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fan speed control with temperature

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pankajwillkillu

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OK , directly expressing the problem scenario .
I have a temperature sensor [IC LM 35], comparator [IV 741 opamp] , a fan [18V D.C fan], thyristor [SCR TYN612M].

Now, i want to use them over to form a circuit capable of controlling my fan speed in proportional to temperature.
Is there a way out to form a simple but effective circuit out of this?
HELP !



i know what to do but don't know how to do (eg, i don't know what values of L-C pair should i employ for simple commutation of SCR)
 

The temperature sensor puts out a voltage that is a linear response to the temperature. That voltage can be compared to a reference voltage that correlates to a temperature through the op amp. For example, if the output of the temperature sensor was 10mV per F and you wanted it to kick on about 50F then you would want the reference to be 500mV. That feeds into your thyristor to switch on the fan which is the load. That's one way to do it. There maybe some other translational things from the output of the opamp to the thyristor but that I am not sure of. Hope that gives you an idea.
 
Audioguru said:
The SCR is a DC switch that cannot control the speed of a DC motor.
Click to expand...

As SCR needs to be ruled out of scene i am thinking of using BJT transistor with enough rating to start working with LM35 output being fed to its base.
Will it work?

IMG_20140322_213417.jpg

If yes, then what rating of transistor should i choose and are there any other pheriferals(resistors,capacitors etc) required
 

You forgot to tell us how much is the maximum current used by the fan. You show a resistor in series with the fan so I guess you want to limit its maximum speed and limit its maximum current.
You also show a little transistor with a maximum allowed heat dissipation of 0.625mW.

Assuming that the resistor reduces the maximum fan voltage to 12V and assuming that the fan has a current of 200mA at 12V then when the transistor reduces the fan voltage to 6V, the current is 100mA and the transistor heats with 6V x 100mA= 0.6W then the transistor will be very hot.

The LM35 has an output voltage that is too small to drive a transistor. The LM35 can drive a single-supply opamp that can drive a transistor.
 
pankajwillkillu said:
Is there a way out to form a simple but effective circuit out of this?
Click to expand...

A control based on linear conduction of a transistor can achieve design requirement for 'simple' as you determined, however such approach increases overall power dissipation, due circuit consists of dissipate surplus heat on transistor.

A more 'effective' concept would require some pulsed conduction of transistor as done with PWM so that could work on saturated region of conduction.



+++
 
Audioguru said:
You forgot to tell us how much is the maximum current used by the fan. You show a resistor in series with the fan so I guess you want to limit its maximum speed and limit its maximum current.
You also show a little transistor with a maximum allowed heat dissipation of 0.625mW.

Assuming that the resistor reduces the maximum fan voltage to 12V and assuming that the fan has a current of 200mA at 12V then when the transistor reduces the fan voltage to 6V, the current is 100mA and the transistor heats with 6V x 100mA= 0.6W then the transistor will be very hot.

The LM35 has an output voltage that is too small to drive a transistor. The LM35 can drive a single-supply opamp that can drive a transistor.
Click to expand...


I am using 18V DC motor with Continuous Current(A): 0.15A free-run , see specs here of motor
https://kinmore.en.alibaba.com/product/736034524-212371283/Permanent_Magnet_18V_High_Speed_Micro_DC_Micro_Motors_RS365_.html

regarding power supply, i may be using eliminator with standard 2-3 Ampere current suppy which just maintains the voltage.
So, a good transistor for this is my need.
I may use opamp IC741 as suggested that LM35 has lower values of output ; something similar to this


temperature sensor.gif
 

Just for advice that although you did not mentioned what is properly the target for cooling ( and consequently, did not mentioned its requirement ), you must consider that such control system is based on direct loop back cut off of heat source, what means that temperature measured on LM35 intrinsically displays a delayed value, inherent for heat diffusion on each material junction, and potentiometer must be adjusted a little bellow maximum allowable temperature.






+++
 

Before designing a circuit, you need to complete the specification.

"controlling my fan speed in proportional to temperature" suggests a linear to temperature to motor voltage relation, but the characteristic points must be specified.
- a temperature for 0 V motor voltage
- a temperature for 18V motor voltage
 

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