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Topic: What did I not understand about transistor amplification? (Read 1 time) previous topic - next topic

a_lehmann

Hey there,

this my first transistor amplification and I'm running into some trouble of course.
In my original circuit I'm using a NPN transistor to pulse an IR LED with higher current than the arduino can provide.
The power source is 4 AAA batteries, so it's 6 V and up to 4,5 A (short circuit, limited by inner resistance).

The amplification was not that good so I made this easy circuit (attachment) to measure where the problem is.
When I got the transistor thing right there must be a high resistance between collector and emitter when there is no voltage on the base and nearly no resistance when I put voltage on the base. Right?

On my breadboard I have 30 mV when the base has no connection to anything and the full 6 V when I connect the base to +6V with a 10 K-Ohm resistor. That's cool.
But, now I'll describe my problem, there is only 100 mA flowing where the LED is. I measured it by taking the LED out, so there is only my multimeter, the transistor and the 10k resistor on the base. Nothing else. I expected the 4,5 A (which I had at the short circuit of the 4 AAA) but only got this 100 mA. Why?

The transistor is rated for 15 A / 80 V (it's a 2N6488). But I've also tested another npn with the same result.

So I would be glad if you could tell me what I didn't get about it. And how can I achieve to draw the full 4,5 A.

Greetings

Alex

pwillard

#1
Jul 21, 2011, 06:03 pm Last Edit: Jul 21, 2011, 06:26 pm by pwillard Reason: 1
1) Never a good idea to short circuit a transistor, regardless of rating.
2) I doubt you will find this exact configuration you have in ANY amplifier designs
3) Never assume there is some "inner resistance" in a transistor.

In short, I feel that you have less understanding about NPN transistors than you believe you do.  That's OK... but I'm not really going to try to explain transistors in a sentence or two. Are you sure you mean "amplify"?

How about we give an example of how you can get an NPN to amplify?  Good place to start?   Maybe I will draw something and do a follow up post.

A problem you have right now is this.  A transistor can SWITCH ON and OFF and it can amplify... sometimes it can both... but it's related to base biasing.   You *can* make a voltage amplifying switch this way, (if the collector V+ voltage is higher) though there are some issues with how you have things connected.  in other words... you have sort of the right idea... but a less than optimal hookup.  You can make a current handling switch in cases where you want a device to handle more current than a micro can handle.


Ok, rather than draw up something new... I'll give you a drawing I already created that does what you want.   Obvuously, change out my values of transistor and LED for yours.

http://pwillard.com/files/driver2.pdf

Erni


a_lehmann

Thank you very much so far for your long answer.
I'm glad you try to help me!

Quote from: pwillard
1) Never a good idea to short circuit a transistor, regardless of rating.

Why not?

Quote from: pwillard
2) I doubt you will find this exact configuration you have in ANY amplifier designs

Yes, I know, I only build this to measure the current.

Quote from: pwillard
3) Never assume there is some "inner resistance" in a transistor.

How else would you tell it?

Quote from: pwillard
In short, I feel that you have less understanding about NPN transistors than you believe you do.

That's right I don't know much about any component. Very new to it.

Quote from: pwillard
Are you sure you mean "amplify"?

As you say it... No? I'm sorry, my English has got rusty. I don't want to amplify something like an audio signal. I just want more power than the arduino can provide. Like you say, an ON/OFF SWITCH . I'm using a timer interrupt to switch the LED ON/OFF every 13 us to achieve the 38 kHz the reciever needs. It already works on my breadboards but not with the desired current.

Quote from: pwillard
How about we give an example of how you can get an NPN to amplify?  Good place to start?   Maybe I will draw something and do a follow up post.

It would be great if you could help me to solve my problem. :)

Quote from: pwillard
A problem you have right now is this.  A transistor can SWITCH ON and OFF and it can amplify... sometimes it can both... but it's related to base biasing.  Using just 1 resistor won't really do it for you.  You *can* make a voltage amplifying switch this way, (if the collector V+ voltage is higher) though there are some issues with how you have things connected.  in other words... you have sort of the right idea... but a less than optimal hookup.

Yeah unfortunatley I don't get what I have to change yet. Do you understand me at least? I want this IR LED to be pulsed at 38 kHz. In the datasheet there is a description that says that you can use it with 3 A (for 10 us in the example). Because you don't get that much from an arduino pin I wanted to use a transistor. But I only get about 100 mA.

Quote from: pwillard
Ok, rather than draw up something new... I'll give you a drawing I already created that does what you want.   Obvuously, change out my values of transistor and LED for yours.

At first view I would say I've done it this way.

So why do I get only 100 mA when I short circuit a transistor? How do I get the transistor to let the whole current pass?



Quote from: Erni
This thread might help you:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1230677423

Do I have to use a MOSFET instead?

pwillard

#4
Jul 21, 2011, 07:22 pm Last Edit: Jul 21, 2011, 07:25 pm by pwillard Reason: 1
Starting at bottom.  Mosfet not required.

Quote
At first view I would say I've done it this way.


Look again.

1) you have a high value resistor when 1K is better.  You want the transistor to conduct.
2) You have connected the Emitter and Collector differently.
3) you do not have a current limiting resistor on the LED.

Essentially, let the Collector feed the load... (LED), connect Emitter to GND potential.

Quote
Never a good idea to short circuit a transistor, regardless of rating.


To put it simply... that's how you let out the magic smoke that makes it work.  A transistor will not limit itself to it's maximum rated current.  It will break (go poof) when it's maximum rated current is exceeded.  A short circuit will allow these conditions to exist.

This is also why you need to always limit LED current (with a resistor) to keep the  value below the maximum rated value for that LED.

An LED with no resistor current limiting will allow an effectively unlimited amount of current to flow through it... until it "fries" itself.  Same is true of transistor.




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