My fans are not working on my car because i have ECU problem.So i am trying to set something up so they will kick on at a certain temperature,and my plan is to piggy back off of the temperature sensor on my car.
Now how the sensor works is as the temperature gets hotter the voltage drops.
So my question is.Is there such a thing as a relay that sends a signal when the trigger drops below a certain voltage? But the highest voltage a have seen on my temp sensor is around 2v and it drops from there as the temp gets higher.So the relay needs to have a very low trigger voltage.

I hope this makes sense to you.Thank you


ANSWER: The approach you are suggesting is very risky because the voltages around the temperature sensing system are probably not that exact and a relay that will drop out when the voltage is at a certain point is not a reliable engineering solution.  You can try it but you may find it difficult to find a relay that stays ON till the volt falls below 2volts......  

Better solution is a transistor operated relay.  The transistor can be biased to switch at the voltage point you have selected; the switching of the transistor to either OFF or ON will operate or release the relay.  The relay contacts can be either NO or NC which ever you need to operate the fans.

Just google this phrase "transistor switch driving a relay" and you will see many, many transistor relay drive circuits come up.

Hope this sets you off to a workable solution. Let me know if you need more.

---------- FOLLOW-UP ----------

QUESTION: That makes a lot more sense,and would work great.Excuse my ignorance,but im having a hard time finding the right transistor for my application.
By my understanding i need a transistor that will turn off once the base reaches a certain volts (0-2v)so that way when my temp gets hotter the volts drop down to say 1v and my transistor turns on sending a signal to my relay.
Or i could get a transistor that turned off once it dropped to 1v,and use the NC contact or the relay.
I would rather the first option,but both will work.So what im looking for it an adjustable base volt transistor,because i dont know at what volts i want the fans to come at so im going to have to do a bit of trial and error.Do they make such a thing?

Thank you

Any garden variety transistor will do so long as it has current rating for the current rating of the relay.  The sensitivity to voltage turn on point is a function of the biasing and gain setting of the transistor circuit.  If you want a 2v switch the biasing of the transistor base circuit can be set by using the resistors to turn on the transistor at 2 volts.  For example, if the supply voltage is 12v to the circuit, you could use a 10k resistor from 12v to base and 3k from base to ground.  Or any ratio that will give you a 2 volt ratio plus about 1v which is the voltage from emitter to base.  If you put in some emitter resistance you will have to add some more resistance in the biasing circuit but the transistor will be safer operating point with some emitter resistance.  It is all ohms law calculations.  Hint:  The transistor will turn on as soon as the base voltage is about  0.7v greater than the emitter voltage.  And you have the relay in the collector circuit of the transistor........   Good Luck.  


All Answers

Answers by Expert:

Ask Experts




Electronics questions about AC, DC and digital theory.


Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

BSEE University of North Dakota

©2017 About.com. All rights reserved.