x Turn on thread page Beta
 You are Here: Home >< Physics

# Phototransistors watch

1. Can someone explain the stats of this, or any, phototransistor.

I don't do electronics so excuse my naivity but basically I would like to know the range that this transistor will operate away from an infra red source, and also the range of voltages induced? Does this little component really induce 70v Or am i reading that wrong.

Thanks for any info
2. My knowledge of phototransistors is fairly poor though I'll assume there's a lot in common with transistors etc.

It doesn't induce 70v. Rather it can be 'supplied' with a voltage difference of 70v between its collector and emitter terminals. In a normal transistor there would be a base terminal which is used to regulate the collector-emitter current, in this case I assume that the light is used to regulate it.

With respect to operating distances, this may well have to be determined by experimentation - it would depend on your source, phototransistor, their packaging and whatever's inbetween.

As for trying to figure out voltages induced, this component is more about current change in the collector-emitter line. This could be translated to a voltage by using a resistor for example.

(I'm sure there are people on here with more understanding about this than me, but this is my best interpretation!)
3. I tried using a Phototransistor in my GCSE Electronics project, since LDRs didn't give a linear output... 1/2 of my 10 weeks was figuring out how to use it!! It was so very complicated... This one looks a lot nicer and user friendly though!! The voltages induced don't really matter, since you can use an op-amp if they are too small, and the voltage will not be too big!! They would be order of mV, I should think... And for the distance... Well, I think you can experiment, and use the fact that Intensity prop. 1/distance^2 (I think that works for Infrared)... Anyhow, my advice would be to use an LDR if it is convenient, but this looks quite simple, so it doesn't matter if you can't!!
4. Hi,

So, a phototransistor is just a regular transistor with a window in. A transistor has three terminals, right? Base(B), Collector(C) and Emitter(E). It lets through C-E current in proportion to the B-E current. In a regular transistor, the B-E current is simply a current you provide with a regular voltage. However, in a phototransistor, the C-E current becomes proportional to the power of the incident light, which allows a B-E current to flow. (This light creates holes in the silicon which allows current to flow, but that's not really necessary knowledge for how they operate...)

The 70V, as someone has said, is not "induced" by the phototransistor. This is not a solar cell, which has a voltage induced by the incident light. All the 70V means is this is the maximum voltage you can put across the C-E pins before the device would start to behave strangely (non-linear effects, or breaking!)

If you want to calculate how far away the thing will work, you need to know two things 1) intensity of your source (because clearly you can be further away if the source is brighter, and yes 1/r^2 works for ALL radiation), and 2) the responsivity of the phototransistor.

The responsivity is defined as the current flowing per unit power incident on the device, so it's measured in Amps per Watt. It basically tells you how much current flows you get when you shine a certain amount of radiation on it.

e.g. I don't know exact figures for the device, but if the intensity of the source at some distance is comparable to the sun i.e. 140mW per cm^2, your responsivity is of order 1A/W, and the area of the detector is 0.1cm^2, then that'll be 14mW of power on the device, and so 14mA will flow.

Whether or not this is enough to make the device "work" is dependent on the sensitivity of the electronics around it. However, since the "dark current" is of order nanoamps, you can safely say that currents of the order 10mA are way above the dark current, and so the device is likely to behave properly with these sorts of currents. As someone else said, if the effects are not sensitive enough for your circuits, you can always amp them up.

Watch out, because responsivity may be different at different wavelengths of light. Infrared can be around 800nm, and I believe silicon phototransistors have maximum responsivity at around this wavelength, but I'm not sure.

Hope this helps. I'm off to bed!
5. (Original post by Worzo)
This is not a solar cell, which has a voltage induced by the incident light.
Yes it is. It is a Photovoltaic/diode attached to a Transistor. However the important effect is that is it a transistor that can be used to switch 70V.
Sorry just being anal.
6. What do you mean by photovoltaic/diode?

Is it actually a small photovoltaic cell (like a solar cell) that produces a small current in the base then? A voltage is definitely induced by the light? In this case, I'd assume the device would only have 2 pins. Is this correct?

If it was simply a photodiode on the end of the base, then I'd still expect it to have 3 pins because it would not be able to produce a voltage of its own. I'm just wondering what is actually the case. I suppose both systems could work...which are more common?
7. (Original post by Worzo)
What do you mean by photovoltaic/diode?

Is it actually a small photovoltaic cell (like a solar cell) that produces a small current in the base then? A voltage is definitely induced by the light? In this case, I'd assume the device would only have 2 pins. Is this correct?

If it was simply a photodiode on the end of the base, then I'd still expect it to have 3 pins because it would not be able to produce a voltage of its own. I'm just wondering what is actually the case. I suppose both systems could work...which are more common?
8. (Original post by Worzo)
What do you mean by photovoltaic/diode?

Is it actually a small photovoltaic cell (like a solar cell) that produces a small current in the base then? A voltage is definitely induced by the light? In this case, I'd assume the device would only have 2 pins. Is this correct?

If it was simply a photodiode on the end of the base, then I'd still expect it to have 3 pins because it would not be able to produce a voltage of its own. I'm just wondering what is actually the case. I suppose both systems could work...which are more common?
Both photodiodes photovoltaics and phototransistors can essentially be used in the exact same way. They are all two pin packages, in fact a photodiode and a photovoltaic are the same thing, just that a photodiode is a small package device never intended to generate usable power.
A phototransistor is esentially the same as a photodiode and a transistor combined. Except of course a transistor has all the parts to make a photodiode in the first place. So in a p-n-p transistor, the p-n part of the transistor acts like a photodiode allowing a current to flow from the p part to the n part, (collector to the base). Then it acts just like a transistor with a current flowing to the base.
All phototransistors have 2 pins.
They also can work as a voltage source I think...
9. (Original post by Mehh)
Then it acts just like a transistor with a current flowing to the base.
This might sound stupid, but (at least in FET's) isn't there an SiO2 insulator over the base gate, restricting the current to an unusable amount? Unless this is BJT and I take my comment back completely!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 18, 2006
Today on TSR

### Loughborough better than Cambridge

Loughborough at number one

### Can I date a girl with no boobs?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE