### Author Topic: Calculating min. and max. phototransistor resistance  (Read 5628 times)

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#### MadMax

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##### Calculating min. and max. phototransistor resistance
« on: November 29, 2008, 08:37:12 AM »
I need to get a IR phototransistor. I, however, do not have enough money to buy a couple different ones, test them all, and use the best one. I need to calculate the (rough) minimum and maximum resistance of one. I've looked at different datasheets, and can't seem to understand where I need to look for. Could somebody explain where I need to look for when I'm looking at a datasheet like this one:  http://www.farnell.com/datasheets/70729.pdf

I know the resistance depends on the amount of light shining on it (duh...) that's why I need to know how to calculate the resistance at a normal temperature (20 - 22 degrees centigrade).

Thanks in advance,
Max Henger
« Last Edit: November 29, 2008, 08:42:02 AM by MadMax »

#### szhang

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##### Re: Calculating min. and max. phototransistor resistance
« Reply #1 on: November 29, 2008, 02:43:42 PM »
phototransitor's as rated by current, not resistance.  What you're thinking of is photoresistors.

See "Figure 4. Collector Light Current vs. Irradiance" in the datashsheet, it gives you rough number on the current at difference light levels.

#### MadMax

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##### Re: Calculating min. and max. phototransistor resistance
« Reply #2 on: November 29, 2008, 05:05:35 PM »
so after that it's a simple R = U / I calculation... thank you very much

#### szhang

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##### Re: Calculating min. and max. phototransistor resistance
« Reply #3 on: November 29, 2008, 07:12:10 PM »
It doesn't quite work that way I don't think, because transitors are current controlled devices, so the current would be about the same at all voltage, therefore the apparent resistance would be different at different voltages.

#### ArcMan

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##### Re: Calculating min. and max. phototransistor resistance
« Reply #4 on: November 29, 2008, 07:39:08 PM »
Just to pile on...

As szhang stated, a transistor is a current device, so it can't be viewed as resistor.  Think of it as a current amplifier.  The base/emitter current is amplified to produce the collector/emitter current.  In the case of a phototransistor, incoming light replaces (or supplements) the base/emitter current.  You can think of it as a light-to-current converter.

#### Admin

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##### Re: Calculating min. and max. phototransistor resistance
« Reply #5 on: December 01, 2008, 11:39:13 PM »
At the bottom of the datasheets you should see a line chart graphing distance vs. output signal strength (current or voltage).

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