How do I know which way around a photo transistor goes?

Thread Starter

McDuff

Joined Mar 26, 2022
4
I understand the diode anode/cathode identification but I have a BPW21R which is a phototransistor. It has two terminals as the base is driven by the light input. Datasheets tell me that there is a cathode connected to the package in a TO5 can but I can't see anything that tells me which is the cathode! My thinking is that the arrow points to the cathode therefore that is the emitter equivalent. Am I right?
Is this a common issue with phototransistors? Has someone worked it out already?
Thank you for your guidance.
 

crutschow

Joined Mar 14, 2008
29,825
The BPW21R is a photo diode.
Why do you think it is a photo transistor?
Yes, the data sheet states that the Cathode is connected to package, so the insulated pin is the anode.
 

Thread Starter

McDuff

Joined Mar 26, 2022
4
Because it uses a transistor symbol with the base not connected. Thank you for sorting that out for me and (Bertus) for the link to the transimpedance amp. Much appreciated.
 

Thread Starter

McDuff

Joined Mar 26, 2022
4
On the Vishay datasheeet. It shows a collector and emitter with the base unconnected (presumably illuminated by the incident light).
 

Ya’akov

Joined Jan 27, 2019
5,678
On the Vishay datasheeet. It shows a collector and emitter with the base unconnected (presumably illuminated by the incident light).
Yes, the base is the light input. Even 3 lead photo transistors don't have the base connected electrically, one pin is NC.
 

Ya’akov

Joined Jan 27, 2019
5,678
The BPW21R is a photo diode.
Why do you think it is a photo transistor?
Yes, the data sheet states that the Cathode is connected to package, so the insulated pin is the anode.
Pricey little bugger too but I must say the specs are impressive and the spectral response approximating human vision is a really cool thing.
 

DickCappels

Joined Aug 21, 2008
8,711
I have many two-leaded IR photo transistors. I suspect the emitter is the lead that is cut short, but the real test is to connect the transistor to a power supply and a resistor (e.g. 10k). Then if voltage across the resistor changes when the light on the input window changes, the polarity is correct. Otherwise...
 

Tonyr1084

Joined Sep 24, 2015
6,894
I can't see anything that tells me which is the cathode!
One pin (on the right) is connected to the PD (Photo Diode) body and one pin is isolated by an epoxy seal. The tab of a transistor tells which pin is the emitter. Since this is not a transistor or a photo transistor it's a PD. Only two leads. The tab end is the cathode.
1648473979900.png
 

Thread Starter

McDuff

Joined Mar 26, 2022
4
One pin (on the right) is connected to the PD (Photo Diode) body and one pin is isolated by an epoxy seal. The tab of a transistor tells which pin is the emitter. Since this is not a transistor or a photo transistor it's a PD. Only two leads. The tab end is the cathode.
View attachment 263795
Thank you all. I've got it working now and appreciate your help and guidance.
 
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