hello im new to this forum and need some help i have a pic circuit connected to a relay which has a DC motor which i need to control using temperature, does anyone have any tips on how to do this? Thanks

 

Couple of recent posts here and also many previous examples.
http://forum.allaboutcircuits.com/t...-reverse-a-2-wire-reversible-dc-motor.106925/
Max.

 

thanks for the reply max but i should of specified it was more the speed of the motor i was looking to control rather that the revering it
Alan

 

Your drawing shows a basic On/Off method?
What is the source and nature of the control signal?
Max.

 

Could use a thermister, neg temp coef., fixed resistor & voltage controlled PWM like this:
; resistor is top of pot, TCR bottom of pot. There are other forms in Bill's Blog?

 

I have a thermistor connected to a pic chip which is programmed to turn the relay on when the thermistor reaches 20°C. I tried connecting another themistor in the relay, in a potential divider set up but still doesn't seem to work .

 

You cant control the speed with a thermistor, you need a pwm or variable voltage supply.

pwm controller.

http://www.dprg.org/tutorials/2005-11a/

 

This sounds like you want to control a motor speed based on temperature, for example as temperature increases motor speed increases. Something like fan speed control based on temperature rise? Would that be where you are going with this? Should that be the case and you want micro controller control run an analog voltage from your thermistor or temperature sensor and have the uC output a PWM to the motor based on temperature.
You really need to better explain what you are trying to do.

Ron

 

here is a simple pc fan controller using a LM317 and thermistor


http://www.eeweb.com/blog/extreme_circuits/pc-fan-speed-controller-for-a-low-noise-pc

 

Thanks everyone who replied i used the link to the simple PWM circuit based on the 555 timer and connected the output pin on my microcontroller to pin 4 (reset pin) on my 555 and now the themistor connected to the microcontroller acts as a threshold temperature and the second thermistor controls the frequency of the 555. Im not really great at explaining things but i hope you understand thanks again for helping

 

Here is what I don't understand. It looks like (your drawing is very small and hard to read) you have the thermistor and resistor forming a voltage divider whose Vout will change with temperature, but it looks like that analog voltage is going to a digital input to your micro controller. This does not seem like you will have a reliable threshold. What micro controller are you using? The 555 RST is tied to a Q output? I don't understand that? Finally the 555 Out is driving a NPN transistor used as a switch for the fan. Wouldn't this be better if the fan was in the transistor collector rather than emitter? Finally you may want to place a 0.1uF cap across the motor terminals and a snubber diode.

Finally depending on the micro controller why not run the thermistor signal into an analog input and let the micro controller drive the fan through a transistor?

Ron