Check unknown part if photo transistor or photo diode - Page 1
Forum section: electronics / beginners

author: Reider
author_rank: Contributor
timestamp: November 15, 2016, 08:08:37 pm
content: HelloI have some unknown optical parts in a device that act as a proximity sensor and some as a photogate. All of them have 2 legs. I can spot the IR leds from the color, others seem more tricky.How can you determine with a multi meter that the part is a photo transistor or a photo diode without breaking it?Also the operational characteristics like max current etc.Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: November 15, 2016, 08:54:53 pm
content: The diode test on the DMM should show something like a 700 mV forward drop for a photodiode, but open or much more (e.g. > 4 V) for a phototransistor.For the parameters one could measure the current for a given light intensity. Limiting values like break down voltage are a little hard to measure without a risk of breaking the part.

author: Reider
author_rank: Contributor
timestamp: November 15, 2016, 09:23:10 pm
content: Thanks, I'm also guessing that when doing a continuity check on the part, the diode will let current through in one direction but not in the other, while the transistor wont in either direction when no IR light is transmitted on it, or just a really small portion. Am I guessing right?

author: grifftech
author_rank: Frequent Contributor
timestamp: November 17, 2016, 02:42:15 pm
content: take a photo

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 17, 2016, 04:43:48 pm
content: Diode check would identify the diodes, but I would measure voltage across each device when exposed to the light source, using series resistors and a power supply. Very low voltage drop for the transistor - pretty standard diode voltage drop for the diodes. This would also hopefully provide some useful application data for you.

author: alanb
author_rank: Frequent Contributor
timestamp: November 17, 2016, 04:45:28 pm
content: Quote from: grifftech on November 17, 2016, 02:42:15 pmHow can you tell by appearance? As far as I can see both photo transistors and photo diodes look the same.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: November 17, 2016, 05:51:26 pm
content: Quote from: alanb on November 17, 2016, 04:45:28 pmI guess there could be some packages which are uniquely used for one of the two versions, but that's just me speculating..

author: Reider
author_rank: Contributor
timestamp: November 17, 2016, 10:10:33 pm
content: This a picture of one of the photogates, maybe that is a D so guessing both are diodes but have to be sure and also about witch is witch...Hmm but also the E and C can stand for Emitter Collector than that is a transistor...Also the other ones are harder because they are not on a board and look like standard 3mm diodes but I have no picture atm of those

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2016, 10:13:45 pm
content: lol, it is clear as a whistle what is what. AK = LED diode (anode kathode), CE = phototransistor (collector, emitter). Read just the markings on the PCB!

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: November 18, 2016, 01:31:05 am
content: I would support that this is a LED + phototransistor. But I wouldn't say that it is clear as a whistle - since Reider posted it at the beginners section it wasn't clear for him.

author: Reider
author_rank: Contributor
timestamp: November 19, 2016, 03:17:16 pm
content: So these players are the harder ones to identify. No board, no markings.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 19, 2016, 04:09:34 pm
content: The continuity/diode test works unless you have bright light. The typical DMMs use something like 1 mA. This already takes quite some light to operate a phototransistor with 1 mA. So a phototransistor will read more or less open, maybe a little sensitive to bright light.From a picture is can be hard to tell - they can look essentially the same. The marks on the board can help.One could also to a test circuit and measure the sensitivity: the phototransistor will conduct about 100-500 times more current and also work a little in the reversed direction.

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 19, 2016, 04:19:31 pm
content: Is the left one dark due to the shadow or is it dark-colored? If the latter, this is the receiver (phototransistor/photodiode). The dark color is an IR filter. It is used to filter away most of the ambient light before reaching the actual receiver.But it is hard to tell from the picture.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 19, 2016, 06:02:31 pm
content: Diode test :Vf around 0.6-0.7v -> Photodiode, Anode is +ve. Will also generate a few hundred mV on volts range when exposed to light.Vf <0.2v when exposed to bright light -> Phototransistor, collector is +ve terminal that gives the lowest VfVf around 1V -> IR LED