I have just built the circuit below

http://www.electroschematics.com/4788/temperature-controlled-dc-fan/

I need the output from pin 6 of the op amp to be variable, rising gradually as the thermistor warms up
instead of switching abruptly from a high to a low, I want to connect a meter to the output so I can
watch the needle gradually swing across the dial as the temp rises, what can I do here to achieve this please?

Neil

 

I have just built the circuit below

http://www.electroschematics.com/4788/temperature-controlled-dc-fan/

I need the output from pin 6 of the op amp to be variable, rising gradually as the thermistor warms up
instead of switching abruptly from a high to a low, I want to connect a meter to the output so I can
watch the needle gradually swing across the dial as the temp rises, what can I do here to achieve this please?

Neil

Can you post the data sheet for the thermistor?
What voltage are you using?

 

The 741 opamp is wired as an analog comparator.
For an analog amplifier, connect a resistor from pin-2 to pin-6. You will have to play around with the value of the resistor to get the right amount of gain. Try 10k to 100kΩ for starters.

 

Thankyou Mr Chips.....

Ron I don't have any info for the thermistor since I stole it from work.....all I can tell you is it is a 10K before connecting it to a circuit.

 

You could also put a low-pass filter (a capacitor to ground, after the resistor) on the output so that it's damped. This would slow the transition when it switches, but it would still flip from on to off. If you want proportional control, you need an amplifier and not a comparator, as noted.

 

You probably have a 10K at 25C (77F) thermistor.

If you go to vishay.com and download document 29049 and look on page 10, you will see resistances for this part at various temps. Measure your part's resistance at 0c, in a warm room (25c) and at 100c (then make a cup of tea).

Your data should match.

Armed with these numbers, you should be good to go.

Note the melting point of ice is 0C, not the freezing point of water.

F = 1.8C + 32 or twice C less 10% plus 32

Vishay.com has a lot of info on thermistors.

 

Here is a chart for a 10K Ntc, if you have a dvm set it to Ohms and measure it at room temperature (25C) and boiled kettle water (100C), that will give you an idea which thermistor type it is, then you can compare with the chart.

 

The 741 opamp is wired as an analog comparator.
For an analog amplifier, connect a resistor from pin-2 to pin-6. You will have to play around with the value of the resistor to get the right amount of gain. Try 10k to 100kΩ for starters.

awesome.....this is working a treat

thanks Mr Chips, I started with 10K..... lots of playing around with variations and component values will take place over Christmas
while the rest of the family are endulging in Christmas TV, and afternoon naps.

Neil.

 

Glad to hear that. You're welcome.
I'll go have an eggnog to drink to that.

 

My recipe for egg nog is one part whiskey.

 

Hello there,

Actually you dont really need an op amp either if you just intend to measure with a meter unless you want to set the scale and offset.

The simplest example is a 10k fixed resistor in series with the thermistor, and connect that to say a 5v DC voltage source with the thermistor on the bottom and 10k fixed resistor on top (fixed resistor connected to +V of the DC source, thermistor connected to ground).
When you measure the voltage at the junction of the thermistor and fixed resistor you will measure half the supply voltage which would be 2.5v with a 5v power supply. As the temperature rises, the voltage will decrease, but if you want the voltage to increase then just connect the thermistor to +5v and the fixed resistor to ground, then you will measure 2.5v at 25 degrees C and as the temperature rises the voltage will rise also. There is a simple formula if you want to use it.

If you do use the op amp then you can set the gain and offset as well. The offset will give you 0v at 0 degrees C if that's what you want, or some other voltage if you want that instead.
The circuit wont be that complicated but the choice of op amp has to be right.

It would be good to state what the range of temperatures you want to measure is.

Connections for simplest circuit:
+5vdc o----TH---+---10k---o GND

measure the voltage at the + to ground ("TH" is the thermistor).