Typical CTR value of Optocoupler  - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: August 08, 2020, 08:39:10 am
content: Hi guys what does it mean by Typical CTR value of Optocoupler.

author: ataradov
author_rank: Super Contributor
timestamp: August 08, 2020, 08:45:13 am
content: CTR is a current transfer ratio. Both sides of the optocoupler are current driven devices. So on the output (transistor) you get 30-90% of the LED current.In most cases it does not matter as optocouplers are mostly used as digital devices. But it is possible to pass analog signals and then it starts to matter and you need to compensate for the undefined loss.Typically compensation is done by amplifying the signal on the other side and passing it back to the first side though another optocoupler. This way you have have a loop and you can drive the input current to achieve predictable output current. There are integrated devices that do this, but they are way more expensive compared to regular jelly bean parts.

author: khatus
author_rank: Regular Contributor
timestamp: August 08, 2020, 08:56:30 am
content: What is meant by the word typical here?

author: ataradov
author_rank: Super Contributor
timestamp: August 08, 2020, 09:00:26 am
content: Have you seen any datasheet for any device? Manufactures often specify minimum, typical and maximum value. Typical means that if you take one optocoupler from a batch, it most likely would have CRT = 60 %. But with smaller probability it may be 30% or 90%. Or anything in between. The distribution it assumed to be Gaussian in most cases, but may not be.The min and max values are probably strict, since devices with lower and higher CTR would be rejected at the factory.

author: nfmax
author_rank: Super Contributor
timestamp: August 08, 2020, 09:44:44 am
content: Minimum&maximumfigures on a datasheet (usually shown inbold) are based on test limits(*), that every device manufactured must pass. Normally, the test limits are slightly 'stricter' than the datasheet values, to allow for measurement uncertainty in the test equipment and its calibration.Typicalvalues are generally measures of the central tendency of the distribution ofpassingtest results: normally the mean, but sometimes the median value or mode will be used - although this it not normally stated on the datasheet.So, if you measure the CTR of a large number of optocouplers (legit ones, not test rejects recycled through the back alleyways of Shenzhen), and average the values you measure, the result should be close to the datasheettypicalvalue. The more devices you measure, the closer it should get. Another way to look at it is that the if you pick one optocoupler at random, there is an approximately 50% chance its CTR will exceed thetypicalvalue.(*) Except where it says 'guaranteed by design' or similar wording, against some parameter

author: Andreas
author_rank: Super Contributor
timestamp: August 08, 2020, 10:32:03 am
content: Quote from: khatus on August 08, 2020, 08:56:30 amread the datasheet. The conditions for typical values are usually hidden in a headline or footnote or a test cirquit.usually it is for Optocouplers:- 25 deg C operating temperature- a certain input current for the led lamp- a certain load resistorand it is always a mean value of a larger distribution.You should design for the min and max values as the "typical values" are only given for information.with best regardsAndreas