# Question about experiment "7 segment display"

#### Unregistered

Joined Dec 31, 1969
0
Hi,

I have a question about the experiment titled "7 segment display" on the site (url : http://www.allaboutcircuits.com/vol_6/chpt_7/9.html ). I was curious about the use of 470 Ohms resistors for the display and went on to check its characteristics on the Radio Shack website. It uses 20mA and 1.7V as an input for each led.

My question is: as the output of the 4511 CMOS component, when it is fed 6V, is about 4.84 volts (assuming output voltage raises linearly when input voltage raises); I should only need to put a resistor that lowers the voltage to 1.7 volt, and according to Ohm's law that would be about 160 Ohm. This is quite different from 470 Ohms.

So, can anyone point out what where I am making a mistake? This is very puzzling...

Thanks,

-mat

#### hgmjr

Joined Jan 28, 2005
9,029
What voltage do you intend to use instead of the 6V?

hgmjr

#### beenthere

Joined Apr 20, 2004
15,819
The point here is that the LED's in the 7 segment display are the devices that take 1.7 volts to operate. After they are in conduction, the purpose of the resistor is to limit current through the LED's to a safe level.

The 470 ohm resistors limit current to just over 9 mills/segment.

#### Unregistered

Joined Dec 31, 1969
0
Ok, I understood that. I was just curious about this "470 ohm" when it seems, according to my computations, that 160 ohm resistors would be enough. So, ok! Cheers.