Hello,

I'm wondering if it is possible to calculate current the collector current of a phototransistor that is paired with an IR Led? I have an IR Led that is 13.6 cm away from the phototransistor and has a radiant intensity of between 65 mw/sr and 125 mw/sr. The phototransistor that I'm using is SFH 3015 FA. It's photocurrent could range anywhere from 160 uA to 800 uA at an irradiance of 0.1 mW/cm^2 and VCE=5V. The phototransistor also has a radiant sensitive area of 0.4 mm^2. I would have a resistor of 1.6kohm from emitter to ground.

I believe that irradiance at the phototransistor can be calculated based on the radiant intensity of the Led but I'm not sure if it would just be a simple radiant intensity/distance^2 or if I need to consider the radiant sensitive area as well? Also, the irradiance that I calculate at the phototransistor will not be 0.1 mW/cm^2. Can I assume a linear relationship between current and irradiance? When I have a range of values for the photocurrent, are those values typically the same as collector current? I would assume that I need to multiply that value by hfe of the phototransistor to get collector current but if that's the case it seems that the phototransistor datasheet is fairly useless since no typical hfe is given. Lastly, does my resistor have any effect on the collector current? Ultimately I will be in the active region of the DC load line so can I assume that the resistance has a negligible effect?

 

How much of the light arrives at the phototransistor also depends on the polar diagram of the LED and possibly the phototransistor too if it has a lens.

 

This question is difficult to give a clear answer to.

But there is no IR LED type.

If we ignore the optical deviation, then there is a worst possible deviation of nearly 1 to 10 at the transfer rate (LED: 65 to 125 mw / sr and Transistor: 160 to 800 uA).

Optical is also the issue of matching wavelengths.

 

This question is difficult to give a clear answer to.

But there is no IR LED type.

If we ignore the optical deviation, then there is a worst possible deviation of nearly 1 to 10 at the transfer rate (LED: 65 to 125 mw / sr and Transistor: 160 to 800 uA).

Optical is also the issue of matching wavelengths.

LED is SFH 4015n. based on half angle of the LED and phototransistor, I can assume that less than 5% of the light at the phototransistor is lost due to half angle. Also, the loss due to wavelength should be negligible.

 

To clarify, I realize that there will be a large ratio between max and min collector current based on the large ranges in radiant intensity of the LED and photocurrent at the phototransistor. I just want to know how to calculate what that range is. IE min and max collector current at the phototransistor. Even if you don't know how to solve the entire problem I've posted, answering portions of the questions that I've asked above would still be helpful.

At this point I'm fairly certain that I can calculate the range of irradiance at the phototransistor by taking min and max radiant intensity and dividing them by 13.6cm^2. The questions I then have are: 1) can I assume a linear relationship between irradiance and photocurrent? For example, if my calculated irradiance is 0.3 mW/cm^2, can I assume that my min and max photocurrent would be 3*160 uA to 3*800 uA? is photocurrent different from collector current? I assume so... and if so, can I use some standard value of hfe to calculate collector current? Lastly, will my 1600 ohm resistor at the load affect my collector current since I will be operating in the active region?

 

To clarify, I realize that there will be a large ratio between max and min collector current based on the large ranges in radiant intensity of the LED and photocurrent at the phototransistor. I just want to know how to calculate what that range is. IE min and max collector current at the phototransistor. Even if you don't know how to solve the entire problem I've posted, answering portions of the questions that I've asked above would still be helpful.

At this point I'm fairly certain that I can calculate the range of irradiance at the phototransistor by taking min and max radiant intensity and dividing them by 13.6cm^2. The questions I then have are: 1) can I assume a linear relationship between irradiance and photocurrent? For example, if my calculated irradiance is 0.3 mW/cm^2, can I assume that my min and max photocurrent would be 3*160 uA to 3*800 uA? is photocurrent different from collector current? I assume so... and if so, can I use some standard value of hfe to calculate collector current? Lastly, will my 1600 ohm resistor at the load affect my collector current since I will be operating in the active region?

You are making this way too complicated.measure the current through your phototransistor by measuring the voltage across your 1600 ohm resistor and apply ohms law.

The 1/(d^2) law applies as long as your two devices (ir emitter and phototransistor) are co-axial ly aligned. Any mis alignment can be a much bigger effect than distance. Look at a random IR emitter DATASHEET to see the intensity variation vs angle.

 

You are making this way too complicated.measure the current through your phototransistor by measuring the voltage across your 1600 ohm resistor and apply ohms law.

The 1/(d^2) law applies as long as your two devices (ir emitter and phototransistor) are co-axial ly aligned. Any mis alignment can be a much bigger effect than distance. Look at a random IR emitter DATASHEET to see the intensity variation vs angle.

I know how to measure my phototransistor current but that doesn't actually tell me much... having data at one specific data point isn't useful at all unless I know what the exact radiated intensity of the LED is and what the exact sensitivity of the phototransistor is and I am not going to get that information when I order a part. I need to know what the potential min and max collector current is for any combination of LED's and phototransistors.

Misalignment is negligible. Given the half angle of both the phototransistor and LED, misalignment would have to be fairly drastic to see a noticeable drop in output. In this application distance actually plays a much greater role in output given that the distance is over 13 cm.

 

Hello,

I'm wondering if it is possible to calculate current the collector current of a phototransistor that is paired with an IR Led? I have an IR Led that is 13.6 cm away from the phototransistor and has a radiant intensity of between 65 mw/sr and 125 mw/sr. The phototransistor that I'm using is SFH 3015 FA. It's photocurrent could range anywhere from 160 uA to 800 uA at an irradiance of 0.1 mW/cm^2 and VCE=5V. The phototransistor also has a radiant sensitive area of 0.4 mm^2. I would have a resistor of 1.6kohm from emitter to ground.

I believe that irradiance at the phototransistor can be calculated based on the radiant intensity of the Led but I'm not sure if it would just be a simple radiant intensity/distance^2 or if I need to consider the radiant sensitive area as well? Also, the irradiance that I calculate at the phototransistor will not be 0.1 mW/cm^2. Can I assume a linear relationship between current and irradiance? When I have a range of values for the photocurrent, are those values typically the same as collector current? I would assume that I need to multiply that value by hfe of the phototransistor to get collector current but if that's the case it seems that the phototransistor datasheet is fairly useless since no typical hfe is given. Lastly, does my resistor have any effect on the collector current? Ultimately I will be in the active region of the DC load line so can I assume that the resistance has a negligible effect?

What you propose might work for one instance on one pair, but would not have good application on a different pair.

 

The short answer is:

Any number you came up with would only be good to one or two orders of magnitude.
Practical opto-electronic solutions require circuitry that can cope with a large range of possible signal levels.

That's why people use fancy modulation schemes, more akin to radio transmission than a simple "current transfer ratio" as one would have in an opto-coupler situation.

At 13 CM distance, small variations in position and angle can have huge impact on CTR, not to mention the part-to-part variation.

Maybe you could elaborate more on what you are trying to accomplish?
There may be some clever solutions.

 

See

 

See