# Thermistor

#### brendyboy

Joined Dec 21, 2005
27
My temperature control circuit uses a 1k thermistor which i understand has a negative coefficient. This causes the relay to switch on when the temperture gets too cold.
However I need it to work the opposite way, so that when the temperature rises it swtiches the relay in the circuit on.

WIll this just be a case of changing the thermistor to a positive coefficient or will I have to do more tinkering (which I cannot afford to do at present) with the circuit.

Is there any other thing i could do? i basically need an electric fan to switch on when the temp rises

many thanks

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by brendyboy@Apr 22 2006, 11:06 AM
My temperature control circuit uses a 1k thermistor which i understand has a negative coefficient. This causes the relay to switch on when the temperture gets too cold.
However I need it to work the opposite way, so that when the temperature rises it swtiches the relay in the circuit on.

WIll this just be a case of changing the thermistor to a positive coefficient or will I have to do more tinkering (which I cannot afford to do at present) with the circuit.

Is there any other thing i could do? i basically need an electric fan to switch on when the temp rises

many thanks
[post=16415]Quoted post[/post]​
Assumng that you are refering to the same circuit you attached in your earlier post, you might try swapping the position of the thermistor and R7 in your circuit.

hgmjr

#### hgmjr

Joined Jan 28, 2005
9,027
It just occurred to me that another solution to your desire to change the sense of the response of your circuit would be simply to move the wire from pin 5 on the relay to pin 3 on the relay or visa versa.

hgmjr

#### brendyboy

Joined Dec 21, 2005
27
Originally posted by hgmjr@Apr 22 2006, 10:48 PM
It just occurred to me that another solution to your desire to change the sense of the response of your circuit would be simply to move the wire from pin 5 on the relay to pin 3 on the relay or visa versa.

hgmjr
[post=16425]Quoted post[/post]​

You see I was getting very confused with the fact there are 3 pins. if im attaching a polarised fan then does the positive go to pin 3 or 4??

When i was tinkering with the circuit i found i couldnt get anything from either pins, instead i connected the negitive end to the negative pin on the circuit and the positive to tr3 im not sure how that came to be, but it worked.
Do you think there may be a faulty connection or bad soldering with the relay?

Can i just say i really appreciate your help as im desperate to get this circuit done so my project works!
many many thanks!!

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by brendyboy@Apr 22 2006, 05:56 PM
You see I was getting very confused with the fact there are 3 pins. if im attaching a polarised fan then does the positive go to pin 3 or 4??

When i was tinkering with the circuit i found i couldnt get anything from either pins, instead i connected the negitive end to the negative pin on the circuit and the positive to tr3 im not sure how that came to be, but it worked.
Do you think there may be a faulty connection or bad soldering with the relay?

Can i just say i really appreciate your help as im desperate to get this circuit done so my project works!
many many thanks!!
[post=16426]Quoted post[/post]​
From your comments above, I get the impression that you believe that the fan is receiving its power from the relay connection. Actually, the contacts of the relay are acting only as a switch that opens or closes under the influence of the thermistor based temperature detection circuitry. I don't know the power requirements of your fan but you will need to create a series circuit that includes the fan, one of the relay closures (normally-open or normally-closed) and a power source. The power source can be the same as the one powering the detection circuitry provided the power requirements of your fan are not too great.

If you have made any circuit connection hookup changes from the one in your original posting, you may need to draw your circuit and post it to clarify what it currently looks like.

hgmjr

#### brendyboy

Joined Dec 21, 2005
27
Originally posted by hgmjr@Apr 22 2006, 11:24 PM
From your comments above, I get the impression that you believe that the fan is receiving its power from the relay connection. Actually, the contacts of the relay are acting only as a switch that opens or closes under the influence of the thermistor based temperature detection circuitry. I don't know the power requirements of your fan but you will need to create a series circuit that includes the fan, one of the relay closures (normally-open or normally-closed) and a power source. The power source can be the same as the one powering the detection circuitry provided the power requirements of your fan are not too great.

If you have made any circuit connection hookup changes from the one in your original posting, you may need to draw your circuit and post it to clarify what it currently looks like.

hgmjr
[post=16427]Quoted post[/post]​
You are a f**king lifesaver man!!!!!

It worked! I know now why it didnt work and it was what you said about the relay not passing on power to the fan! I also just switched the pins so that the fan switched when the temp rose!!!

seriously mate!! I was one week away from f**king up my dissertation and you have helped me out a lot!! Seriously appreciate it!! Il def give this forum a mention in the bibliography!!!

Thanks man!! all the best

#### hgmjr

Joined Jan 28, 2005
9,027
Greeting Brendyboy,

I am glad you were able to figure out what was preventing your circuit from working just in the nick of time.

Keep in mind that getting your problem sorted out was a group effort by several knowledgeable AAC members. All of the members who took the time to reply, contributed to moving you closer to a solution. I just happened to be the last one in the series of responses that you received prior to your successfully diagnosing the problem.

Don't overlook the pivotal role you played in solving your problem. You are the one that had to interpret all of the suggestions provided and apply them to the problem you were facing. Kudos to you.

In the future, feel free to post your puzzling electronics problems here in the forum. The knowledgebase embodied in the member community of this forum is awesome and I continue to be impressed by the quality of the replies they provide.

Good Luck,
hgmjr