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1A transister and beta

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rizwanspirit

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I have to use here in this circuit transister of min 1A current rating.please notify any transisitor and also give its beta,and please tell that in the data sheet of any transistor there are 2 bete Hfe min and Hfe max. we have to use min Hfe or max Hfe.
 

The Zener diode is 19V at a rated current of 6.6mA.
It still regulates at a current of 2mA so a few mA will be available for the base current of the transistor.
Without a load the zener diode will dissipate 19V x 6.6mA= 125mW which is fine because its max allowed dissipation is 500mW.

The load is 18V at 40ma so any little transistor can be used since it will have only 7V across it and only 40mA which is a dissipation of only 0.28W. A 2N3904 can dissipate a max heat of 0.62W.

A 2N3904 transistor has a minimum hFE of 60 at 40mA. So its base current can be 40mA/60= 0.7mA or more. It doesn't matter how high is its max hFE.

So calculate the value of the resistor.
 

    rizwanspirit

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min Hfe would be used to guarantee a minimum output current, but max Hfe can be interesting to check short circuit behaviour.

I assume from the post title, that you intended a higher output current than 40 mA, but you didn't tell exactly. Apart from output current, also the power dissipation should be considered when selecting a suitable transistor.
 

In my example with the little 2N3904 transistor, it will disappear into a puff of smoke if the output is shorted.

At first I thought of using a TIP31 or TIP41 power transistor but then the little zener diode would disappeatr into a puff of smoke if the output load is disconnected.
 

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