urgent....transistor help required

[vishu412k]

i need to know how to use 'transistor as a diode'......and its applications. i know of transistor as a switch, but as diode.....dos'nt make sense, unless its the same thing! the problem is, someone else has been asked to speak on 'transistor as switch'! seminar's in 10 hours ! HELP PLEASE!!!! :shock: :shock: :roll:

 

[samcheetah]

i dont understand your question. first you are asking about transistor as a diode and then you are asking about transistor as a switch. the two of them are different. a diode has two terminals and it lets current to flow in only one direction while a transistor switch has three terminals in which the voltage at one terminal determines the switch behaviour i.e. either there is a short circuit between the other two terminals or there is an open circuit.

if u want to use a transistor as a diode then yes u can bias any of the two PN junctions in the transistor as you wish. infact the base-emitter junction is always forward biased and the I-V curve of that junction is simillar to that of a diode.

now if u want to use the transistor as a switch, that is a different story. you bias the transistor at saturation for operation as a switch. a voltage at the base will determine the behaviour of the switch. digital circuits basically use this transistor switch as a means to establish logic levels.

im sorry if my answer was wrong because there is a bit of confusion in your question

 

[Russlk]

The transistor consists of two diodes, a base-collector diode and a base-emitter diode. The base-collector is normally reverse biased when the transistor is used as a switch and the base-emitter diode is forward biased. To use the transistor as a diode, two methods are used: 1. connect the base to the collector and use the base-emitter diode. This configuration allows high current but has a low reverse voltage rating because the base-emitter diode breakdown is in the range of 5 to 8 volts. 2. For higher voltage rating, use the collector-base diode, but the current rating will be low because the base is not designed for high current. The emitter is open in this configuration.

 

[Oznog]

i need to know how to use 'transistor as a diode'......and its applications. i know of transistor as a switch, but as diode.....dos'nt make sense, unless its the same thing! the problem is, someone else has been asked to speak on 'transistor as switch'! seminar's in 10 hours ! HELP PLEASE!!!! :shock: :shock: :roll:

It's not hard to explain how you can, but I don't recall any good reason to do so. An actual diode makes a far better diode. I saw something about using it as a temp sensor-
https://www.electro-tech-online.com/custompdfs/2004/10/MAX6627-MAX6628.pdf
**broken link removed**
I can think of several conventional thermal sensors I might use before that, but apparently it's pretty accurate without knowing the properties of the particular transistor.

 

[vishu412k]

connect the base to the collector and use the base-emitter diode. This configuration allows high current but has a low reverse voltage rating because the base-emitter diode breakdown is in the range of 5 to 8 volts.

can u explain this a bit in detail. is it that if base collector is shorted, then flow of electrons from collector to +ve supply (& from emitter to +ve sypply) increases current, but base region being smaller, excess electrons will cause breakdown early? :roll:

 

[zachtheterrible]

i guess if ur short on parts, you could do as russ said and use the base emitter junction. i'd clip off the collector pin cuz u probably wont even be needin that

 

[plot]

Use the base-emitter junction as suggested, don't wire the collector to anything. In my experience you'll get a drop of about 6 volts doing this on most transistors.

 

[Russlk]

As Plot said, you can use the base-emitter junction reversed biased as a zener diode but if you want to use it forward biased, connect the base to the collector because the saturated collector-emitter voltage is about 0.2 volts and the base-emitter is .5 volts, so the transistor is not saturated and can carry as much current as its rating. If the base is not connected to the collector, the diode resistance will be much higher and the current rating much reduced.