With 22 ohms across 5 volts, you have a constant 230 milliamps regardless of V-in.The regulator is NOT setup as a constant current source: it is setup as a constant VOLTAGE source. The only reason it is providing (fairly) constant current to the LED is due to its constant VOLTAGE output on a constant load (that resistor).The advantage I see is that by wiring the led pre-regulator, you have lessened the voltage drop thus power dissipation over the regulator.The drawback obviously is that the voltage drop over the led + regulator is now much higher.
This isn't a drawback.
QuoteThis isn't a drawback.What happens if you attempt to power this whole thing up with a 7v source?