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Transistors - does size matter?
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Transistors - does size matter?

by jgunn on Fri Nov 12, 2010 2:23 pm

So, I'm trying to move beyond the stage of understanding electronics in general terms and copying other people's work, to understanding the specifics so I can make smart choices in my own designs.

I did a project last week where I harvested a circuit board out of a toy lightsaber and used the output to the rather anemic LEDs to drive a TIP42 transistor, switching a bigger power source to power a 1W superbright LED. The lightsaber is much cooler now (well, not temperature-wise). The transistor choice wasn't really a choice - I simply used what someone else had used, knowing it would work.

But for future projects, I want to know how to determine the "right" transistor for the job. Was the TIP42 massive overkill? Was it barely enough? Was it a choice born of convenience because the first person to do this knew the TIP42 was available down the street at Radio Shack?

I would expect that looking at a data sheet, (http://www.fairchildsemi.com/ds/TI/TIP42.pdf) I'd find specs like the minimum current required across the base and emitter to switch my circuit across the base and collector, and maybe the maximum current and voltage the part can handle in general. I'd think these numbers would be what I would base my part selection on. However, either I'm reading the sheet wrong, I don't know what the labels mean, or my concept of the relevant ratings for transistors is way off.

For my example circuit, the harvested board is pushing something like 2.5-3v at ~35mA. I'm switching a circuit of about 3v (a pair of AAA batteries) into a micropuck driver that is driving a 1W LED.

Thanks for your continued help in this forum, I really appreciate it.

-Jeff
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Re: Transistors - does size matter?

by jbisjim on Fri Nov 12, 2010 2:29 pm

Size always matters

so i am told
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Re: Transistors - does size matter?

by zener on Fri Nov 12, 2010 2:42 pm

The maximum switching voltage and collector current are under "Absolute Maximum Ratings" on page 1: Vceo and Ic. Specs for PNP and P channel devices are always negative, go figure.
You also need to worry about power, what they are calling Pc (datasheets don't always agree on these names). This is where you start to face the realities of design, and you start getting into details. That is why it is often just as good to use a known good design, or a transistor "definitely big enough". But you will benefit greatly from digging into the details.

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Please be positive and constructive with your questions and comments.