# Phototransistors

Discussion in 'General Electronics Discussion' started by idris, Jul 17, 2011.

1. ### idris

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0
Jun 19, 2011
I'm new to phototransistors, and having looked at a few datasheets, it appears that unlike ordinary transistors they don't seem to list hfe.
I'm probably missing the point, but what basic equations and/or parameters do you need when using a phototransistor as a voltage devider as an input to a comparator?

2. ### Resqueline

2,848
2
Jul 31, 2009
Hfe is current gain. Phototransistors have photons instead of electrons as the base input and so you can't define gain that way.
Phototransistors might have a Collector current [mA] per light intensity [Lumens] parameter instead.

3. ### idris

33
0
Jun 19, 2011
What are the important parameters when choosing which transistor to use?
As far as I can tell the most significant will probably be ...
• Emmiter Collector Voltage
• Collector Emmiter Voltage
• Collector Current
• Collector Peak Current
...but I can't find info on how to factor these values into circuit calculations.

4. ### Resqueline

2,848
2
Jul 31, 2009
Neither of the above (in ordinary applications). Try with sensitivity [mA/(mW/cm2)], peak wavelength [nm], max dark current [nA], half sensitivity angle [degrees], etc.

5. ### idris

33
0
Jun 19, 2011
To be fair, I've been Googling on and off for several days and struggling to find the ansewer. But from your list, sensitivity sounds like the one I'm most interested in. Or possibly max current, if such a parameter exists.
Thanks

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,802
Jan 21, 2010
Large current carrying capacity is generally not an issue. Phototransistors are very rarely used to switch an actual load, they are generally used to create a signal which is amplified.

In that respect, the greater sensitivity (current per unit of illumination) allows a greater change in a relatively small current. The phototransistor generally has a resistor in series with it and the change in current through the phototransistor cases a change in voltage across the resistor -- the signal that is amplified.

Sometimes a phototransistor is used with another transistor in a darlington configuration. In this case the transistor is acting as a variable current source feeding the base of another transistor which switches a load.

7. ### idris

33
0
Jun 19, 2011
I'm expecting to use one in series with a resistror as one input for a comparator (LM393 with hysteresis feedback resistor) to problvide a logic output.
But since I have no idea what amount of light to expect to be incident on the transistor for my application, I can't figure out what voltage to expect at the comparator input.
And then it all gets a bit chicken and egg.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,802
Jan 21, 2010
You simply make the reference that is compared to the phototransistor variable.

That way you need not know in advance exactly how the phototranssitor will perform.

Of course, if you find the adjustment too critical, you will know the approximate voltage you are getting and you can add resistors either side of the pot to make the range of adjustment smaller (and hence less sensitive).

9. ### idris

33
0
Jun 19, 2011
(Obviously I'm missing something really simple.)
I know Vcc. I know Gnd=0. I know the series resistance. If I knew the current through the resistor I would know the voltage between the resistor and the transistor. But since I don't know the wattage of light falling on the transistor I don't know the current flowing though it. So I can't work out the voltage going into the comparator. Unless I plug it in. But if I can plug it in I've already decided what transistor to buy.

What obvious factor am I missing?

10. ### BobK

7,682
1,686
Jan 5, 2010
The obvious factor that you are missing is the range of light intensity that you want the phototransitor to work in.

Start this way:

Is it direct sunlight compared to shade?
It is normal room lighting vs total darkness?

i.e. characterize the range you need. Then use google to find a chart giving the typical light intensity for the scenario you are interested in. I was looking a such a chart recently, but I will not do your googling for you.

Bob

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
Or more simply, just connect up the phototransistor and measure the range of voltages you get for a given value resistor for the given lighting conditions.

12. ### idris

33
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Jun 19, 2011
There's an old saying that goes something like this: when a new guy comes to town who doesn't speak your language, no one expects you to wipe his arse. But it's good karma to tell him which shop sells toilet paper and what to ask for.

Yeah, I had thought of that Steve, but how do I choose which phototransistor to test? (I'm not trying to be facetious, this is a major part of my difficulties.)

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
No problems. Tell us what you want to use it for and as many details as possible to make it clear what the problems are. I'm sure we can suggest something.

If it's not especially an critical application then you may well find that the choice of phototransistor is likewise not critical.

It may also be useful to tell us where you are so we can suggest a supplier that is relatively local to you.

14. ### idris

33
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Jun 19, 2011
Thanks very much Steve.
Long story cut short - sensing a white flash for 1/25 sec on an TV screen in a semi darkened room and using it to trigger a timing circuit acurate to 1/100 sec.

Full details on my other thread.

(I can cope with suppliers - most likely CPC/Farnell.)