Ace, sorry, yes, you're right... I thought you had the other end to V+. However, you don't have enough resistance if your backlight is drawing ~677 mA, as you say it is, and if that is the case, it won't last long. I'd try Sarma's suggestion, but make sure to get the current down.
I'm not so sure that an unregulated supply for the backlight is a good idea. How stable is the unregulated source? Fluctuations in the voltage will cause changes in the brightness of the backlight.
also, what does the datasheet say about the backlight current? Given that you didn't quote that, I would guess that you don't have a current limiting resistor for the BL. Hint, an LED will draw as much current as you can give it. The spec'd current is a limit that you have to enforce.
So, there are two issues to deal with but first and foremost, get your BL current under control. Then see if you need to deal with the vreg at all.
You could also consider using a separate regulator like the LM317 in constant current mode for the blacklight.
I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.
Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.
1. Infact i use 16*2 modules with back lit and we provide input supply 12V directly fpr the backlit with an external series resistor limiting the current to manufacturers spec. in this case we may use 78l05 for the processor itself. it is a nominal input voltage that matters- it can be around 9 v Dc depending on the ac main window applicable to the country and the quality of the transformer used for the adapter.
i also use the same arrangement on 5V regulated DC (7805 and not the L05) with a 220 ohm series reistor . the 7805 will of course have a heat sink made of aluminum metal sheet.
Sarma
Last edited by mvs sarma; 28th August 2007 at 10:37 AM.
you did not eaborate the other loads on this 7805 where lcd with backlit is a partOriginally Posted by AceOfHearts
what is the load currentof the regulator under concern without LCD and with LCD .please try to describe the scenario more detailed,
Sarma
Regards,
Sarma.
Thanks for this explenation, much appreciated. However, the actual point I wanted clarification on remains unclear. I know what the numbers 11.06-5=6.06 represent.
The question is, why do we use the 'excess voltage', if I can use this term to find the power dissipated? Why cant we just use 11.06 and multiply that with 0.7?
mvs sarma,
I did say on a later post that with the backlight off, current output was 23mA. I would immagine this is all that would matter.
But anyway, the other device connected to this 7805 is an 8051 microcontroller.
peace
PS.
I have installed a clip-on heatsink which is working rather nicely. The fins of the heatsink are not heating up too much indicating adequate heat-loss to the surrounding.
The backlight looks just right when connected to +5V and GND directly. Does the LCD not have a current limiting resistor built-in?
Thanks to all for all the help. I hope you understand I cannot reply to every single response/suggestions individually due to shortage of time, but I have taken all the advise onboard.
Thanks again.
Usually not! - because it depends on the voltage you're feeding it from. As suggested previously in this thread, measure the current the backlight is taking, if it's too high you MUST add a series resistor, otherwise it will blow fairly quickly.
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk
11.06V x 0.7A is the total power dissipated by the regulator plus dissipated by the LEDs in the display.
5.0V x 0.7A is dissipated by the LEDs and 6.06V x 0.7A is dissipated by the regulator.
Uncle $crooge
I came across LCD displays 16*1 and 16*2 with on board smd resistors on the led circutry and solderable loops for controling the current. Instead we used to prefer external one for the safty purpose only.
Regards,
Sarma.
Thanks i got your message in earlier posts. cool working of LM7805 alone is not sufficient perhaps. LCD back lit alone taking so much current is rather unhealthy for the device's life.
we have too to reduce it to around 100 to 120 mA toteal load with back lit.
In cidentally which is make and model no of the LCD that is claing so much backlit crt-- according to few spcs i went thro, even the blue color backlit has a limit at 220mA.
try by a series resistor on the LED+ line with 125mA approximate and see whether the illumination( for an yellow-green color) is nominally sufficient. at this current the LCD may live longer. No pot is needed once the value of the resistor is known.
they are taking voltage drop across the LEDs as 4.4V or so, i may interpret that this value is with 2 Yellow LEDs in seires and 4 or 5 such combinations in parallel. thus we may calculate
(5-(say)4)/.12mA= 11hm:
please try a 11 or 12 ohms 0.5 watts resistor and things would be safe.
Last edited by mvs sarma; 30th August 2007 at 03:34 PM.
Regards,
Sarma.
Older LCD (LED) backlights can draw quite a bit of power. Newer LCDs with HE LEDs draw much less current 65ma for a 2x16 blue / white.
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