Hello Everyone:

I am a complete newbee never built anything electronic and until 2 mos ago thought a diode was something that happened when you passed away and owed someone money - you "die owed" .

Ahem well... in any case I want to build a differential temperature controller. Heres what it should do - there are two temperature sensors "A" and "B" and two 24 volt zone valves "X" and "Y". WHen the temperature difference is less than 5 degrees then valve 'X' should open and valve 'Y' close and when the temperature difference is greater than 5 degrees then valve "X" should close and valve "Y" open.

Using a prexisting 24 volt transformer to power the system. I would then go to two thermistors. THese would then go to a voltage comparator. THe voltage comparator would then act on two transistors , one an NPN and the other a PNP to have opposite effects thus allowing one valve to open and one to close . AN LED to show me which circuit is active and a resistor before each LED so it doesnt get fried

PLease see pic.



OK so any suggestions on what type of voltage comparator to use why this wont work, what else I should do etc etc would be most sincerely appreciated. Remember I am a newbee

MIke

 

Well I just found one problem the thermistors will need resistors too so they dont burn out

mike

 

Is there a way to get the image into the thread?

 

save the .bmp as a .png file. then you can use the larger version in the thread.

 

.png images are much preferred. They are very compact compared to .bmp and .gif, and not "lossy" like .jpg.

Once you upload the image to the forum, right-click on the link that will appear, and post that as an image in the thread.

 

OK I have added two voltage potentiometers before the thermistors will post pic soon

mike

 

well heres a bigger problem the zone valves are 24 volt AC - so I guess its back to the drawing board - will have to use relays instead I guess

 

HI Guys well I revised the idea. I have a 12 ac dc off the shelf converter and I will run that to the transisitors and use a 12vDC 24vAC relay

Only thing is I cant find a relay liek that - do they make em ??

MIke

 

There ain't no sucha thing as a coil that will use 24AC just the same as 12 DC. If you want to use 12 volt DC relays with a 24 volt supply for the coils, just put in a bridge rectifier, a capacitor, and a resistor to make the 24 AC into 12 DC (one for each relay).

If you're talking about 24 VAC contacts, forget everything I just said. The contacts don't care about the coil voltage.