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How to select transistor? Answered

Hi guys!

How do you know which version of transistor to select when it comes to doing some projects?

I mean, lets say i have all the parameters of my project, the output current and voltage, so how do you know which specific model of transistor to select from the data you have? Is there some kind of list from which we can select the required model from our data?

For example, i have an LED array which draws about 90mA of current in total and has a total operating voltage of 15V, and from past instructables that i read on this site, i know that BC547 has a current rating of 100mA and can tolerate the voltages, so i can use it.

My question is, that suppose i didn't read these instructables, then how will i know which transistor or FET i have to select that has a current rating of about 100mA and can tolerate the voltages(i.e how will i know that i can select the BC547 or alternative)?

Also, when selecting a diode for example, i know from reading few instructables that 1n4007 has a rating of about 1A and can tolerate good amount of voltages. Suppose i don't know about the diode, then how will i know that based on my requirements, i can select the 1n4007 diode?

This is an example in general, and it applies to all cases of load currents and voltages, and other semiconductor devices.


Normally i check the website of the manufacturers (NXP, Linear, National and so on) or the shops (like Farnel, mouser, digikey) and they normally have a rooster where you can select and adjust (with sliders) some values to narrow the population of the rooster.

So you indeed go the other direction:
You tell them what you need at least and they sort out what does not fit.
Also every electrotechnician has its favorites he knows very well and used them in the past.
Often for a first test.build, he then uses those parts because they are already at hand (normally) and he know them. Later on if the principal design works, he will swap those favorites to the cost-minimals which just can do the job. See, it is not costeffective to use a 4A-FET where a 0.1A would do because a 4A-type is much more expensive than the 0.1A.