[SOLVED] problems with TIP 122 as switching transistor.

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boiler95

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Hi,

I'm trying to control a LED panel using a PIC micro-controller PIC16F72. I have configured RB4 on PORTB as O/P. Please find the circuit diagram attached.
My LED panel consists of approx 30 Rows of led, all rows in parallel.
Each row has 3 LED in series
Only 1 row has 2 LED's

When i send a high on the output port all the LED glows. When i send a low on the O/p Port all are off except the row with 2 LEDs which glow slightly. When i measure voltage across the panel it measure ~5.4 V.


These LEDs have a Fwd voltage of around 3.2-3.4V and Forward current of 20mA.

I don't want those LEDs to Glow and need all LEDs to be completely off when the o/p of the micro-controller is low.

can someone please let me know.

Thanks

 

Hi,

And if you put an extra led, of the same type, on to that column with 2 leds to make 3 leds, it works ok ?
 

I assume you also have current limiting resistors in each chain of LEDS.
Try connecting a resistor (~ 15K) between the base and emitter. The TIP122 is a darlington with quite high leakage current which could be enough to light the LEDS.

Brian.
 
betwixt said:
I assume you also have current limiting resistors in each chain of LEDS.
Try connecting a resistor (~ 15K) between the base and emitter. The TIP122 is a darlington with quite high leakage current which could be enough to light the LEDS.

Brian.
Click to expand...

Thank you, that worked !!
 

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