# Selecting the correct transistor

#### Smoke_Maker

Joined Sep 24, 2007
126
I'm having trouble with selecting a PNP transistor to turn on the TIP31, the voltage detector will sink a max of 20ma.

I want to flow 1.5 amps through the TIP31 so R5 will be about 3 ohms.

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#### Wendy

Joined Mar 24, 2008
23,062
Unless you are wanting 1.5A though the base figure a minumum beta of 10 (being way conservative) you need 150ma through the PNP. Almost any transistor will do that, my first choice would be a 2N2907A, but that is because of personal bias. I am assuming the transistor is saturated, which cuts down the wattage requirements.

Like the icon.

#### Smoke_Maker

Joined Sep 24, 2007
126
Thanks for the help Bill, I looked at the data sheet and I'm haveing understanding the Vbe(sat) numbers.

I guess what I'm saying is that I still can't read data sheets.

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#### Wendy

Joined Mar 24, 2008
23,062
The number you are interested in is the collector current, which is -0.6A. The 150ma I mentioned is well within that window. Beta is also important, but this number varies so much that you are better off figuring absolute worst case.

You've already defined what your collector current for the TIP31 will be. I took this number and divided by 10, which is extremely conservative. If the 2N2907A is also calculated using 10 (which is extremely low, 50 is more typical as a minimum) then that transistor will take 15ma to switch completely on.

A loose rule of thumb, Ic = Ib X β

β is the gain of the transistor, but the exact number is usually uncertain and wanders all over the place.

#### Smoke_Maker

Joined Sep 24, 2007
126
I think what your saying is to get 1.5 amps collector current through the TIP31 I will need 150ma through the base of the NPN?

For the 2n2907a to put out 150am out the collector it needs to sink 15am through the base.?

and the way to find the base current is to divide the collector current by anything from 10 to 50 and this is called beta?

#### Wendy

Joined Mar 24, 2008
23,062
Close enough. The math can get hairy, but that is the simplification.

#### Smoke_Maker

Joined Sep 24, 2007
126
I'm learning now, where can I find info on how to read data sheets. Those negative numbers in the data sheet is over my head.

I'm using SMT parts and found the MMBT3904 and MMBT3906 as favorite parts, so I need to find a compatable SMT part for the 2n2907a.

#### Wendy

Joined Mar 24, 2008
23,062
There is nothing special about that transistor. Use what you have if the Ic is .4A or more.

Personally I like feed though components, they are easier to breadboard.

Bill's Index

#### DickCappels

Joined Aug 21, 2008
9,495
Hey! The CR2032 is only rated for a maximum (pulsed) discharge current of 20 milliamps, NOT 1.5A!

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