# read temperature, and control a valve actuator

Discussion in 'The Projects Forum' started by chiswilson, Dec 7, 2008.

1. ### chiswilson Thread Starter New Member

Dec 6, 2008
2
0
Hi all,
I think this is a fairy simple circuit I need to build, but I'm not sure. This is my first post here, but I'm SO glad to find these forums.
What I need to do is compare two temperatures, and depending on which one is higher, control a valve (in a solar water system). It is a 3 way valve, which only rotates through 90 degrees.

So, I need to use two thermistors to compare two temperatures. If temp 'A' is higher than temp 'B', then I need my valve actuator in position 'a'. If temp 'B' is higher, then I need my valve actuator in position 'b'. As stated, the valve only rotates through 90 degrees, so my valve actuator (or servo) or whatever I use, only has to rotate 90 degrees.

My thermistors will likely be standard 10Kohm thermistors (I believe that resistance is at 77 degrees F). I dont actually care what the temperature IS, I only care which temp is higher. I'm assuming the temperature part of the circuit is the easy part.
As far as turning the valve...I could use a Goldline actuator 'GVA-24', which apparently runs on 24Vac. However, I have not yet found the specifics on how to control it. I think it is a 3-wire device.
Should I use a servo motor instead? Maybe the Goldline actuator is in fact a servo (?). I realize that whatever I use has to have enough torque to control the existing valve.
If anyone can point me in the right direction, I would appreciate it.

thanks,
Chisolm Wilson

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The valve actuator is a motor itself which can rotate 2 and 3 way valves. You need a comparator circuit to compare the voltages of the two thermistors and output either a high logic or low logic according to which temperature is higher.

3. ### chiswilson Thread Starter New Member

Dec 6, 2008
2
0
Right, but I'm assuming I cant just feed a logic high or low directly into the actuator, can I? Dont I have to generate the 24 Vac and control the actuator that way?
CW

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
A voltage comparator can be used to signal the temperature difference. Depending on the current draw of the actuator, it might be ll you need to pull in a SPDT relay to control the valve.

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You can control a relay with the comparator which will control the actuator. The actuator will be supplied to whatever voltage it needs (between the relay limits of course).

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You will need some hysterisis on the comparator, or when the temperatures are close, the valve will be constantly cycling back and fourth which will rapidly wear out both the valve and the actuator.

It would help to have a fairly complete diagram of the system.