# help needed understanding datasheets

Discussion in 'General Electronics Chat' started by ancient, Jan 14, 2013.

1. ### ancient Thread Starter New Member

Jan 14, 2013
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0
Hello all. I'm rebuilding an old project I did in college and I'm trying to understand why I used the resistors I used. The real reason I chose these resistors was because everyone else in the class was using them, but now I want to really learn. Anyway, I'm using port2 pins on the MSP430FG4618/F2012 Experimenter's Board connected to the inputs of 74hc374n flip-flops. The output of the flip-flops are connected to 470 ohm resistors which are then connected to segments on a 7-segment display. I remember that the voltage supplied to flip flops and 7-segs was 3.3V.

I'd like to understand why resistors are needed and how one can determine what values for the resistors are needed based on the data on datasheets.

http://www.ti.com/tool/MSP-EXP430FG4618

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2. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
The resistors are used to limit the current flowing through the LEDs. Without the resistors you are in danger of blowing your LEDs. Basically, you need to drop the voltage to a safe operating voltage of the LED and then you use Ohm's law to calculate the series resistance.

For example, if your source voltage is 5V and you wish to operate your LED at 2.5V @ 5mA,
the series resistance R = (5V - 2.5V)/5mA = 500 ohms.

3. ### ancient Thread Starter New Member

Jan 14, 2013
3
0
Thanks MrChips, I want to understand this a little more. Do these 7-segs need to be connected to Vcc (3.3V from microcontroller)? I just can't remember if that is what common anode means. I believe the 7-seg I'm using is the MAN72. Since the LED is common anode, does that mean I have to look at the forward current and voltage for my calculation?

4. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
Common anode means that the anode has to be connected to the supply voltage. The driver chip output connected to each digit has to go LOW in order for the LED segment to be lit.

LEDs are generally rated to run at about 20mA for full brightness. Highly efficient LEDs can operate with much less current. I like to run around 1 to 5mA. The thing to do is start at a low current and then adjust to suit your brightness preference.

Let us assume the supply voltage is 3.3V and the LED turn on voltage is 2.5V. The series resistor has to drop the difference of 800mV. At 4mA this would call for a 200Ω resistor. You will need seven such resistors for the seven segments.

Try 470, 330, 220 and 100Ω and see which one you prefer.

Instead of using the driver chip, you can temporarily test the LED by grounding the end of the resistor that would normally be connected to the output of the driver chip.

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