Which transistor do I use?

Question

Hello, another newbie question about electronics, sorry.

I've been looking for weeks but I can't find a straight up, plain English, Layman's terms explanation as to which transistor I should use for my circuits. I understand how transistors work, but I'm unfamiliar with how they really differ. How do I know if I need to use PnP or NpN? What's the difference between a 3904 and a 1318? Is it just the amount of current required on the center pin to allow the current to pass through?

Any plain, simple answers would be much appreciated. Thanks!

Answer 1

The different types have different properties, like max collector current, max voltage ratings, frequency response, speed, current gain, etc. From the beginner's perspective it's best to pick a good all-round general purpose one that you like and stick with it. It will do 90% of what you need.

I use BC548 for NPN and BC558 for PNP. They are "complementary" which means they have similar but opposite properties. They have the base in the middle, which is the sensible place for it as far as I'm concerned. And they are cheap.



If you don't know the difference between NPN and PNP, you need to first get an understanding of how transistors work. You don't have to get into semiconductor chemistry or quantum physics, just understand how a transistor behaves as an amplifier. Start with NPN transistors. Once you understand how an NPN works, a PNP is easy, it is just the exact opposite. Current goes into the base of a NPN, but comes out of the base of a PNP, and so on.



There are plenty of introductions to how transistors work on the web with varying clarity and complexity. The thing that switched the light on for me about how they work was a little picture in a manual for an electronics project kit. I can't find anything like it so I'll have to describe it.

You had a semi-circular channel like a long trough, with a solid barrier across it like a sluice gate. The barrier stopped water flowing down the channel. Coming into the big channel at the side was a small channel. Water flowing along the small channel moved a paddle. The paddle was attached to a lever such that the flow of water in the small channel caused the barrier in the big channel to raise, letting water flow underneath it out of the bottom of the big channel.

So, no water flowing in the small channel = no water flowing in the big channel. Put a bit of water in at the small channel, and you get a lot of water flowing in the big channel. Double the amount of water flowing in the small channel, and the water flowing in the big channel also doubles.

Now, for water current think electric current, and you have a NPN transistor! The small channel inlet is the base, the top of the big channel where the water goes in is the collector, and the outlet for the combined base and collector currents is the emitter.

Answer 2

The first question you should ask is FET or Bipolar.



The key difference there is that a FET is a voltage driven device, you apply a voltage to switch it on or off, the gain depends on the voltage difference between two pins.

A bipolar transistor is a current driven device, a current must flow into (or out of) the base in order to turn it on/off. The thing you are controlling is determined by the current flowing not the voltages.



bipolar seem to get used more in home electronics for some reason but personally I find FETs a lot easier to work with. A lot of the time you could use either type with minimal circuit changes.



Once you have decided that the polarity of the transistor you need (p channel / n channel for a FET, PNP or NPN for a bipolar) depends on the polarity of the thing you are trying to switch.



E.g. for a FET Vgs(threshold), the voltage difference required to turn the FET on, is positive for N channel and negative for P channel.

So if I needed to switch on a FET connecting something to ground I would use an N channel (my control signal isn't going to be below ground voltage so I'll need a positive Vgs to have any control). If on the other hand I was trying to use a FET to switch power on to a part then both the input and the output of the FET (source and drain) are going to be up at the power supply voltage when it is on. In that situation your control signal is going to be less that the source voltage and so you need a negative Vgs.



Similar logic can be applied for bipolars, if you use the wrong type you end up needing current to flow against a voltage difference for it to switch on. A little bit of basic design checking and it becomes fairly obvious you have the wrong type selected.



As for which of the thousands of each type to pick, the simple answer is that it normally doesn't matter too much. Just use the first one you find where the voltage and current ratings are within the range you need.

Yes, you could spend a long time doing an exhaustive search and find the perfect part to use. 99.9% of the time it's not worth it.

Answer 3

no longer basically are the different solutions superb, one significant element has been surpassed over, going to a extra effective transistor is going to get you no the place, different than possibly to a burned out means supply, or worse. EP because of the fact that's obvious which you haven't any longer have been given the talents needed. stay with what you have. To effectively use a extra effective output transistor, of the BJT variety, overlook the MOSFET, you may improve the stress from the present drivers, strengthen the means supply, be certain that the output circuitry will cope with the recent load with out changing output impedance, and that the warmth sink is rated for the recent load. One accessible answer for the driving force point is to apply Darlington pair form output transistor. on a similar time as this seems after the driving force matters, it does no longer something for the means supply, output impedance matters, nor the heatsink. If those issues are no longer scaled dazzling, or changed as needed, you are able to finally end up with a small scale version of the Hiroshima mushroom cloud, yet with out the radiation. save what you have, and go purchase a clean amp.