Help with Differential Temperature Switch

[jfuredy]

I have been bouncing this idea around in my head for years, but I have never been able to find enough information about how to make it work to make me comfortable enough to try it.

I am trying to build a fan controller for my attic that will turn on the fan in the attic if the temperature in the attic is greater than the temperature outside.  I think this is a much better solution than a simple temperature switch to activate the fan when it's "hot" in the attic, especially here in AZ where the ambient air temp can be 115*F.

I suspect that what I want to use is an LM358 to compare the signal from two temperature sensors, and use the output of the op amp to trigger a relay tyo activate the fan.  I know the logic that I want, but my problem is in determining all of the exact parts that I need to put together to make it work, including pull-up/down resistors, diodes, etc.

Can anyone direct me to something that can do this (off the shelf would even be fine) or to a circuit diagram for a circuit to do this?

Thanks in advance,
Joe

[Rob]

Welcome to the boards.

Do you know about the LM335? It's a direct-readout temp sensor. So your comparator has one of these at one input, and a pot at the other for your temp setpoint. So two chips for the whole enchilada, plus the relay you mention. I'd sort of expect you to need a transistor to handle the current to close the relay contacts, and a clamping diode on the relay coil, but we're still talking about a <$20 project.

[jfuredy]

Yeah, I guess it would be something like that.  But I don't want it to react to a temperature setpoint.  I want it to trigger the relay only when it is hotter in the attic than outside.  So I think would need two of these LM335s, each feeding into one input of the LM358?

It's the other stuff like the clamping diode that you mention and adding some resistors? to add some hysteresis and a pot or two to adjust the start and stop temperature differentials that I don't know how to hook up.

I was finally able to find this circuit http://www.radiolocman.com/electrical-engineering/circuit-cache.html?di=11169 .  Does anyone have any comments on using the circuit in Figure 2?  Does this include any hysteresis to avoid relay chatter?

Thanks again for your guidance,
Joe

[justDIY]

you won't find a circuit any simpler than that, without cheating and using a microcontroller!

most comparators have some degree of hystersis in them, but I don't know off the top of my head how you'd add more easily.

[cpemma]

LM335 and similar semiconductor sensors would do the job, but they're a bit expensive compared to two similar thermistors which would also work in a much simpler circuit. At worst you'd need to add a series trimpot to null out the two's tolerance variation at around the operating temperature.

Looking at the link, I wouldn't start from there; he's had to go complicated with a PNP switcher and biasing resistors as the 741 won't swing below about 1.5V so won't turn an NPN switcher off without extra components. An LM358 will swing down to 0V and is cheap and common.

A bit like this (the CA3140 will also work)



but the second thermistor in place of VR1 and a 10k pot between the two, wiper to inverting (-) input, to balance the thermistors.

Hysteresis can be added with a high-value resistor between opamp output pin and R1/R2 junction - it will be set in parallel with either R1 or R2 (depending on whether the opamp output is high or low) changing the reference voltage on the non-inverting (+) input whenever the switch operates.

If you're switching the fan through a relay, add an inverse diode across the coil as shown at the link.

The circuit's expanded on here.