# Voltage used up by a transistor and shift registers

Discussion in 'The Projects Forum' started by yash02, Apr 10, 2015.

1. ### yash02 Thread Starter New Member

Nov 19, 2014
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i am on a project 8x8x8 LED cube and I am stuck on the value of register i should use.

I am using 2N5296 transistor for common cathode and 8 shift registers for controlling 64 anodes.The LED I have used are from robocaze.com which are 3mm Blue LED undiffused.

Now the problem arises between shift register and anodes of LED I need to connect resistor and I want to calculate its value.But I dont know that what amount of voltage and current are used up by the transistor and the shift register (74HC595).

If someone can help me figuring it out then I will be very thankful to him.

Transistor 2N5296

Shift Register 74HC595

-YASH LOTAN

2. ### DickCappels Moderator

Aug 21, 2008
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How much LED current will you run through each 2N5296?

3. ### yash02 Thread Starter New Member

Nov 19, 2014
12
0
LED can tolerate a max of 35mA. I wish to run 64 LED array at 20mA i.e., 20mA * 64 = 1.28A which is the current I want from each transistor.

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,980
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Provided one shift register will be driving only one LED at a time, value = (Vdd-Vf-Vsat)/0.02 Ohms, where Vdd is the shift register supply voltage, Vf is the nominal forward voltage of the blue LED and Vsat is the saturation voltage of the 2N5296.

5. ### yash02 Thread Starter New Member

Nov 19, 2014
12
0
The saturation voltage is max 1V so much I should take to calculate resistor value?

6. ### ScottWang Moderator

Aug 23, 2012
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Attach the schematic will be more easy to see the whole picture and to help the discussion.

7. ### yash02 Thread Starter New Member

Nov 19, 2014
12
0

I want to know the value of R1.
Q1 is 2N5296 Shift register output is 5V and 40mA(i.e. from arduino mega 2560 digital pin)

8. ### ScottWang Moderator

Aug 23, 2012
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I still can't see the whole picture only can guess, the output of register can't offering enough current for 8 leds, I_led = 20mA * 8 = 160mA, for the saturation current of Q1 that the hFE should be count as 1/10, and b of Q1 should providing Ib = 160mA/10mA=16mA.
Calculating the values of R1:
Assuming that the output voltage of register is 5V and labeled it as Vq.
I_R1 = (Vq - Vbe)/16mA
I_R1 = (5V - 0.7V)/16mA
I_R1 = 4.3V/16mA
I_R1 = 268.75Ω = 269Ω

269Ω is for the full duty cycle, but the leds display is using scan, so you can using the values less more than that.

9. ### Alec_t AAC Fanatic!

Sep 17, 2013
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A single 5V shift register output can drive only one blue LED; not three in series.

10. ### yash02 Thread Starter New Member

Nov 19, 2014
12
0
corrected schematic

i want to know the value of R1

11. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,980
1,138
You're still showing 3 LEDs in series. That won't work unless your shift register can output > ~9V at 20mA.

12. ### ScottWang Moderator

Aug 23, 2012
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777
Does it like lp stylus jumping?