Selection of CTR of opto isolator

Hi,
I just want to isolate the 500 Hz digital signal ( High 24V, Low 0V) , Hence i want to use the PC817. But in the datasheet i found a term called "CTR" and they have given some Rank marks for them.

I will connect the isolater's LED in series with 1.5K resistor and the given input signal voltage toggles between 0V and 24V.
Signal( 24V or 0V ) - 1.5K - Opto LED - Gnd.

Output side of PC817:
5V - 10K - Collector; Emitter - Gnd. The output will be taken from the collector of PC817.

What value of CTR( RanK ) i need to choose and how does the CTR value selection affects the circuit design?

CTR is current-transfer-ratio; basically gain. For your case, I think you've got a lot more current flowing in the LED(14mA) than you really need. You only need 5/10K=0.5mA in the collector, so it looks like any of the "ranks" would work.

Actually what is the use of CTR. How does an optocoupler having 50% CTR is differ from other having 600%.

If you look at the datasheet, you will see that the CTR will vary with temperature, and to some extent, on applied voltage at the collector output.
It is up to the designer (you!) to choose the components in the circuit so that the current requirements are met across the entire temperature range that you expect to use the design. If it is only going to be used in your home workbench (25C), that is fine - but if it must also work at the arctic circle or in a freezer at -30C that changes the requirements as you can tell by the graphs. The CTR is basically a ratio to describe the efficiency of coupling between the input current and the output current of the device. 600% is alot more efficient than 50%. You can think of it as current gain.

For example, a device rated for 1mA input current and a CTR of 50% means that an input current (diode side) of 1mA may produce an output current on the collector/output side of 0.5mA (assuming that the voltage and resistor connections on that output side will support that). A device with the same current rating but with a CTR of 600% means that the input current of 1ma could support 6mA of output current on the collector side, again if the voltage and components on that side allow for it. If the output is connected to VCC = 5VDC and has 1K resistor pullup, it will only support/provide 5mA.
So, in this example, the 50% CTR device with 1mA going in would have 0.5mA output, but the 600% device would have 5mA.

I have designed like this..
Input side:
24V-4.7K-LED of PC817-Gnd

Output side:
5V-1K-Collector of PC817; Emitter of PC817 - Gnd
Output taken from collector..

I have tried with PC 817 Rank A and PC817 Rank D.

Both are producing same output voltage and Same current. Then how does the Rank( CTR ) play here...

Hi,

With 24V and 4k7 you may expect about 5mA if LED current.

With 5V and 1k gives about 5mA.

So you need a CTR of 100% or more for the output to saturate.
CTR of a new optocoupler will be well within specified values.
But CTR may degrade with time and temperature. Additionally CTR will vary from piece to piece.
So if you operate an optocoupler at it's limit you shouldn't be surprised if it fails in the field.
So please add some headroom to ensure proper operation.

Example.
If your weight is 70kg and you want to go climbing.
Now you have a rope that is specified to break with 50...200kg.
You test a new one of this rope at home and it carries your weight.
Would you use this rope for climbing?
Can you be sure the rope carries your weight after years of use?
Would you use another rope of same type for climbing without testing it?

I personally would answer three times "No".

From my experience with optocouplers.
I build a circuit and it failed after about 5 years of use 7/24, 50% duty cycle. Driven within specified limits.
The CTR degraded to about 30% of the specified value within this time.

Since this time I add a lot more headroom with my optocoupler circuits.

Klaus

Murugesh_89 said:

I have designed like this..
Input side:
24V-4.7K-LED of PC817-Gnd

Output side:
5V-1K-Collector of PC817; Emitter of PC817 - Gnd
Output taken from collector..

I have tried with PC 817 Rank A and PC817 Rank D.

Both are producing same output voltage and Same current. Then how does the Rank( CTR ) play here...

Click to expand...

Once your transistor saturates, CTR is irrelevant. That's kind of the definition of saturation: the output stays the same as the input rises.the CTR defines your minimum requirements.