| Author |
Message |
Keyser Soze
Guest
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 Posted:
Wed Nov 02, 2005 5:35 pm Post subject:
Re: Phototransistor with three pins |
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"Tim Williams" <tmoranwms@charter.net> wrote in message news:KV39f.32591$E17.32551@fe03.lga...
| Quote: | "Keyser Soze" <noreturn@nothere.com> wrote in message
news:_aW8f.6904$BZ5.4384@newssvr13.news.prodigy.com...
The collector current of phototransistors is usually very low, in the
range of 400 to 800 microamps with full illumination. Darlington
phototransistors can switch more collector current but rise and fall
times increase from a few microseconds to several milliseconds.
Odd, my 4N35 says 20mA minimum Ic for approximately 20mA in the LED.
Tim
The 4N35 is an optocoupler not just a phototransistor. |
See: http://www.fairchildsemi.com/ds/4N/4N35.pdf for data sheet.
You may want to note that while the current transfer ratio for the 4N35 is specified to be 100% at 25C it drops to 40% at the high
and low temperature extremes.
The CTR performance of related devices (4N25-4N28) is so poor (10-20%) that they have no specification for temperature extremes at
all.
Optocouplers tend to have the optical design optimized for best performance.
When using phototransistors on the other hand the optical path does not lend itself to convent optimizations.
Manufactures of phototransistors seem to try very hard to obscure the specification for photon sensitivity of their products. This
makes it quite difficult for a designer to compare products from various vendors.
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Watson A.Name - \"Watt Su
Guest
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| Posted:
Sat Nov 05, 2005 1:35 am Post subject:
Re: Phototransistor with three pins |
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"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:_yE8f.1180$8c5.717@newsread3.news.pas.earthlink.net...
| Quote: | Tim Williams wrote:
"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:Odk8f.2916$yX2.2374@newsread2.news.pas.earthlink.net...
A simple-minded way is to connect the transistor in the DCT mode,
making it look like a diode, but still act like a transistor:
No?? If it's NPN, it's going to be forward-biased! I don't see how
that
could ever work.
My understanding is the transistor has an additive (OR) choice
between base
current and light turning on the collector.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Start with a transistor.
If the base is connected to the collector, it is still technically
and pracitcally an active device, with the base current controlling
the
collector current.
Take one step backwards, close your eyes to that.
Shine a light on a silicon PN junction and notice that an
electrical
voltage is produced if open circuit (or hi Z load) and that an
electrical current is produced if shorted circuit (or low Z load).
Partly open eyes, take one-half step forward.
Shine a light on a transistor die (that is how the vast majority of
phototransistors were made; a lens that focused the light on a 2N2222
or
equivalent die).
In effect, charge is being injected in the base.
You now may open the eyes all the way and finish stepping forward.
|
My understanding is that phototransistors are not used as photovoltaic
devices. They are used just as a PIN photodiode is used, reverse
biased, but with current amplification. |
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Watson A.Name - \"Watt Su
Guest
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| Posted:
Sat Nov 05, 2005 1:35 am Post subject:
Re: Phototransistor with three pins |
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"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:Ifk8f.2917$yX2.1069@newsread2.news.pas.earthlink.net...
| Quote: | Tim Williams wrote:
"Geir Klemetsen" <regeirgarbagespam@start.no> wrote in message
news:djrd72$khn$1@services.kq.no...
But with a 3 pins phototransistor, I don't know how to assemble it
to do the same as in the schematic above.
If the third pin is base, leave it floating, or connect it to
emitter. If
for some reason you wanted to operate the phototransistor as a
regular
transistor as well, that's what it's there for.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Poor choices.
1) floating base: NO GO
|
No go electrically, but that's not what it's used for. It's used for a
phototransistor, and normally the base is left floating.
| Quote: | 2) base tied to emitter: less sensitivity than if base tied to
collector. |
Then it will have no photo sensitivity. It'll just be a forward biased
diode.
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Robert Baer
Guest
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| Posted:
Sun Nov 06, 2005 3:35 pm Post subject:
Re: Phototransistor with three pins |
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Watson A.Name - "Watt Sun, the Dark Remover" wrote:
| Quote: | "Robert Baer" <robertbaer@earthlink.net> wrote in message
news:_yE8f.1180$8c5.717@newsread3.news.pas.earthlink.net...
Tim Williams wrote:
"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:Odk8f.2916$yX2.2374@newsread2.news.pas.earthlink.net...
A simple-minded way is to connect the transistor in the DCT mode,
making it look like a diode, but still act like a transistor:
No?? If it's NPN, it's going to be forward-biased! I don't see how
that
could ever work.
My understanding is the transistor has an additive (OR) choice
between base
current and light turning on the collector.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Start with a transistor.
If the base is connected to the collector, it is still technically
and pracitcally an active device, with the base current controlling
the
collector current.
Take one step backwards, close your eyes to that.
Shine a light on a silicon PN junction and notice that an
electrical
voltage is produced if open circuit (or hi Z load) and that an
electrical current is produced if shorted circuit (or low Z load).
Partly open eyes, take one-half step forward.
Shine a light on a transistor die (that is how the vast majority of
phototransistors were made; a lens that focused the light on a 2N2222
or
equivalent die).
In effect, charge is being injected in the base.
You now may open the eyes all the way and finish stepping forward.
My understanding is that phototransistors are not used as photovoltaic
devices. They are used just as a PIN photodiode is used, reverse
biased, but with current amplification.
...and tying the base to the emitter gives a sensitivity almost three |
orders of magnitude less than with a floating or biased base. |
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