Phototransistors

[Sling]

Regarding a phototransistor such as the KTIR0821DS can someone kindly explain what the anode and cathode are for exactly?

Seems like one of those things that are simple if you know how, and I will probably kick myself once I know it, but I thought it was to power the component. I understand that light acts as the base, but the anode and cathode do not seem to power it, as a power line through these seems to make no difference to its operation.

Also wondering how I can make it operate so that there's more of a difference in voltage output depending on light.

Basically I'm just trying to figure out how they work exactly.

 

[crutschow]

The anode and cathode are to power the LED light source for the interrupter. You run a current (typically 20mA) through the LED and a series resistor to generate the light. The phototransistor conducts current between the collector and emitter proportional to the amount of light falling on the base region. Thus the transistor conducts current when not interrupted since it receives light from the LED. When interrupted, the transistor receives little light and it conducts no current.

 

[Sling]

The anode and cathode are to power the LED light source for the interrupter. You run a current (typically 20mA) through the LED and a series resistor to generate the light.

So the cathode and anode are kind of superfluous then? I mean, the light source could be from anywhere. Why does a line have to go through the phototransistor for it?

 

[crutschow]

The LED is not superfluous for this particular device. It is designed for interrupter type operation where it detects an object by the presence or absence of light from the LED source.

But you could use some other light source if you desire. That's just not using the device the way it was intended. If you don't want to use the LED source than you might consider buying a phototransistor without it.

 

[Sling]

Well I've connected an LED (with series resistor) through the anode and cathode line, while a +9v line was powering a load through the collector/emitter, and the LED did not receive any power.

I had to connect the voltage source for this LED to function, as it did not function from the phototransistor only. So it seems completely pointless to have an anode and cathode as far as I can see.

 

[BeeBop]

Well I've connected an LED (with series resistor) through the anode and cathode line, while a +9v line was powering a load through the collector/emitter, and the LED did not receive any power.

I had to connect the voltage source for this LED to function, as it did not function from the phototransistor only. So it seems completely pointless to have an anode and cathode as far as I can see.

That is because you bought a photo-transistor opto-coupler, rather than just a photo-transistor.

I think what you wanted was something like these:
phototransistor - Google Search or this https://www.electro-tech-online.com/custompdfs/2010/01/nte3031.pdf

 

[crutschow]

You connected an external LED?

The LED I'm referring to is built into your device and it's connected to the anode and cathode terminals. Read the data sheet.

Your comment about the LED not functioning from the phototransistor only makes no sense to me. It doesn't function from the phototransistor. It generates light that operates the phototransistor.

 

[Sling]

The LED I'm referring to is built into your device and it's connected to the anode and cathode terminals. Read the data sheet.

I'm terrible at reading datasheets. I've been trying to find where it says that but I must be blind or something.

Possibly I've blown the inbuilt LED by accidentally passing too much current through it. Is there anywhere on the datasheets that suggests resistors to use for it?

It suggests 2v and 4mA for the phototransistor component (if I'm reading it correctly) but I can't for the life of me find where the inbuilt LED is referenced.

I could swear that's the symbol for a diode, not an LED, in the line is not connected to the transistor component line...


*** After some more digging on the internet, it seems that 220 ohms would be about right for the LED line. Still wondering where this information is on the datasheet though !?! Could they make these things any more obtuse.

 

[Reloadron]

OK first the device you linked to is a photo transistor with a built in light source. Think of it that way. Their general applications are:

Applications
Cassette tape recorders,VCRs
Floppy disk drives
Various microcomputerized control equipment

Take a look at your data sheet. Look at the device pin out and note the light source (LED) anode is pin 1 and the Cathode is pin 4. Look below the illustration and note the small arrows pointing down then hitting a surface and pointing up. The light source is focused to bounce off an object and be reflected back to what is the base of the transistor. These devices are used primarily to detect things.

The forward voltage is nominally 1.2 Volts and the forward current is 50 mAmps. If for example you wanted to power the light source with a 6 volt source you would need to drop 6 - 1.2 = 4.8 volts. We know the current max is .050 Amp so 4.8 / .050 = 96 so you want about a 100 ohm resistor in series with the light source. If you placed a voltage greater than 1.2 volts across pinss 1 & 4 you likely toasted the light source (LED). Also remember Anode + and Cathode -.

Does that help?

Ron

 

[Sling]

Yes, that is actually very kind of you to help.

Forgive me for being such a novice at reading datasheets, but hey everyone has to start somewhere right.

Thanks to everyone who replied.