daemonik Posted December 23, 2010 Report Share Posted December 23, 2010 I've got 2 of these LCDs: digikey optrex (datasheets at digikey!) black dot's and white LED backlight My general problem is, they are not equally illuminated and have unequal contrast! thus I probably have to use two separate illumination/contrast control circuits. As for contrast, this is not difficult... but I have some uncertainties with illumination: 1. I'm no expert but I guess I could also drive the backlight LED's with 12V??? 12V are used since I want to connect an AOUT_NG-Module and also need 12V Gate Outputs (Roland System100 and hopefully some other in the future). furthermore, 12V for LED backlight would be much appreciated by the 7805 which gets really hot (~90°C at 250mA backlight current [spec. stats a max current of 125mA for each modules backlight]) 2. could I use just a resistor and a pot in serial instead of the constant current source for each module? or would this lead to unstable illumination? I'm using a 12V 1.5A switched PSU. any help is much appreciated!!! and merry x-mas to all Quote Link to comment Share on other sites More sharing options...
phunk Posted December 24, 2010 Report Share Posted December 24, 2010 (edited) 1. I'm no expert but I guess I could also drive the backlight LED's with 12V??? 12V are used since I want to connect an AOUT_NG-Module and also need 12V Gate Outputs (Roland System100 and hopefully some other in the future). furthermore, 12V for LED backlight would be much appreciated by the 7805 which gets really hot (~90°C at 250mA backlight current [spec. stats a max current of 125mA for each modules backlight]) 2. could I use just a resistor and a pot in serial instead of the constant current source for each module? or would this lead to unstable illumination? I'm using a 12V 1.5A switched PSU. You cannot drive them with 12V directly but as you suggested with a resitor in series that regulates voltage and current. The forward voltage for this type of LEDs is 3.5V according to the datasheet. Current is 120mA. That means: R = (Vin-Vled)/Iled = (12V-3.5V)/0.12A = 62,5Ohm. BUT... the Problem you face now is that you cannot use a standard 1/4 Watt resistor, since it would melt away due to its power rating. P= (I^2)xR = (0,12A^2)x62,5Ohm = 0,9Watt. Using a pot in series would be an option and convenient for adjusting the backlight...same problem with power though. You gotta calculate that. Edited December 24, 2010 by phunk Quote Link to comment Share on other sites More sharing options...
Hawkeye Posted December 24, 2010 Report Share Posted December 24, 2010 <missonary mode> ok, this may not be helpful, but you could always switch to vfds and sell the lcds to some new-schoolers :-) </missionary mode> merry x-mas! Quote Link to comment Share on other sites More sharing options...
daemonik Posted December 27, 2010 Author Report Share Posted December 27, 2010 You cannot drive them with 12V directly but as you suggested with a resitor in series that regulates voltage and current. The forward voltage for this type of LEDs is 3.5V according to the datasheet. Current is 120mA. That means: R = (Vin-Vled)/Iled = (12V-3.5V)/0.12A = 62,5Ohm. BUT... the Problem you face now is that you cannot use a standard 1/4 Watt resistor, since it would melt away due to its power rating. P= (I^2)xR = (0,12A^2)x62,5Ohm = 0,9Watt. Using a pot in series would be an option and convenient for adjusting the backlight...same problem with power though. You gotta calculate that. Thanks for your helpful explanations. Indeed I would have missed about the power ratings (btw: (12V-3.5V)/0.12A = 71Ohm; and P= ~1Watt). For now I will go for the Resistor and Pot. if some illumination jitter appears I can switch to the Transistor based current-source Can you please explain me the purpose of the 10k Resistor between 5V and the BC337's collector in the core32 schematic?? Is it really needed? ok, this may not be helpful, but you could always switch to vfds and sell the lcds to some new-schoolers :-) merry x-mas! Dam...indeed VFDs do look old-skool... would make a great design accompanying retro synths however my first MB SEQ will sit in front of a NordModular, a NordModular G2 and a Blofeld... so no need for style yet Quote Link to comment Share on other sites More sharing options...
Futureman Posted December 28, 2010 Report Share Posted December 28, 2010 Sorry to burst your bubble about VFD's.. but with the bunch of VFD's I got, I had to go through them all to find matched pairs. Some were brighter than others. I had the same issue with a few of my SEQ's regarding LCD's (Not all matching). It is fustrating, but as mine were just boring green ebay cheapies, I just bought another & hoped it would match. For the $20 I spent on another screen it offset the arsing around trying to electrically match the ones I already had. Time = money type thing. Best of luck Mike Quote Link to comment Share on other sites More sharing options...
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