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Jan 2008 Ingenuity Unlimited novice q...

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neroku
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Username: neroku

Post Number: 1
Registered: 09-2007

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Posted on Saturday, 29 December, 2007 - 06:43 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello, I have a novice question about IU column of Jan 2008 issue.
In the circuit diagram there is a 10k resistor (R1) in parallel with transistor emitter and base. I use to see such a resistor in this kind of transistor configuration (PNP and NPN disposed in that way), so, what is the purpose of R1?

Thanks in Advanced

(Message edited by Neroku on 29 December, 2007)

(Message edited by Neroku on 29 December, 2007)
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dave_g
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Username: dave_g

Post Number: 371
Registered: 05-2005

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Posted on Saturday, 29 December, 2007 - 07:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

TR1 is a PNP transistor. R1 would hold the base to the positive rail and therefore keeps TR1 turned off.

Pressing S1 brings the potential on the base of TR1 towards the negative rail, so turning on TR1 and, eventually, the microprocessor in this application. The microprocessor then turns on TR2 (an NPN transistor, so taking the base high turns it on), maintaining the base current to TR1 even when S1 is released.

Without R1, the base of TR1 could "float" high resulting in the microprocessor being turned on when not expected, since it would effectively be open circuit. Think of it as reducing the impedance of TR1's base.

Hope that helps!

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