I have been working on a circuit which involves a photosensitive element soldered on a PCB. I am confused whether it is a photodiode or a phototransistor. Is there any means by which I can figure out whether its a photodiode or phototransistor without desoldering it? its a two terminal device. I am attaching its picture.
There are a few ways you can do this:
- Connect a voltmeter across the leads and shine a bright light or a laser pointer on it. If the voltmeter reads a voltage (not just noise) then it is probably a photodiode.
- Connect a diode tester across the leads. While this isn't ideal as they are in-circuit, it may still work. If your diode tester doesn't read anything, swap the leads around. If you still don't see a reading, then it's probably a phototransistor.
- If the board has some sort of amplifier (an op-amp, most likely) connected close to these devices, they're probably photodiodes. Otherwise they could be phototransistors, which are just switches and wouldn't need amplification.
- Do the PCB designators show a "D" or a "Q"? "D" is for diode, "Q" is for transistor
In order to get a more accurate reading (particularly with the diode test) it would be best to desolder one leg of the device (to take it out of the circuit) before measuring. However, it looks like there is some sort of gunk holding it to the board so you may need to remove that first.
Obviously a better view of the (entire) board and part numbers would be very helpful in this case.
Just from the looks of the active area, it certainly looks like a photodiode.
However, since it's a 2-lead device, it doesn't matter which it is - it will be used the same way in either case. The difference between the two is generally that photodiodes are faster but less sensitive, while phototransistors are more sensitive but slower. Phototransistors also have greater dark current. In any case, each is used in a circuit which converts photocurrent to voltage. In the case of a phototransistor, this is usually a simple resistor, while photodiodes (with their lower currents) ordinarily use some sort of amplifier.
As I say, the large active area which is visible says they are probably photodiodes.