Differential Temperature Controller

[mburman]

Long ago in a life Far, Far Away...

I took an electronics course... but that was High School... and now it's more than 20 years later...

I'm trying to build a Differential Temperature Controller for a solar water heater. The idea is that the controller measures the temperature at the solar panel and at the storage tank. When the temperature at the panel is higher than in the tank, a relay closes and turns on a circulation pump. After the temperature is balanced the pump turns off. I found a schematic here... http://www.redrok.com/images/diftemp.gif and used the one on the right (the one with thermisters). From that I made the circuit diagram. I built it up on a board and it looks amaturish, but all the connections are good. Here's my problem... I'm not getting sufficient voltage from the op amp output to switch the relay. I measured with a Voltmeter and the output is showing the voltage thru the relay coil. When I disconnected the relay from the output I can measure the voltage and see it change with the temperature difference so it is working...

Now... my question... How can I get the output high enough to switch the relay without being so high that the relay won't turn off when the temp calls for it to be off...?

Thanks for all suggestions!!

[bertus]

Hello,

The picture at the redrok site does not show.(cannot connect error).
The picture you posted in the site is very tiny.

Bertus

[mburman]

Sorry... resized the image... if its too small still let me know... thanks for looking! I don't know why the link to the schematic didn't work... it works for me...?

I appreciate your help!

[wayneh]

I think you need a transistor (MOSFET would be good) to amplify the current from the op-amp to something high enough to power the relay. Or, your relay is shot. Does it trigger when activated manually?

The diagram is still confusing. Are there unshown connections to power the op-amp?

[someonesdad]

You haven't given a schematic, at least one that shows the parts you're using and the connections. The question is the relay -- what is the current its coil requires and what load is it switching?

If you're switching a line voltage load with the relay, a solid state relay could be used as they can usually interface directly with the output from an op amp or comparator. If not, then you'll need something like a transistor like wayneh suggested. Remember to put a diode around the coil to help clamp transient inductive switching spikes.

[mburman]

The relay does work and all of my connections are on the diagram. The schematic i built it from does not show powering the op amp but I did temporarily try it. It did put out 12 volts when powered, but the output never changed when the thermisters were warmed. Without the power to the chip I get 6 to 8 volts depending on the temp difference between the thermisters. So the circuit seems to work... it just doesn't reach the 12 volts the relay needs to trigger. If I added a transistor (or mosfet) where/how would I need to connect it and what type would I need to buy?

Thanks again for the help!

[iONic]

Here is the schematic from the site:

[wayneh]

Originally Posted by mburman

The relay does work and all of my connections are on the diagram. The schematic i built it from does not show powering the op amp but I did temporarily try it. It did put out 12 volts when powered, but the output never changed when the thermisters were warmed. Without the power to the chip I get 6 to 8 volts depending on the temp difference between the thermisters. So the circuit seems to work... it just doesn't reach the 12 volts the relay needs to trigger. If I added a transistor (or mosfet) where/how would I need to connect it and what type would I need to buy?

The op-amp MUST be powered for the circuit to have a chance. It's meant to act as a comparator, with the output swinging all the way to one rail (+12v) when the "+" input voltage exceeds the "-" input voltage, and to the opposite rail (ground) when the inputs reverse polarity. The datsheet will specify how much current it can source (acting at "+") or sync (at "-") "Hysteresis" is the middle ground, where switching would be chaotic back and forth if you don't have at least a little hysteresis to quiet it. A small feedback from the output gives a little hurdle the input has to overcome to make a switch.

If you didn't see the output change, first make sure you're looking at the right pins (check the datasheet) and then look at the two inputs and the output. If it really isn't working, try one of the other op-amps available on the same IC.

If the relay needs more current than the op-amp spec says it can handle, you'll need the transistor. Let's get the op-amp working first. Try using an LED with a 1K resistor in place of the relay, until the rest is sorted out.

[iONic]

In other words try this:

[wayneh]

Sweet! What he said.