Why use different transistor numbers for switches and amplifiers

[Billy Mayo]

At work on the circuit boards these use these types of transistors , they use transistors are a SWITCH

2N2484
2N3700
2N3440
2N6287
2N4093
2N5566
2N5116
2N3251
2N2222
2N2904

My question is why do they use different type transistors , when the transistor is used as a SWITCH?

What difference does it make when using a different number type transistor

Why don't they just use 2N2904 through out the whole circuit and use it as a switch , why use all these different numbers of transistors? what for?

 

[dr pepper]

The number depicts the ratings and package of the transistor.
An off mains switch mode supply needs a transistor of at least 700v rating.
A car 19v laptop switching converter only needs one of around 50v.
You might find the switching trans in each to look the same, but as I said will have different ratings, therefore will have a different type number.

 

[KeepItSimpleStupid]

One comment, because I don't feel like looking at all of the data.

The 2n2222 and 2n3904 are similar, but not identical. The one thing you can never guess is cost and availability. That could be a determining factor.

Tabularize the parameters.

 

[crutschow]

What makes you think that, since a transistor is used as a switch, the various transistor parameters that affect it's use as a switch and differ between transistors aren't important?

 

[Billy Mayo]

the various transistor parameters that affect it's use as a switch

What various parameters affect the transistor as a switch?

 

[BioniC187]

Ib saturation curents, Ice MAX. Temperature vs current, cut off current - many things. It depends on the input signal, and load you are switching. Imagine if you have a really low current signal, and wish to sample it to use as a switch. You would want to use a high gain transistor and low cut off current to sample the line to achieve saturation.

 

[crutschow]

What various parameters affect the transistor as a switch?

I rather expected you to came back with that question. I realize your favorite hobby is thinking up more questions to ask on a myriad of subjects, but as a little homework exercise I suggest you look at a transistor data sheet and see if you can determine which parameters might be important in using a transistor as a switch (such as this 2222A). That's how I learned after I started doing design work after leaving school, it was not taught to me. I believe, for the most part, the answer to your question is fairly obvious. Information obtained that way is much more useful then if it's just handed to you. Much of engineering involves logical thought and this is a good exercise in honing that process.

 

[crutschow]

Ib saturation curents, Ice MAX. Temperature vs current, cut off current - many things. It depends on the input signal, and load you are switching. Imagine if you have a really low current signal, and wish to sample it to use as a switch. You would want to use a high gain transistor and low cut off current to sample the line to achieve saturation.

Aww, you spoiled my post.

 

[BioniC187]

I just realised lol

 

[dr pepper]

The most important thing about transistor numbers is that the designer and anyone messing with the circuit understands them.