# Need help to understand this circuit

Discussion in 'General Electronics Chat' started by sonda, Dec 20, 2012.

1. ### sonda Thread Starter New Member

Oct 30, 2012
10
0
Hi,
can any one please tell me, that is this circuit is a correct one? is this a non inverting one? thank you.

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2. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
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The circuit would work - need to know what you think the circuit does...

Here's a start.
The LM339 is a comparator. The two 10k resistor's form a voltage divider to set the voltage on the - input to 1/2 the supply voltage.

The transistor is PNP transistor.

Look up comparator and PNP transistor on this site.

One more hint:
The 15k ntc is a variable resistor which varies in resistance with changes in temperature.
ntc = negative temperature coefficient.

Last edited: Dec 20, 2012
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3. ### sonda Thread Starter New Member

Oct 30, 2012
10
0
Hi ,
thank u for your kind reply. i m going to use this circuit as cooling device, what i didnt understand is why it is controlling the + side? i dont know that what i m mention is right or wrong. i saw in many slimier circuits controlling the - in the op amp. this may be a pointless question, if so i m sorry. thanks again.

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
No need to apologize! You won't learn if you don't ask questions
The LM339 is not an op-amp, it's a comparator.
The + and - inputs are used differently depending on what the circuit is used for.
A comparator compares the voltages on the + and - inputs.
In this circuit the - input is set at 6vdc by the two resistors.
When the + input rises above the - input the output goes High,
when + is below -, output goes low.

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5. ### mbohuntr Senior Member

Apr 6, 2009
431
32
Look up "Voltage dividers" and how to calculate the voltage drop.(you will need Ohms Law)
Notice the voltage at each input. (hint) When the resistors are equal, the voltage is cut in half. Now the NTC resistor is greater than it's partner, but will become less so as the temp goes up. As the resistance changes, the voltage at the input will grow until it exceeds the other input. When that happens, the output will change state.
Note that this particular circuit uses a 339 output that actually "sinks" current like a ground. The transistor conducts(turns on) when it's base is negative "PNP" whereas an "NPN" type transistor conducts when it's base is positively biased. The arrow on the transistor tells you it's type.

Ask away, it's how you learn...

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6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
7,517
1,240
As the temperature gets hotter the output of the LM339 goes low and triggers the transistor which puts the fan on, when it cools down the ic turns the transistor off and so the fan stops...

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7. ### GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,785
I am guessing your problem is that the transistor is not turning off (that is, the compariitor's output is not getting a high enough voltage to turn off the PNP transistor).

Comparitors usually need some help pulling high, then they can pull low on their own. To help them pull high, the output is normally connected to +V rail with a 4.7k resistor. In your case, the transistor is preventing the output from seeing the help from the + rail. Try adding a smaller resistor from the base of transistor to V+ to help it pull high.

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8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
673
I agree that a resistor from base to emitter should always be used in this type of circuit.
I suspect the fan current requirement will demand a base resistor lower than 10k. We need to know how much current the fan requires.

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9. ### sonda Thread Starter New Member

Oct 30, 2012
10
0
Thanks guys...
i try this out and let u all know if there is any trouble... i did understand the circuit now,, many thanks again..