Help with choosing a transistor

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 ;)

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

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.

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.

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

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.

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

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 ;)

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

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

These people sell into Sweeden

http://se.farnell.com/

Thanks... That seams to be a good site :-D

also elfa is swedish, might be less expensive incl postage. www.elfa.se

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?

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.

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.

Well actually IKEA is selling transistors.

They must have some radio or something that's got them inside ;)

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.

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

I got some questions on the similar topics.

I want to drive a 12V PC Fan speed using an Arduino PWM pin.

I read a few examples on the net and got the following components from the local electronic stops to control the 12V PC Fan...

Darlington :TIP-122, NPN, 100V, 5A, Base current 0.12A

IRFZ44, N channel MOSFET, 60V, 50A ( i think this is overkill, also the most expensive )

IRF630, N Channel MOSFET, 200V, 9A

I got it working with the TIP122 to control the fan speed using PWM... hv not tried the two MOSFET yet..

My questions, is it the "correct" component to use a Darlington TIP122 just as a switch for 12V fan, should I use something like TIP31C, NPN, 100V, 5V Base current 1A instead ?

Is the Base current the max it can take, for my TIP122, I had a 1K resistor from the PWM to the Base.

Thanks

If it works then it’s probably the right device. Typically for a transistor used as a switch your main concern is it’s on/ off frequency response and then if it will withstand the operating voltages and currents. If your concerned about power dissipation and not so much about frequency (switching a motor is low frequency) i.e. battery operation I would think about using the Mosfets.

Ideal conditions you want are as follows minimum transistor turn-on current and voltage (base to emitter current for NPN). Near zero voltage drop across the transistor (Coll to Emitter volt.) when transistor is saturated. Zero current flow when transistor is OFF. All within your frequency (Bandwidth) of your application.

Datasheet TP122
http://www.fairchildsemi.com/ds/TI/TIP121.pdf

I can go on and on.