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Help with transistors

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(@garnold)
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Joined: 5 years ago
Posts: 81
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I'm trying to control 9v power to a boost converter. I'm using a 5v Pro Mini as the brains. I have the following questions...

1. How do I know if the transistor can control the power draw from the boost converter?

2. Is the 5v logic from the Pro too high for a transistor? I read I should use a 1k resistor.

3. Should I be using a FET?

4. How do you even figure out what transistor to use? There are way to many to pick from!

I realize this is something I should just know and it is basic. I'm still learning and the amount of transistors to pick from is hard to understand. I purchased a small kit from Amazon that seemed to have a good selection to choose from. I was using the N2222. From what I was reading it seemed like the right choice. Why, I have no idea but the post said it was. 


   
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robotBuilder
(@robotbuilder)
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Joined: 5 years ago
Posts: 2037
 

@garnold

4. How do you even figure out what transistor to use? There are way to many to pick from!

Do you have a circuit diagram of what you are doing?

You choose a transistor that meets the specifications of your circuit design.

I won't give a tutorial on transistors for there are no doubt some good ones on the internet.

 


   
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(@pugwash)
Sorcerers' Apprentice
Joined: 5 years ago
Posts: 923
 

@garnold

It is basically down to the question "how much current am I running through the transistor?".

It sounds very much like you need a MOSFET i.e. a transistor with low internal impedance. My experience is that this impedance is about 2 to 6 ohms. BJTs are much higher.

You could try a BJT, but if the current draw is too high, the worst that could happen is you will get the dreaded "magic smoke".


   
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ZoolanderMicro
(@zoolandermicro)
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Joined: 4 years ago
Posts: 144
 

There are a couple of common MOSFETs used with 3 - 5 volt outputs. The FQP30N06L and RFP30N06LE N-channel MOSFETs both can handle 30A continuous current. A Darlington transistor like the TIP120 can handle about 5A continuous current. I have never used a Darlington, so I don't know how to bias the base. If the load you are trying to drive is 1A or less, a NPN bi-polar transistor is fairly easy to use. The BC337 or P2N2222A can handle up to 800mA. A 1k ohm resistor at the base from a 5v output pin will work. That allows about 5mA through the base. You might try breadboarding a bjt transistor and experimenting. A PN2222A can handle up to 1A max. I have used a 680 ohm resistor with these successfully (aprox. 8mA base current). What is a 'Boost Converter'? 

ZoolanderMicro, where small ideas are a big deal


   
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robotBuilder
(@robotbuilder)
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Joined: 5 years ago
Posts: 2037
 

I like to have some understanding of how components work and what their role is in a particular circuit otherwise you will have to keep asking someone else to do it for you. This starts with an understanding of basic electronics which begins with a simple circuit consisting of a electrical source and electrical load. Power (watts) equals voltage by amps so you need to know if your transistor can handle that without getting too hot. You don't have to guess what components to use you can calculate it from their specifications.

Poor circuit design can have serious consequences when it comes to overheating. Take for example a battery that catches fire while being charged because the charger was badly designed resulting in your house going up in smoke.

 

 

 


   
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