proper way to connect 2 pin transistor & limit its power?

Other Hardware General Electronics
1

i have an IR photo transistor (2 pin) and the datasheet gives it a max of 100mw (LPT2023)
Another (not IR) phototransistor (2 pin) the datasheet gives it a max .5ma (3DU5C)
(Yes, one was mw and the other ma)

I am working with 3.3v arduino pro.

What would be the proper way to wire up the phototransistors with the arduino and limit it's power?

thanks.

2

Do you know the voltage drops for these two devices?

3

Make a divider using a fixed resistor and the transistor. If the limit really is 0.5mA, then the
resistor should be 10k minimum. For the other anything down to 100 ohms should be OK, but
it all depends what sort of light levels are involved.

4

The LPT2023 has a maximum power dissipation of 100mw (milliWatts). The maximim current is 8 mA according the the data sheet. The maximum collector-emitter voltage is 5 volts.

So your eyes are completely OK when it comes to reading (mw vs. ma).

And do not forget that the wavelenght of your light source and your photo transistors have to match. Otherwise they will not work very well or even not at all.

Let us assume that the voltage drop across collector and emitter can go as low as 0.1 volts, you need a resistor that takes care that the remaining 3.2 volts will not give you more than 8 mA. So a 470 ohm resistor should be the minimum value.

Depending on the behavior of your photo transistor higer values might give you a better respone to light changes.

5

ieee488:
Do you know the voltage drops for these two devices?

They are photo transistors, they act roughly as light-programmed constant-current
devices until near saturation.

The datasheet I found had no abs max current rating for LPT2023, just 4 forward current entries
presumably for the different gain classes for a fixed illumination level. I would imagine it
will happily handle 20mA or more.

The spectral sensitivity depends on the material, but if the material is transparent extends to all
shorter wavelengths than the nominal wavelength. However IR phototransistors often have
a visible-light filter to reduce sensitivty to visible.

6

I have something to go on now. I am learning that just because I can get something to work today, if I don't do it right, it may not work later!! I am having a hard time getting around 2 wire transistors for some reason.