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Resistors for a NPN transistor? Answered

In the circuit diagram(which I've uploaded),I intent to use a NPN transistor(BC 547) as a relay.The 9 VDC is stepped down to 5 VDC by a LM7805 and should be the trigger to the transistor,completing a circuit which needs the 9 VDC.The Collector is given the common negative of both the voltages and the trigger gets the positive 5VDC.I need a bit help to calculate R1 and R2.I had used two random resistors previously but the Load got powered by the 9 VDC even before the trigger was applied to the transistor.I figured out that the problem might be related to the resistors used.
(I haven't included the caps used at the terminals of the regulator in the schematic)

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steveastroukBest Answer (author)2013-04-15

You don't need the 7805. You don't need R1. What is "the load" ? Can the transistor handle the current needed ?

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user

Its not that simple.Actually this is a part of a bigger project.The LM 7805 is simply inevitable.Yes,the transistor can handle the current needed."The load" is the same buzzer I've been talking 'bout yesterday.I enquired and found out that it needs about 100 mA of current.

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user

Then the 547 isn't really big enough. I'd want a margin on top of the "Ic max" figure of at least 25%, usually more.

You don't need R1, and if the buzzer is electromagnetic, it needs a diode across it.

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steveastrouk (author)2013-04-15

Please leave the Best answer set. That's two questions I've answered, and you've cancelled the BA.

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user

I'm terribly sorry Steve.I always make the mistake of clicking the"select as Best Answer" whenever I really want to click the"Reply button".I don't choose a best answer soon after asking a question because thats an end to further comments from other users coming in.I assure you that the best answer will be selected soon but please do wait for a few more replies.

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It doesn't stop other users replying, it stops other users replying UNDER the BA, for some reason.

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Any way ,I seem to have solved this problem too(special thanks to you)and yes!time for the best answer.

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See what I mean ? I can reply to THIS comment, even though BA is set.

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Oh!my mistake.Thanks for letting me know chief.

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steveastrouk (author)2013-04-15

The resistor should be related to the maximum load and the figure "Hfe" on the datasheet. Make sure the maximum current for the transistor is >> load current. Then base current = load current /Hfe
and resistor value = (Vin-0.6)/ base current. Then reduce it by 20% to allow the base to saturate.

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Adarsh_tronix (author)2013-04-15