# How to properly read an optocoupler datasheet?

#### ti_chris

Joined May 22, 2018
5
Hi,

I've been looking at a few opto-coupler datasheet and I've really been struggling with some maker's way of giving me the data. Case in point, let's take the 4n37 (a pretty popular one).

I'm looking at Vishay's datasheet (http://www.vishay.com/docs/81181/4n35.pdf) and they claim that it has a CTR of 100% on the first page. But then if I scroll down to look at the graph of If vs CTR, I see in figure 5 that generally, the CTR won't be > 1.0 (100%) which seems to contradict what I read on the very first page.

What does make sense to me is a graph as it provided by Toshiba for the same part (http://pdf1.alldatasheet.com/datasheet-pdf/view/30834/TOSHIBA/4N37.html). In that diagram, (at-least for 10v) I do see that the CTR ranges from slightly less than 100% to 300%.

I would be surprised if these parts differed that much in spec that I'm assuming that I'm reading these wrong. What's even more puzzling to me is that the freescale part seems to suggest that the opto is actually significantly below 100% in figure 2 (http://pdf1.alldatasheet.com/datasheet-pdf/view/2850/MOTOROLA/4N37.html).

Any help in properly reading these datasheet would be appreciated,
Thanks

#### Ian Rogers

Joined Dec 12, 2012
817

#### OBW0549

Joined Mar 2, 2015
3,566
But then if I scroll down to look at the graph of If vs CTR, I see in figure 5 that generally, the CTR won't be > 1.0 (100%) which seems to contradict what I read on the very first page.
I think you may be misinterpreting that graph:

That graph isn't plotting CTR vs. If. Rather, it's plotting normalized CTR-- that is, CTR at the stated condition (Ta = 85 °C) relative to the CTR at Vce = 10V, If = 10 mA and Ta = 25 °C.

I think that's where the confusion lies.

#### ti_chris

Joined May 22, 2018
5
I think you may be misinterpreting that graph:

View attachment 181389

That graph isn't plotting CTR vs. If. Rather, it's plotting normalized CTR-- that is, CTR at the stated condition (Ta = 85 °C) relative to the CTR at Vce = 10V, If = 10 mA and Ta = 25 °C.

I think that's where the confusion lies.
Ah yes you are correct. Thanks. I'm looking at the wrong figure here. I see that I completely glanced over the "CTR" section on page 3. I'm interested in getting the CTR under 'normal' condition (aka: what they normalized it to). That seems to be If = 10mA, 25°C, Vf = 1V. It reads a minimum of 100% with no typical or max. Am I then correct in understanding that it's 100% as a baseline.

I understand that I would have to take temperature, forward current, voltage and age into account ontop of the baseline.

#### OBW0549

Joined Mar 2, 2015
3,566
I understand that I would have to take temperature, forward current, voltage and age into account ontop of the baseline.
That's the way I see it, too, although I don't know what an appropriate derating factor for aging would be. Fifty percent, perhaps?