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HELP - resistors and dip-switch


Virus
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audioguru

I have built the unit (Reply #19 on: May 12, 2006, 05:44:34 PM ) on a breadboard, 1 ste time not lucky.  The mosfet gets steaming hot instantly, if I clip + 12v to the circuit, with no out put to the led that I connected. I can hear the 555 oscillating. How can I test the opamp to see of it is working.

I have checked and again checked and again checked and …. the wiring can’t find the fault.  Should I rip it all up and redo it? Build it to a PCB ?

Attached photos.

Please help.

Thanks Virus

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I can't remember how much current your fan draws, but it is a lot more than what the lousy contacts on your breadboard can handle.

What resistor value did you use for the 2N3906 current source?
Is the 555 oscillating?
Does your power supply voltage drop too low when it is loaded, therefore the Mosfet doesn't have enough voltage on its gate to turn on properly?

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I can't remember how much current your fan draws, but it is a lot more than what the lousy contacts on your breadboard can handle.


I only had the led connected, no fans at all, wanted to see if the circuit at least is working.

What resistor value did you use for the 2N3906 current source?.


Resistor RF = 90Kohm


Is the 555 oscillating?


Yes, for the 555, I can hear it load enough.

Does your power supply voltage drop too low when it is loaded, therefore the Mosfet doesn't have enough voltage on its gate to turn on properly?


I use the 0 - 30 VDC power supply from this forum, it does not even blink.


audioguru,

is the theory of this circuit good ???,

regarding the post of allvol

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It is a lousy circuit.
The transistors T1 and T2 aren't needed.
Pin 3 of the 555 can drive a resistor in series with the timing capacitor and the triangle wave on the capacitor can feed the input of the opamp.
I would use a faster, newer opamp or a proper comparator instead of a slow opamp that causes the Mosfet to heat-up. 

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audioguru

Thanks for the circuit update, will build it this weekend and give you feed back.

What should the value of D1 be ? ???

I will bug you a bit more later, I would like to get rid of the RDC 10K and rather use a solid resistor, with a 10 K thermistor, iaw, make it a circuit with a solid predetermined value for the temp sensor to react on.

Thanks

Virus :D

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I could not get that circuit to work.... and probably never would.

Which circuit? Did you try the circuit I fixed?

I have a DC motor speed control circuit using PWM that works perfectly. I use a dual opamp as an integrator and schmitt trigger to make a triangle wave and use another opamp as a comparator of the triangle wave to the DC setting of a pot like in this circuit.
The 555 circuit that I fixed should work perfectly the same. 
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Guys

I HAVE FOUND THE PROBLEM, I had the Mosfets supply and drian short-circuited on the breadboard as soon as the circuit switches on. DEAD SHORT INSTANT HEAT.

With that sorted out I have successfully built the circuit in post number 19 (with the MC34071 opamp) and the one in number 34 audioguru’s circuit.

I choose for now to go with the audioguru’s circuit, with less components, and is still on the board. As I got the same results from both circuits.

I have replaced the 4.7k resistor with a 1k resistor, the fans where making a very lousy high pitch sound, then measured with my MM, I get 21kHz, on pin 6 of the 555.

First I replaced R5 with a 10K resistor (because the thermistors value is changing and need a fix value to work from), and adjusted RDC to give an output of 4.5 volts, with the fans just start turning.

Then I replaced R5 with the 10 k thermistor, the output voltage varies with

MC34071-D.pdf

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Hi Allvol,
"My" 555 circuit is the classic "triangle wave into a comparator with the other comparator's input connected to a variable DC voltage" PWM circuit that usually uses a opamp integrator and a Schmitt trigger oscillator to make the triangle wave.
The 555's timing capacitor is charged and discharged symmetrically by a resistor from the 555's output pin 3. The capacitor is also connected to pins 2 and 6 for its threshold voltages and has a curved triangle wave across it.

"Your" 555 circuit with its steering diodes also works well for PWM.

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Hi

P1 is the 555 circuit of audioguru and measured on my MM. (Was running the fans etc.)
P2 is the “textbook version” of allvoll and as measured after replacing the 47uF cap with a .47uF cap. My MM does not read if it is to slow. Yes, I did get the LED to flash, with the 47 uF cap.

As mentioned before both circuits are working. The original 555 with the opamp and the 2 transistor, and the audioguru version are working. The fans are running OK.

Now lets get the range with the thermistor in place, where to from here?

Thanx

Virus

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Allvol

I had a ball playing with the fan’s.

Did the 10k trim pot as per your diagram fan1.gif, works excellent, can get the fans down to just- just- just- turning (long pulses). I also left the other trim pot in place, between the 2 pot’s and changing the cap C2 with different values, what a range?, with some setting nice (annoying) music on the fans !

I have a question though, how much of what, here, will do the trick?

This project started out with resistors and a dip switch, because of the circuit in

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Changing the DC voltage of pin 5 on a 555 changes both the frequency and the duty-cycle of the pulses. Using the steering diodes circuit or the triangle-wave circuit changes the duty-cycle of the pulses without changing the frequency much. Then the frequency can be high enough that it isn't audible.

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