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Making switching NPN with PNP

crazybuoy

Sep 19, 2010
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Sep 19, 2010
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There is a small circuit schematic attached where I am using voltage regulator 7812. I input 24VDC and 12VDC comes out from OUT pin. I want to make switch off & on NPN transistor with 12VDC buy using PNP transistor before NPN. And I will use PC printer port to provide the 1 & 0 pulse of 5VDC then how I should wire the circuit I want to just get the idea. Or is there any other way to do the same?
 

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  • Switching PNP.JPG
    Switching PNP.JPG
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Last edited:

barathbushan

Sep 26, 2009
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Sep 26, 2009
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i dont know why you have to use the pnp transistor
but if your goal is to switch the power supply to the load then you can use the circuit below

of125d.jpg
 

crazybuoy

Sep 19, 2010
13
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Sep 19, 2010
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I have tried the following attached circuit in the same structure having fixing the components on a paper card. The real schematic is also attached. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with – of power supply. It showing always 24 volt on both switch on & off. I think at switch off, it should show zero volt at voltmeter. Please tell me where I am wiring wrong in paper card. And also tell me the method of testing switching process.
 

Attachments

  • Paper card circuit.JPG
    Paper card circuit.JPG
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  • Sechmatic.JPG
    Sechmatic.JPG
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I have no idea why you need the 7812. It seems to be a very convoluted design.

You are missing resistors in the base of the first and third transistor. This may cause damage to the transistors or the parallel port (or both).

A pull-down resistor on the base of the third transistor is probably a good idea too.

The collector of the TIP42 should go to 0V when it is switched on (well, it won't go completely to 0V, but it should fall dramatically. I presume the 4k7 resistor in the collector of this transistor represents the load.

I would also check how much current your printer port can source. You probably only need a fraction of a mA. At present it gets shorted to ground through the first transistor's BE junction which is baaad mkay.
 

crazybuoy

Sep 19, 2010
13
Joined
Sep 19, 2010
Messages
13
I have try again it by changing the values of resistors in the following way as changed in attached circuit. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with emitter of TIP142. At 0 pulse from parallel port, voltmeter show 24VDC and at 1 pulse it show 12VDC. I want to make it show closer to zero volt at 1 pulse. So how to edit it to do so?
 

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  • Paper card circuit.JPG
    Paper card circuit.JPG
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