JLD 612 for a lp gas valve

[topdogs]

I will be installing this controller and the PT 100 thermocouple in my new smokehouse to control the gas valve. In prior posts, this was talked about but not solved. Can this controller be used alone without a seperate SSR to control a 24 volt gas valve? If so, should I use the secondary output of the transformer (24V) as the power to the controller rather then the 120V primary? According to the controller specs this is not in the proper operating voltage of 85V plus? Will it still work with the 24V side and do I really need a seperate SSR? Thanks.

[richiem]

Run the controller from the 120VAC line. What comes next depends on three or four things --
1) the operating voltage of the gas valve -- is it powered by 24VAC? Or does it need DC to operate? Or does it need 120VAC?
2) the current the valve requires -- if it's more than about 1 ampere, then you should not use the internal relays in the controller, in my opinion -- but one of those relays can control a second, higher-current relay to then control the valve, and an SSR may be the best choice.
3) if the valve closes completely, how will you relight the flame? Is there a pilot light?
4) if you can't relight. how can you prevent the valve from closing completely?

This can be made to work, but these questions need answers.

I would set the system up to act as a simple thermostat with controlled hysteresis.

[topdogs]

Thank you for the help and here is more information.

1. The valve is 24 vac with and electrical rating of .23 amps. I will be using a stepdown transformer. Probably should've stayed at a 120 valve.?

2. The first will be controllong the smoker temp only. Can this controller be used by itself or do you think that it would be better to use an SSR. I don't mind the money part, I want it to be right the first time. I will also be installing a second one in series to the first for internal temp of the meat. Will this be a problem and can I run this one thru the same SSR as well or is that a bad idea?

3. This gas valve has a standing pilot with a thermocouple as a loss of flame safety device which is not an option but a necessity, and the first controller will be used as the smokhouse temp for a typical thermostat set up.... on and off with the main gas vave. I would like to have a max of a 5 degree temperature drop before opening.

4. In your opinion is there a better choice for a controller since I will need to step up the temps for different periods of time until the proper internal temp of the meat is reached?

Thank you again for your help!

[richiem]

OK on the 24V valve -- it's good to have a low voltage supply running such things and the isolation provided by the 120V>24V transformer is a good thing. The low current rating of the valve means that you can use the internal relay(s) in the controller to run the valve.

As to question 2, I'm not sure there's a way you can use two controllers to control a single valve. If I understand what you want to do, it sounds like this: Control the gas to keep the smoker heat within a 5 degree F range (I assume so the meat doesn't get too hot too fast). Then if the meat temp gets too high at that smoker temp, turn the gas off to keep the meat from getting too hot. So the control paradigm is run the gas to keep a constant temp in the smoker, then vary the heat to keep the meat at a constant temp.

Doing the first part is easy. But the second part is a challenge because of the thermal lag in the meat temp due to the thermal mass of the meat -- slow to heat up, slow to cool down.

I think the solution is to use two controllers connected to two gas valves in series in the gas line (note that I have no idea about how to control the lighting of the gas when using two valves) -- one controller sensing the smoker cabinet temp, and the second controlling the heat of the meat; it will require some careful measurements of heat rise and fall to get the temp spread (hysteresis) of each controller right. This is a challenging application -- if you get it right, you'll really have accomplished something.

As to the right controller, if you need to ramp up or down the heat for specific durations, then the JLD634 is the one you want.

You've got me out of my depth here as to connecting the two controllers and valves, but it seems to me that you want to have the smoker at a fixed maximum temp within a larger hysteresis spread so that the controllers aren't fighting one another -- this means having a large enough hysteresis spread, say 10 or 20 degrees, so that the smoker cools down enough to let the meat temp drop, then turn the gas off and on to hold the meat at a constant temp in a tighter spread, say 2 degrees F, and this second modulation will have a fairly long period. So it would work like this -- kind of thinking out loud here -- smoker cool, valve 1 and valve two are both on. Smoker reaches valve 1 limit, and gas turns off, but the meat temp keeps rising because the smoker is still hotter than the meat. As the meat reaches the valve 2 limit, if valve 1 is on, then valve 2 turns the gas off. the meat temp continues to rise, then begins to drop; as the lower limit for valve2 is reached it will open, and if the lower limit of valve 1 is also reached, the valves are both on and the heat begins again.

I think this would work, but as I say, I'm way out of my depth here.

Good luck.

[topdogs]

Thank you again for the help. I might have confused you by leaving an important item out. The second controller wired in series for the internal meat temp would have it's own pt100 probe and only have a maximun set point of lets say 154F, then it would open that relay and shut off the gas valve when it reaches the set point. While the smoker temp would have to be increased and should always be above the internal temp of the meat, with its own probe, it will be able to raise above the temp of the meat without opening the second controllers relay. Sorry about that! Two controllers each having their own probe but connected in series. I think this will be ok as I will be just linking the 24 volt power from the transformer to both controllers, then the valve for a conplete circuit. Then connect the neutrals and connect it to the gas valve just like a regular thermostat would be. It's late so I may be wrong. I may have to hook them up individually. I will fugure it out when I am not so tired. Thanks again and I will be checking back again for you input but after some sleep after work. Thank you again.

[topdogs]

Richiem, you posted in a different question that the controller should be connected to the secondary of the transformer. At 24vac, this below the normal operating voltage of the controller. Am I missunderstanding you? Do you mean that the controller should be connected 110 but the output should be connected to the secondary of the transformer? Thank you for the answers to this and our previous conversations. I hope you read the last reply from me it should explain things better.

[richiem]

The controller, if 120V model, gets power from the AC line. However, the relays in the controller need external power -- they are just switches that turn on and off. So to control a gas valve, for example, the 24V secondary of the transformer would route through the controller relay(s) to the gas valve. Sorry for any confusion in terminology.

Now, about the controllers in series -- I presumed that each controller would have its own probe -- that is the only way they will work. What isn't clear to me is the proper control hierarchy for temp settings. I've never tried to do this, so I'm in uncharted waters personally. So, connecting the relay of each controller, the gas valve, and the transformer sec. all in series should do the job.