I am wondering what value of current limiting resistors to use given a 5 volt supply for:

1) LDD-C812RI
https://www.alldatasheet.es/datasheet-pdf/view/1088955/LUMEX/LDD-C812RI.html

2) HDSP-F503
https://docs.broadcom.com/doc/AV02-2556EN

Thanks

 

It depends on how bright you want them.
R=(Vsupply-ForwardVoltage)/DriveCurrent.

The datasheet gives the Forward Voltage and the maximum drive current.

 

It depends on how bright you want them.
R=(Vsupply-ForwardVoltage)/DriveCurrent.

The datasheet gives the Forward Voltage and the maximum drive current.

The point is what values on these sheets does one use?

 

The point is what values on these sheets does one use?

FORWARD VOLTAGE is listed on both datasheets.
The maximum current is called DC FORWARD CURRENT on one and STEADY CURRENT on the other, it refers to continuous current.

 

FORWARD VOLTAGE is listed on both datasheets.
The maximum current is called DC FORWARD CURRENT on one and STEADY CURRENT on the other, it refers to continuous current.

In the case of STEADY CURRENT it specifies per chip. So does that mean that IF = 25 mA / 8 = 3.125 mA per segment or DP?
That seems like a very low value for a 0.8 inch display with 2 digits in 1?

 

In the case of STEADY CURRENT it specifies per chip. So does that mean that IF = 25 mA / 8 = 3.125 mA per segment or DP?
That seems like a very low value for a 0.8 inch display?

There will be a chip in each segment, so that is the value per segment.
(With modern LEDs you might find that 3mA is plenty bright enough. LEDs have improved dramatically since the times when everyone ran them at 20mA.)

 

There will be a chip in each segment, so that is the value per segment.
(With modern LEDs you might find that 3mA is plenty bright enough. LEDs have improved dramatically since the times when everyone ran them at 20mA.)

OK. thanks!

 

since the segments are controlled independently, combined current is large specially when powering several digits.
so good idea is to try them out if possible and start with some conservative value like 5mA per segment.
you may want to use slightly higher current if digits are multiplexed.

 

example segments drawing 10mA, have a Vf of 2.05V. using 5V supply.
(5V-2.05V)/0.010A = 295 Ohm. today one can pick 300 Ohm value. in the past one would pick nearest one such as 270 or 330 Ohm.

same display using 5mA per segment may have a Vf of about 1.95V
(5V-1.95V)/0.005A = 610 Ohm. this nearest common value is 620 Ohm

suppose you are multiplexing displays and they are only on 25% of time. and this may need to be readable in daylight. so you try to pick higher current such as 35mA. this is still safe value even if multiplexing fails and one display is operating at 100% duty cycle. at this current Vf is about 2.2V so

(5V - 2.2V)/0.035A = 80 Ohm. Nearest common value is 82 Ohm.

 

I seem to have got rather blasé about LED current limiting resistors. They get 330Ω. 330Ω on a 3.3V supply, 330Ω on a 5V supply, 330Ω for a red LED, 330Ω for a green LED, 330Ω for individual LEDs, 330Ω for 7-segment displays and they always seem to be about right.

 

example segments drawing 10mA, have a Vf of 2.05V. using 5V supply.
(5V-2.05V)/0.010A = 295 Ohm. today one can pick 300 Ohm value. in the past one would pick nearest one such as 270 or 330 Ohm.

same display using 5mA per segment may have a Vf of about 1.95V
(5V-1.95V)/0.005A = 610 Ohm. this nearest common value is 620 Ohm

suppose you are multiplexing displays and they are only on 25% of time. and this may need to be readable in daylight. so you try to pick higher current such as 35mA. this is still safe value even if multiplexing fails and one display is operating at 100% duty cycle. at this current Vf is about 2.2V so

(5V - 2.2V)/0.035A = 80 Ohm. Nearest common value is 82 Ohm.

View attachment 353707

Thanks!