# Resistor value

Discussion in 'LEDs and Optoelectronics' started by 01sporty, Aug 19, 2018.

1. ### 01sporty

16
1
May 1, 2012
I'm trying to decide on a resistor value to use with some cpc1017n opto relays.
The datasheet is here: http://www.ixysic.com/home/pdfs.nsf/www/CPC1017N.pdf/\$file/CPC1017N.pdf
It specs it at 50ma but I don't see a voltage.
Some eBay listings give the voltage as 1.4 but that seems a bit out of the norm to me.
I'm using 12v for the input and if I use the 50ma and the 1.4v that gives me a 220 ohm, 1 watt resistor.
Does that sound like a safe bet?

2. ### Minder

3,006
640
Apr 24, 2015
The 50ma is the absolute maximum, I would normally go with around 25ma to play it safe and this should fall within the operating range.
Using 12v for the operating voltage would need a 390 to 475 ohm resistor.
M.

3. ### 01sporty

16
1
May 1, 2012
Thanks Minder,

So how does a person go about picking the right values out of the spec sheet?
I did finally find the input voltage drop on the spec sheet: .9 min, 1.2 typ and 1.5 max. That's pretty cut and dry.

I don't get the 'Input Control Current to Activate' at all. First the sheet lists 100mA which sound insanely high but then goes on to list 0.4mA typ. and 1mA max. Pretty wide variances there.

4. ### Minder

3,006
640
Apr 24, 2015
I see this
Absolute max 50ma.?
M.

5. ### 01sporty

16
1
May 1, 2012
My Snippit tool decided to take a vacation so I copied this from the Electrical Characteristics Table:

Input Characteristics
Input Control Current to Activate 3 IL=100mA IF - 0.4 1 mA
Input Control Current to Deactivate - IF 0.3 - - mA
Input Voltage Drop IF=5mA VF 0.9 1.2 1.5 V
Reverse Input Current VR=5V IR - - 10 A
Common Characteristics
Capacitance, Input to Output VIO=0V, f=1MHz CIO - 1 - pF
1 Load

This whole deal got started because I had made a circuit very similar once before. I had some of the left over relays in a bag and had marked 1k2 resistor on the bag. I started to solder those resistors into my new circuit and decided to double check that value. When I plugged in 12v input, 50mA and 1.5v I got a very different answer.

The old circuit worked fine but for it to be a 1k2 resistor I must have used around 10mA. No idea where that might have come from.

Thanks again for your help.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
2,780
Jan 21, 2010
The datasheet says that this device is rated for a continuous load current of 100mA, but allowed a peak of 350mA for 10ms.

For a load current of 100mA, the required input current is 0.4 to 1 mA.

On the next line they give the typical forward voltage of the input at 5 mA.

Based on all of that, and a 12V supply, even a series resistor of 10k would work. It sounds like 5mA is a reasonable input current unless you have some special requirements, but anything up to 25mA would not raise my eyebrows.

So a resistor between 470Ω and 10kΩ is workable, but I probably wouldn't make it much higher than 2k2 unless (again) there were some special requirements.

Edit: fixed output current

Last edited: Aug 20, 2018
7. ### Minder

3,006
640
Apr 24, 2015
I assumed it was the Opto diode resistor the OP needed!???
M.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
2,780
Jan 21, 2010
That's what I was giving him.

9. ### 01sporty

16
1
May 1, 2012
Correct. I had no idea the values could be so broad.

I wish the spec sheets would make it a bit simpler for us non-electrical engineers. The first thing any led needs is that resistor. How hard would it be to put an ideal forward current & voltage right at the top for those of us that have no idea what most of the stuff on that sheet is. I suspect even the engineers ignore most of it.

10. ### Minder

3,006
640
Apr 24, 2015
In a a case like this I have always used the values such as I suggested 390 to 475 ohm.
Keep it somewhat under the 50ma.
M.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
2,780
Jan 21, 2010
Their job is to give people a lot of information, not simple "acceptable" use cases.

However, with a small amount of knowledge you can extract enough information to use the device as long as you don't need to get close to the edge cases.

I recommend looking at maximum continuous values and not exceeding half of these. Where there are minimum values keep a significant margin above these too.

If there are suggested values, consider using them. You will notice that in several places, values are specified for If of 5mA. This is a really good indication that this value is probably generally suitable.

12. ### hevans1944Hop - AC8NS

4,535
2,112
Jun 21, 2012
Spec sheets for electronic parts are intended to be read and understood by electrical engineers.

The only thing any led needs is appropriate forward operating current. The forward operating voltage will be supplied by the designer's circuit and may include an appropriate current-limiting resistor or some other means of setting the forward operating current. It is the "some other means" that requires engineering design.

ratstar likes this.
13. ### BobK

7,682
1,686
Jan 5, 2010
I guess you missed this, in the very first table of the datasheet:

LED Current to operate 1 mA

Or this line in the features section right underneath:

• Only 1mA of LED Current Required to Operate

B0b

14. ### 01sporty

16
1
May 1, 2012
Done.

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