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Optocoupler: CTR and REL_CTR; Use

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Akanimo

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Hi everyone,

I am studying optocoupler for first-time use. I understand the CTR parameter but not the Relative CTR.
Please help with answers to these questions:

1. What is Relative CTR and what is it used for?

2. Is there a relationship between CTR and Relative CTR? If there is, what is the relationship?

3. I am designing a simple on/off control system with a process switch as sensor.
I require to isolate the field side (which has 15 Vdc process switch as input) from the controls side of the system (which has 5 Vdc).
I desire a cost effective optocoupler for the isolation and I chose the Vishay's TCMT4106. Below is the URL to and a picture of the datasheet and a picture of the schematic.

https://www.vishay.com/docs/84181/tcmt4100.pdf
EDABOARD.PNG
pic.jpg

Is this optocoupler appropriate for this requirement?
 

I could not locate the term Relative CTR in the datasheet; the meaning may be referred from the context.

If you really do not need the high frequency response you may use a transistor to switch on and off the IR LED (use the transistor ground side).
 

I didn't found and Relative CTR on the datasheet you provided, however the only time I heard about relative CTR was when mentioning the dregadation of the CTR, which happens after many hours of using the optocoupler.
 

Hi,

1) can not be answered, because you don't give enough information.

2)
2. Is there a relation ship between CTR and Relative CTR? If there is, what is the relationship?
Just from the wording you could see that the "relative value" is the relationship

Example: Nutrition information. Maybe you find "10 % fat". Then the "10%" is the relative value. It gives the relationship between total_weight and fat_weight.
If you have 500g of food with 10% fat, this means that 50g fat is included in this food.

*****
Your example:
There are a lot of informations missing. Therefore I use some "example values", but you have to calculate with your true values.
Let's say Rq is 10k
--> then the collector current is about 0.5mA when active.
Let'say the initial CTR is 200%,
--> then this means you need at least 0.25mA of LED current.

But now you need to know:
* for switching applications you should overdrive with a factor of 5...10
* aging may reduce the CTR to 30% ( of initial CTR) within a couple of years.

Therefore I recommend to multiply the 0.25mA with 5...10 to 1.25...2.50mA
And further divide this result by 30% to get 4.17 ... 8.33mA.
--> let's choose 7mA.

Now with a forward voltage of 1.35V you can calculate Rd = 1950 Ohms.

Klaus
 
Thank you c_mitra, gumattos and KlausST for your responses. They were very helpful.

I understand that the chart shown in the picture attached can be used to determine the initial CTR for a design with the optocoupler
from ambient temperature.

tcmt4100_tcmt4106Capture.PNG

But, how is the ambient temperature determined? Why I am asking this question is that a system is normally cased and the temperature
inside the case while the system is running is likely to be higher than that outside.
It may seem a very basic question but please bear with me. Thanks.


---Akanimo
 

Thank you c_mitra, gumattos and KlausST for your responses. They were very helpful.

I understand that the chart shown in the picture attached can be used to determine the initial CTR for a design with the optocoupler
from ambient temperature.

View attachment 135161

But, how is the ambient temperature determined? Why I am asking this question is that a system is normally cased and the temperature
inside the case while the system is running is likely to be higher than that outside.
It may seem a very basic question but please bear with me. Thanks.


---Akanimo

Helo Akanimo, the ambient temperature can be measured using a specific circuit that changes it's characteristics with the temperature, but I don't think you will need to worry about that, take the temperature of the place you will put your circuit, leave any component that can get hot away and you should be fine
 
An on/off switch should be designed around the minimum CTR and then temperature variations won't matter.

In the circuit shown, because of the 3:1 voltage difference the same resistor value on both sides will work fine even with a minimum 50% CTR.

Consider 4.7k with 15V will provide about 3mA into the LED which, assuming 50% CTR, means the transistor output can sink up to 1.5mA. 1.5mA is sufficient to pull 4.7k to ground from 5V meaning your IC will see a good 'low'.

And just to be clear, the current that's 'transferred' is an 'up-to' value. The transistor can only pull down to ground. If the output node reaches ground at less current than the CTR ratio describes so be it, current stops at that value.
 

Hi,

Consider 4.7k with 15V will provide about 3mA into the LED which, assuming 50% CTR, means the transistor output can sink up to 1.5mA. 1.5mA is sufficient to pull 4.7k to ground from 5V meaning your IC will see a good 'low'.

Be careful: The CTR in the datasheet is given for 5V Vce....but you now talk about less than 0.7V Vce.
It win't be able to drive 1.5mA down to 0.7V. I know you just need less than 1mA...
--> don't go too close to the limit

Klaus
 

You can avoid the problem if you use a transistor to switch on/off the LED. The LED can be driven with up to 50mA but you can safely use it at 20mA. By the way, ambient temp is the outside temp and is different from the junction temp. You need to derate the device power if the ambient temp is much higher than 20-25C.
 

Hi,

just to add:

The initial confusion was because you asked about "relative CTR" but then you refer to "normalized CTR".

--> Please try to use exactely the same words.

Klaus
 

Thank you guys for your contributions. You have moved me forward.

Based on Klaus' post as quoted below, I did some calculations and arrived at some component values.
I simulated the circuit on SPICE based on these component values and I believe the results were successful.
However, I am posting the calculation, circuit and simulation waveforms here so you can look at them. I'll appreciate
any contributions on something I may not have done right.

Your example:
There are a lot of informations missing. Therefore I use some "example values", but you have to calculate with your true values.
Let's say Rq is 10k
--> then the collector current is about 0.5mA when active.
Let'say the initial CTR is 200%,
--> then this means you need at least 0.25mA of LED current.

But now you need to know:
* for switching applications you should overdrive with a factor of 5...10
* aging may reduce the CTR to 30% ( of initial CTR) within a couple of years.

Therefore I recommend to multiply the 0.25mA with 5...10 to 1.25...2.50mA
And further divide this result by 30% to get 4.17 ... 8.33mA.
--> let's choose 7mA.

Now with a forward voltage of 1.35V you can calculate Rd = 1950 Ohms.

Klaus

The Calculation:
If = 0.5mA
CTR(initial) = 90% = 0.9 (obtained from the CTR vs Ambient Temp Chart I posted earlier)
If(overdrive) = 0.5mA*(5...10)
= 2.5mA ... 5mA​
If(degraded) = 2.5mA/0.3 ... 5mA/0.3
= 8.333mA ... 16.667mA​

Taking 12,5mA as drive current,
Rin_field = (15V - Vf)/12.5mA = (15V - 1.35V)/12.5mA = 1.092kohm = 1.1kohm

Ic = If*CTR(initial) = 0.5mA*0.9 = 450uA
Rcontrol = 5V/450uA = 11111.11ohm = 11.11kohm

N/B: Ic = I(Rcontrol) = I(RQ) ...in plot
If = I(Rin_field) = I(Rd) ...in plot
Rd ...in diagram = Rin_field ...in plot
RQ ...in diagram = Rcontrol ...in plot
Vforward = Vnode(switch+diode+Rd) ...in diagram
Vc_e = Vnode(IC_pin+transistor+RQ) ...in diagram

pic.jpg


MBH_opto_Capture.PNG

Thanks.

---Akanimo.
 

I suggest you stick to LED current in the range of 1-10 (or perhaps 20) when the curve is approx linear. As you have sufficient headroom, it is not wise to use that.
 
Hi c_mitra,
Please pardon my improper designation. It's 12.5mA that I calculation for the diode drive. Thanks for contributing.

--Akanimo.
 

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