Can you explain exactly why PWM cant be used? I didn't understand that bit, how many strings are using?, many is a strange number
Ritesh wants to connect 4 red LEDs in series which makes one string then connect many of these strings in parallel. He thinks the total will be 8.8V because each red LED has a typical forward voltage of 2.2V. But he doesn't know that each LED has a DIFFERENT forward voltage from about 1.8V to about 2.4V.
If he does not test each LED and find four that total 8.8V and do that over and over then he will have some strings of (1.8V x 4=) 7.2V that will quickly burn out and he will have some strings of (2.4V x 4=) 9.6V that will not light.
He says he "wants to control the current in the transistor". He tried a little BC337 but found it overheating (of course, because it was a linear heater, not a switch and nothing limited the current).
Ritesh and members here trying to help are completely confused about calculating base current. They do not understand that hFE is a variable number that is high for some transistors and is low for other transistors (even when they have the same part number) and is used when the transistor is an amplifier with plenty of collector-emitter voltage. hFE also changes when the temperature changes. Because it is a variable number then the circuit usually has some negative feedback to reduce the amount of difference between different transistors. His circuit used no negative feedback.
When a transistor is used as a saturated switch then it is turned on as hard as it can get so the base current is increased to 1/10th the collector current. hFE does not work at such a low collector-emitter voltage (maybe 0.1V or less). Something (a series resistor) must be used to limit the current.
In post #27 Ritesh explains an experiment but did not show a schematic and did not say its supply voltage. He used a base current of 2.1mA and we hope he connected the polarity correctly for the collector and emitter. He wrongly thinks that the transistor will limit the current to 10 times the base current because he does not understand that it is a SWITCH. Luckily he used a series current-limiting resistor with the LEDs.
He used one white LED (maybe it has a forward voltage of 3.2V), a 110 ohm resistor and got 83mA which is enough to burn out an LED. Then the supply was probably (110 ohms x 83mA=) 9.13V + 3.2V for the LED + 0.1V for the transistor= 12.43V.
He used two white LEDs (maybe they have a forward voltage of 3.3V each), the 110 ohm resistor and transistor and got 51mA. Then (110 ohms x 51mA=) 5.61V + (3.3V x 2=) 6.6V + 0.1V= 12.31V.
He use three white LEDs (3.2V each?), the 110 ohm resistor and transistor and got 24mA. (110 ohms x 24mA=) 2.64V + (3.2V x 3=) 9.6V + 0.1V= 12.34V.
You can work it out with the resistor at 55 ohms if you want. Of course its low value limited the currents to be much higher.
His experiment shows that the transistor is turned on hard as a switch but it is the resistance that controls the current.
Ritesh does not explain what he is doing with "20 characters controlled individually". He does not say what the PIC does.
In post #27 Ritesh explains an experiment but did not show a schematic and did not say its supply voltage. He used a base current of 2.1mA and we hope he connected the polarity correctly for the collector and emitter. He wrongly thinks that the transistor will limit the current to 10 times the base current because he does not understand that it is a SWITCH. Luckily he used a series current-limiting resistor with the LEDs.
He used one white LED (maybe it has a forward voltage of 3.2V), a 110 ohm resistor and got 83mA which is enough to burn out an LED. Then the supply was probably (110 ohms x 83mA=) 9.13V + 3.2V for the LED + 0.1V for the transistor= 12.43V.
1st off you should be multiplexing our LEDs so you only need one pic. Start here;In led display there are more than 20 characters each is to be controlled individually so, 4 PIc16f676 will be used taht why...!
The LED used is Oval which has high brightness than normal ...work on 3mA to ~150mA for good brightness i have noted the current by testing.
No, it work also at 150mA ..EDIT: If the current-limiting resistor is 55 ohms then the current in the LEDs is about 42mA then don't they burn out?
its not scrolling just flashing...1st off you should be multiplexing our LEDs so you only need one pic. Start here;
yes may be same for good brightness..So the LED's draw a max of 150 ma then i guess. Is that for each string then too?
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