7seg display transistor and resistor values.

Discussion in 'The Projects Forum' started by evol_w10lv, May 1, 2015.

1. evol_w10lv Thread Starter New Member

Feb 19, 2013
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0
I want to connect PIC16F690 (http://ww1.microchip.com/downloads/en/DeviceDoc/41262C.pdf) with 7seg display. Have got this model - datasheet: http://www.datasheets360.com/pdf/-3210061775093412092
Forward current-Max: If = 0.025 A
Forward voltage-Typ: Vf = 2 V
R = (5-2)/0.0250 = 120 Ohms (150 Ohms, if If = 0.02 A)
Resistor wattage:
P = I * V = 0.02 * 3 = 0.06 Watts
Is this correct calculation for current limiting resistor value for anodes of the display?

I'm going to multiplex these segmets (only two of three digits will be used). Idea is to use two NPN transistors to sink current from each of the two common cathodes.
8 * 25 = 200 mA current for the collector. It means I need ~20 mA base current, but maximum output current (any I/O pin of PIC) is 0.025 A. So can I use any NPN transistors which are rated for 200mA collector current?

2. ScottWang Moderator

Aug 23, 2012
4,925
777
Attach the circuit will help.

3. dl324 Distinguished Member

Mar 30, 2015
3,377
651
Hi,

Beta decreases at higher current so you could max out the load on the PIC. You could use a darlington and change your resistor value to account for an extra diode drop.

Since you're going to be multiplexing, you should consider increasing LED current to account for a 50% duty cycle. Just stay below Imax and max average power dissipation for the displays.