LED not britght enough

[daviddlc]

I'm working in a display project, which has a microcontroller controlling 7 segment displays made out of LEDs.

It has 10 ~ 7 segment displays multiplexed and the brightness on the segments is too low.

I already have changed the multiplexing frequency, and the resistor value on the swithching transistor, but stillwould like to have more brigthness, and I have no option to change to high intensity LEDs due cost $$$$ reasons.

Any suggestion ?




Added after 3 minutes:

I'm using a solid state relay for multiplexing and the transistor to turn on and off the segments.

 

[medra]

hi
i cant see the diagram but i think that the microcontroller cant support current to make the leds bright enough

 

[flatulent]

Take an oscilloscope to the bottom bipolar transistor collector. See if it brought down to nearly 0.3 V above ground. This will let you know if the transistor is capable of drawing current.

Then put the scope probe on the top diode and see how low the voltage goes and write down the number. Then put the scope probe on the B+ and see if it goes down when current is drawn. Finally subtract these last two readings to find the drop across the current setting resistor. You may find that you need a higher B+ value.

 

[ARoque]

It´s not clear the voltage supply, but if you consider tha a Led need 1.2V you need minimum 6V as power supply also to drive Led you need to put an extra transistor PNP in order to have a Darlington pair.

Hope that helps.

 

[BeTZ]

U can Use ULN 2803 (Hi-Voltage and Current, Darlington Pairs), It can buffers MCU pinout.

 

[melc]

It´s not clear the voltage supply, but if you consider tha a Led need 1.2V you need minimum 6V as power supply also to drive Led you need to put an extra transistor PNP in order to have a Darlington pair.

Hope that helps.

1.2V is not enough for a brighting LED. Voltage drop across the LED depends by the LED colour and current flow: about 1.4V red, 1.7V green, yellow etc, to 4V for bright green, white or blue

 

[nguyennam]

Roughly the LED needs 3V for the white LED, 2V for the GREEN/BLUE, and 1.7V for the RED/YELLOW ones. In some big LEDs, it is commonly 2 or 3 LEDs connected in series in a package, so the voltage drop would be double or tripple.

But the most important is the current fed through the LED. For the most LEDs, they can light safely with the current of up to 20mA. For the normal light, the odinary LED needs about 10...15mA, the super bright needs about 2...5mA.

For your case, you need to detach your LED chain and connect it to the DC power supply, no multiplex, and adjust the resistor in series with LEDs to get the necessary current basing the desired bright level. Then, reconnect the LED chain into your circuit.

I do not know which microcontroller you are using, and what is the voltage of the LED circuit, but for the Microchip PIC, a output with 20/25mA of source/sink capability is good enough for driving LEDs directly for the good results. And if you use 5V to drive the LEDs, you may connect maximum only 2 LEDs in series depending the type of LED.

nguyennam

 

[papu]

see the stamp micro controller book

 

[mondunno]

hi, why hav u connected all leds in series?

whats the vcc? 12V? better think of designing using some standards. just reply with the resistance value and the transistor used.

the flicker rate? tel that too.

mon

 

[cameo_2007]

increase the Vcc or connect those in parallel n boost the supply current,provided,within the limits of transistor.
thats it

 

[bigaussie]

May be microcontroller can drive transistor bases of value which you are assuming

 

[Mike K8LH]

Please remember that if you need 10-ma to light a column of LEDs to full brightness at 100% duty cycle then you will need approximately 80-ma to light it to full brightness at a 12.5% duty cycle when it's part of an 8 digit multiplexed display. This can easily translate to 140-ma or 160-ma per digit when lighting all 7 or 8 segments on a single digit so make sure your power supply can handle it.

Your high-side switching relays could easily take 1 or 2 msecs to close and this may be reducing the duty cycle considerably. Consider using a medium current P-FET or PNP high side switches for each digit.

Make sure your sinking drivers can handle the higher "pulsed" current required at low duty cycles.

Kind regards, Mike

 

[daviddlc]

Thanks to everybody, sorry I missed the voltage is 12 volts.

What I have done is to replace the relay by a PNP transistor.

Originally I was multiplexing 10 segments one by one. Now what I have done is to multiplex by 5 and using 2 rows.

These changes improved the brightness without adding additional components, just the replacement of the relay by a couple of transistors.

 

[jhbbunch]

You might want to buffer the microcontroller output with something like an HC or ACT variation of a 74374 latch. They can source and sink 25 to 35 mA directly, and they don't tie up seven or eight lines of the micro. With a 74138 decoder you can control an awful lot of 374 latches with 8 data lines and just 3 (or up to 6) output lines for control of the latches. Also, you can wire the latches up for serial input. (D1 is input, you already have a clock pin, then tie Q0 -> D1, Q1 -> D2 ... Q7 ->D8. That's 64 pipns of output using 5 pins on the micro. For an extra 3 pins on the micro you get 512 pins of output.