7 Segment Display LED Specs?

[CrazyChris]

Hello,

 

I need the voltage and current, of a 7 segment display LED,

used in the following circuit. Power source is a 9V battery.

 

[Stephen Moss]

Well the logic devices are CMOS so will working with a power supply voltage from 5 to 18V.

When you say the power source is a 9V battery, is that what you want to run if from or what it is running from? If the latter, then unless there is some form of voltage regulator being used that is not shown on the schematic you provided then it must be running off 9V. Personally I would use a voltage regulator otherwise as the battery voltage drops so does the display brightness.

 

As for the current that is a little more difficult, The typical forward voltage drop of a standard LED is 1.8V so 9-1.8 = 7.2V / 100R = 72mA * 2 = 150mA when all segments on both displays are illuminated.

In theory that works out to about 10mA per segment which is the typical current consumption of a standard LED, peak is 80mA but only when strobed quickly.

 

I personally do not like putting one current limiting resistor in the display Com as opposed to one for each LED as the brightness should vary depending on the number of segments that are illuminated, i.e. two segments to display the number 1 in theory would take 36mA each were as all segments to display a number 8 take 10mA each.

It is possible that in the above example of displaying a number 1 the circuit designer is relying on the current limits of the device output to result in an output voltage drop that keeps the apparent brightness approximately level for all digits but that puts unnecessary strain on the device output pins and may result in premature failure of the device.

 

So if you are looking to purchase a 7 segment LED to use in such a circuit the specification I suggest looking out for are...

Forward Voltage (Vf) 1.8-2.2 (typical value)

Forward Current (If) 10mA (often specified as "typical" and/or "test" current in the data sheet)

Peak or Maximum Current: 60mA minimum but ideally 80-100mA if you can get it

Type: Common Cathode

[CrazyChris]

Yes, the circuit is being run off a 9V battery. And I did notice the 1 resistor for the LED.

 

Here's the webpage:

MB-09 2 Digits Counter Up Hobby Kit

 

They would charge me $50 for shipping.

 

Thank you for your help!!!

[CrazyChris]

Here's a video:

https://youtu.be/g6JxGd5kQas