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Author Topic: Help with choosing a transistor  (Read 1986 times)
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Hey everyone,

I have received my Arduino UNO Sparkfun Inv kit a couple of days ago :-D yay.

I am new to electronics so try to be nice smiley-wink

I am making a LED light (probably 10-20 LEDs) that is connected to a motion detector to go in my hallway.I have everything running nice and smooth. The code is done and everything is running just the way I want on my bread board. But its only running 2 LEDs at the moment. The LEDs I will use are red 3volt 120ma diodes and I have read that the transistor (P2N2222A) can take 600ma, but I am not sure if I am understanding things right?

I have been reading about transistors for 30-40 min now and I didn't get much smarter. The data sheets are so hard to read if you don't know all the stuff in them :-( All transistors I can find seams to work with really high voltages or with very low amps.

Could someone plz explain some basics and help me to understand what I need to get something that will do the job?

Thanks loads.

David
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The data sheets are so hard to read if you don't know all the stuff in them
I'll let you into a secret most of the time you only look at a few of the things.

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All transistors I can find seams to work with really high voltages or with very low amps.
Don't confuse the maximum voltages with the working voltage, typically a transistor might have a collector emitter breakdown voltage of 60V but you wouldn't use it at this voltage, it would be fine for using up to 30V or so.
There are only a few things you need to know:-
Transistor type - NPN or PNP
Package type - surface mount or leaded
Collector current - how much they will pass.
Saturation voltage - the voltage across collector / emitter when the transistor is on. This is important for seeing how much current you can really pass through it. Because this will determine how much heat it burns when it is switching a certain current.
Power dissipation - how good the package is at getting rid of heat.
Gain - ratio of collector current to base current. Normally the bigger the current rating the smaller the gain.

You don't want to know about frequency or bandwidth - you also don't want to pay for it.

Hope that helps.
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Thanks...

You have helped  quite a bit. I think. I need to google some of the terms to get an understanding of what they are.

I think I will be able to figure the rest out, but I would need to know one thing.

If a transistor is rated at 40-50v can I still use it on the Arduino board that is using 5v and LEDs at 3v?

David
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Well what they are is a sort of electronic dimmer switch. But you should be finding out how and why they work. I know some people snicker but I bought Electronics for Dummies and it really helped me understand a lot.

I bought about 50 2n2222's at my local electronics store for working with LED's and Arduino just to have plenty around. They should work fine.
« Last Edit: February 09, 2011, 04:10:36 pm by biocow » Logged

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If a transistor is rated at 40-50v can I still use it on the Arduino board that is using 5v and LEDs at 3v?

Yes, a transistor can be used at less then it's maximum working voltage & current ratings. It's common to select a transistor the has much higher voltage and current ratings then your circuit will utilize. If I had a circuit designed to switch on a 1 amp load, I would feel best using a transistor rated for at least 3 amps or more just so it's not working too close to it's maximum ratings.

Lefty

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Yeah I realised the same thing and I am actually looking at book right now. Found 2 that look to be good and complement each other I think. Dummies is one of them smiley-wink

Well About the P2N222A, I have 2 of them in the Sarkfun kit that I bought. But I cant find anyone in sweden that are selling them.

And if I would be able to use one of these Darlingtontransistors that would be easy, simple and cheap. Maybe the books will tell me if they would work.

http://www.onsemi.com/pub_link/Collateral/BD675-D.PDF

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Oh another post :-)

Yeah that's sort of my problem..... Most transistors with low voltages had really low amps. But as long as I can look at 40-60 volts transistors it wont be a problem to find a transistor that has plenty of extra power. Will need a little heat sink most likely, but thats an easy fix :-D
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These people sell into Sweeden

http://se.farnell.com/
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Thanks... That seams to be a good site :-D
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also elfa is swedish, might be less expensive incl postage. www.elfa.se
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Don't confuse the maximum voltages with the working voltage, typically a transistor might have a collector emitter breakdown voltage of 60V but you wouldn't use it at this voltage, it would be fine for using up to 30V or so.


Let's see if I remembered anything from Solidstate devices


Breakdown voltage is the point where it just allows full current to pass and dies in the process, right?
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and dies in the process, right?

Yes but it might not die. For example a zener diode operates in the breakdown region of the diode.
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Hey doesn't IKEA sell transistors?  Just kidding!

Short and simple a Transistor is a valve.  For an NPN transtor the base is the handle on the valve and the supply voltage is the water pressure applied to the top or collector.  The Maximum Emitter to Collector EC current is the maximum amount of flow the valve can handle.   If you have 10 LED's that handle 10 mA of current in parallel they will need 100 mA of current.  Your transistor must handle at least that much Collector to Emitter current.  If your transistor only handles 50 mA you could use two with 5 LED's each.  Preferably, you should allow some 10 to 50% tolerance though.

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Well actually IKEA is selling transistors.

They must have some radio or something that's got them inside smiley-wink

Thanks for your reply. I think I am starting to get an understanding of it. First the current the "watts" and then you have the heat dissipation as well to consider. If a transistor is made of metal it might have 3-5watts of "cooling" while a normal one only has 0.5. And then you have ones that are much more power full and needs heat sinks.

I have ordered 2 books on electronics to get more knowledge.

And I will actually change the LEDs so the transistors in the Sparkfun kit should be enough :-D Turns out I must have had some really old leds laying around.
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Hi,

Unless I missed something, you want a switch more than you want a proportional valve.  ON of OFF

A MOSFET transistor is a good bet for you.  No resistors on the signal line are required to limit current from the arduino pin.  A resistor is required in series with the LED to control current through the lamp, just like the ones on the shields

a 2N7000 might be a good choice to switch your LEDs.  They can be made to appear to dim the lamp if switched on and off rapidly with a PWM output.

2N7000 s work good in parallel for more current capacity.  They are rated about 350 ma each.  Use half.  No heat sinks should be required.

Join the club.  I have been reading transistor specs for ( ugh!) 45 years.  They still give me a headach and make me crabby
« Last Edit: February 13, 2011, 12:26:31 am by jcarrr » Logged

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