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How does the transistor work in this circuit?

goodanna

Mar 7, 2012
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Hello Everyone.

I have the following circuit set up, http://www.electro-tech-online.com/...ck-understanding-combination-lock-picture.jpg....
.....

however, I need to know how the NPN transistor works. Mainly, why is there a transistor there? I mean, couldnt I just connect the Solenoid directly to the output of the last flip-flop? Initially, the last flip-flop gives an output voltage of 12v. So R11 reduces this voltage suitable for the base of the transistor --- but doesnt the transistor then just increase the voltage back up to 12v.....>>Argghhhh, help, please :)


Thanks
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Post the image here. I can't view that page (and I'm not going to sign up so I can)

edit: The question is probably "The flip flop can't supply enough current for the solenoid"
 

davenn

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Am already a member of that electronics forum ;)


attachment.php


cheers
Dave
 

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davenn

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........however, I need to know how the NPN transistor works.

Its a PNP transistor rather than a NPN type. If it was an NPN then the collector would be connected to the +12V rail not the emitter

Mainly, why is there a transistor there? I mean, couldnt I just connect the Solenoid directly to the output of the last flip-flop?

Steve, gave the answer to that question

Initially, the last flip-flop gives an output voltage of 12v. So R11 reduces this voltage suitable for the base of the transistor --- but doesnt the transistor then just increase the voltage back up to 12v.....>>Argghhhh, help, please :)

Its not so much a voltage thing but a current thing. R11 reduces the current to the base of Q1. Although there may be 12V on the output (Pin1 of IC2b) its probably not enough to drive a solenoid. Using the transistor, Q1, does 2 main things...
1 ... it buffers the output of the IC from any large voltage/ current transients caused by the solenoid
2... it allows a larger current to be provided to the solenoid from the +12V rail via the transistor
3 ... it also allows those extra components R12, C2, D1, D3 to be added for reset feature.

Steve may add more...

Dave
 

TedA

Sep 26, 2011
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goodanna,

I think the previous answers have made clear why the transistor is necessary. I question whether the transistor is sufficient.

Nowhere do I see mentioned any specs for the solenoid. Do you have any? Particularly, we need to know the DC coil resistance.

Even with the transistor in the circuit, you still have a very limited amount of current to drive the solenoid.

I speculate that the solenoid you have for this project will draw far too much current for the circuit shown.

If there is much current involved, you want to run ordinary transistors at a forced beta, or current gain, of about 10. The base drive is about 700 uA. So the circuit might be looking to drive about 7mA through the solenoid. That would mean a 1600 ohm solenoid winding. And a very, very, small amount of force from the solenoid.

With a sensitive solenoid, and a relatively high gain transistor, the circuit may work OK, but the drawing is telling you to use "Almost any small PNP…".

A darlington transistor, and a lower base resistor value might get you close to where you need to be. Or you can use a MOS FET. Or several transistor stages.

You might look up the coil resistance and voltage ratings for your particular solenoid, or just measure across the coil with a multimeter.

Ted
 

davenn

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goodanna,

I think the previous answers have made clear why the transistor is necessary. I question whether the transistor is sufficient.
Nowhere do I see mentioned any specs for the solenoid. Do you have any? Particularly, we need to know the DC coil resistance.
Even with the transistor in the circuit, you still have a very limited amount of current to drive the solenoid.
.........................

Ted

well we dont know the limitations of the current available. That all depends on the type of 12V PSU, it may be capable of many amps. What info is needed, as you said, is the resistance of the solenoid coil. Once we know what current the solenoid needs, then we can select a transistor for Q1 that will handle switching that current requirement. There's plenty of options available.
We also have to recognise that the designer has stated the type of PNP transistor 2N3906, so one would have to assume, initially, that its within the current specs of what's needed to drive the solenoid.

Dave
 
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