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Driving LEDs using a hex buffer

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shrtCrct

New Member
Hi,

I am trying to use a SN74LS17 family hex buffer/driver to drive 5 LEds One LED per output). My problem is that the output from each out pin from the device is at 15V and my LEDs (digikey part 160-1579-1) are rated at 5V with 20mA forward current, 100mA Peak forward current.

My question is that that can I just use a resistor to make sure it just provides the right current. I calculated R to be 715 Ohm (based on 20mA,15V). Is this correct?

I'll appreciate any help in this. Thanks in advance.
 

Diver300

Well-Known Member
Most Helpful Member
The SN74LS17 is an odd choice as it is obsolete.

The output is Open Collector which means that no current can come out of the output. Current can only go into the output.

You are right that you need current limiting resistors. You always need to control the current with LEDs.

The LED voltage is 3.3 V. The 5V is the maximum reverse voltage. The LEDs only light when there is forward current and voltage. No current passes when the voltage is reversed.

15V is the maximum output voltage for the SN74LS17. You can use any voltage up to 15V. 5V would seem more obvious because you must have a 5V supply to run the SN74LS17.

The resistor you need is calculated with:-

(Supply voltage - LED voltage)/LED current so that would be

(15 - 3.3)/0.02 = 585 :eek:hm: for a 15 V supply

or

(5 - 3.3)/0.02 = 85 :eek:hm: for a 5 V supply.
 

Torben

Well-Known Member
Hi shrt,

Not correct, but close enough that it would work. How did you get 715 Ohms? At first I thought you just used Ohm's Law (V=IR => 15 = 0.02 * R => 15 / 0.02 = R) but that gives 750, not 715. Anyway, the calculation you want is:

Rled = (Vsource - Vled) / Iled

. . .so:

Rled = (15 - 5) / 0.02
Rled = 500 Ohms

Your answer would have worked fine. Your LEDs would have been just a hair dimmer, is all (they'd have been limited to ~14mA instead of 20mA).

For more information on LEDs and these calculations you can check out https://www.kpsec.freeuk.com/components/led.htm if you like.


Good luck!

Torben

[Edit: Long work day and too much coffee. I jittered happily right on past the 5V LED voltage thing. Diver300 is right. :) ]
 

audioguru

Well-Known Member
Most Helpful Member
I have some 3.5V blue LEDs that have a forward voltage of almost 5V when 100mA flows through them. Their max continuous current is 30mA but I blink them very brightly with 90mA.
 

shrtCrct

New Member
Torben said:
Hi shrt,

Not correct, but close enough that it would work. How did you get 715 Ohms? At first I thought you just used Ohm's Law (V=IR => 15 = 0.02 * R => 15 / 0.02 = R) but that gives 750, not 715.

Hi Torben,

I used the R = (15 - 0.7)/.02 to get 715. I used 0.7 as the typical voltage drop across a diode/led.

Appreciate your reply.
 

Torben

Well-Known Member
shrtCrct said:
Hi Torben,

I used the R = (15 - 0.7)/.02 to get 715. I used 0.7 as the typical voltage drop across a diode/led.

Appreciate your reply.

Ah, that explains it. 0.7 is the typical drop over silicon diodes (germanium down to 0.2; other types vary). LEDs start around 1.5V-1.7V for IR LEDs and go up from there, depending on colour and composition. There's a handy chart for typical values here: https://www.oksolar.com/led/led_color_chart.htm


Cheers!

Torben
 

MrDEB

Well-Known Member
LED calculator

I may have the wrong figures
15v supply, 4v forward voltage, 100ma max current?
comes out at 120 ohms 1.2watt resistor
 

Torben

Well-Known Member
MrDEB said:
I may have the wrong figures
15v supply, 4v forward voltage, 100ma max current?
comes out at 120 ohms 1.2watt resistor

Yep. Actually 110 Ohms but if you have 10% resistors then the next standard value is 120 Ohms, so you'd use that.


Torben
 

Hero999

Banned
I haven't looked at the datasheet, what's the saturation voltage of the output transistors? Perhaps you need to factor this in to your calculations.
 

audioguru

Well-Known Member
Most Helpful Member
The obsolete driver IC has a max allowed output current of 40mA per output. It blows up with 100mA.

The 15V supply is so high that the max saturation voltage of 0.7V at 40mA and 0.4V at 16mA doesn't matter.
 
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