Author Topic: pnp transitor  (Read 804 times)

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Offline harpoTopic starter

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pnp transitor
« on: October 29, 2011, 05:49:24 PM »
Hello and thank you for coming to my post!

I just got a PNP transistor
when I apply - to the base and collector it lets the electricity flow from the collector to the emitter
but when i give the collector a positive current it gets really hot and does not let anything go to the emitter is this normal? I thought the base was just a activator and the collector could be + or - am i wrong?

Offline jonagik

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Re: pnp transitor
« Reply #1 on: October 29, 2011, 10:19:15 PM »
A *PNP* transistor behaves as follows:

If the voltage of the base is less than that of the emitter and the voltage of the base is less than that of the collector and the voltage of the emitter is greater than the voltage of the base by at least 0.7V then it is in the "saturation" region meaning that it operates effectively like a switch being closed between C and E (the voltage across C and E is usually like 0.2V).

If the voltage of the base is more than that of the collector and the voltage of the base is less than that of the emitter and the voltage of the emitter is greater than the voltage of the base by at least 0.7V then it is in the "saturation" region and the collector current equals beta (this is a property of the device - look at its datasheet) times the base current and the emitter current equals beta plus one times the base current.

Knowing the base, collector and emitter currents, you can then calculate the voltages across the legs.

Hope this helps :)

Offline jonagik

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Re: pnp transitor
« Reply #2 on: October 29, 2011, 10:21:59 PM »
Oh yea and I forgot this.

The power dissipated across the PNP transistor is as follows:

In saturation:
power loss = 0.2 x base current x beta = 0.2 x collector current

In active:
power loss = (voltage at collector - voltage at emitter) x collector current

Transistors are rated for a certain amount of power (60W or something for the ones you buy at an electronics store) so the values above cannot exceed that (well they usually can but it is advisable no to).

Offline Soeren

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Re: pnp transitor
« Reply #3 on: October 30, 2011, 10:43:11 AM »
Hi,

I just got a PNP transistor
when I apply - to the base and collector it lets the electricity flow from the collector to the emitter
but when i give the collector a positive current it gets really hot and does not let anything go to the emitter is this normal?
That depends on the voltages involved and how exactly you connected it up.
Connecting both collector and base directly to ground with the emitter at the positive rail will get it hot very fast as well (assuming you have a power supply that can deliver the current), as you're in effect shorting the supply through the transistor.

When you experience the transistor getting hot, where is the emitter and base attached then?


I thought the base was just a activator and the collector could be + or - am i wrong?
Yes.
A PNP used as a switch should have its emitter connected to the most positive potential (BjT's aren't bidirectional devices).

Connect the positive rail to the emitter, with the load between the collector and ground and you steer it on by a voltage (through a resistor! The base isn't just a control handle) at least ~0.7V below the positive rail/emitter.

You can see a PNP as an NPN flipped upside down, so if you understand the NPN, just sorta swap the rails.


What type is the PNP (what does the markings say) and what do you use as a power supply?
A PNP can be from <100mW to >100W and some will die if you short even a 9V PP3 with it, while other types can stand lots more than the same PP3 can deliver.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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