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    Optocoupler: CTR and REL_CTR; Use

    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.

    http://www.vishay.com/docs/84181/tcmt4100.pdf
    Click image for larger version. 

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    Is this optocoupler appropriate for this requirement?

    •   Alt7th January 2017, 13:54

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    Re: Optocoupler: CTR and REL_CTR; Use

    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).



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    Re: Optocoupler: CTR and REL_CTR; Use

    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.



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    Re: Optocoupler: CTR and REL_CTR; Use

    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


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    •   Alt7th January 2017, 17:44

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    Re: Optocoupler: CTR and REL_CTR; Use

    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.

    Click image for larger version. 

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    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



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    Re: Optocoupler: CTR and REL_CTR; Use

    Quote Originally Posted by Akanimo View Post
    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.

    Click image for larger version. 

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    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


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    •   Alt9th January 2017, 22:27

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    Re: Optocoupler: CTR and REL_CTR; Use

    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.



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    Re: Optocoupler: CTR and REL_CTR; Use

    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



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    Re: Optocoupler: CTR and REL_CTR; Use

    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.



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    Re: Optocoupler: CTR and REL_CTR; Use

    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



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    Re: Optocoupler: CTR and REL_CTR; Use

    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.

    Quote Originally Posted by KlausST View Post
    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

    Click image for larger version. 

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    Thanks.

    ---Akanimo.



    •   Alt10th January 2017, 17:33

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    Re: Optocoupler: CTR and REL_CTR; Use

    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.


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    Re: Optocoupler: CTR and REL_CTR; Use

    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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