Technical Kiln & Controller Help Needed

[Passing Glass]

Hello, I'm hoping someone with some electrical knowledge and/or kiln or controller building experience can help me out.

I have an Aim brick kiln with an infinite switch and I have built a controller for it using a Fuji "brain". I leave the kiln infinite switch on max and I expect the controller to do the heat adjustment as needed.

The controller works fine most of the time, but when the kiln gets pretty hot (like 900), sometimes the kiln continues to heat way past the setpoint even though the controller is signaling to turn the power off - it just keeps going and the power does not cycle off. The kiln will continue to heat forever. It's as if the kiln "locks" the power and just keeps the circuit open and keeps draws power even though the Fuji is signaling to stop the power (and the Fuji IS signaling to stop the power, I've checked it). Also, when I select the "Autotune" function on the Fuji, it almost always locks the power on sooner or more readily than when the autotune process is not running. Not sure if that matters or not. Now, here's a twist. If I turn the infinite switch on the kiln down to maybe 5.5 or 6 (out of 10), the kiln and controller seem to operate fine, but of course the temperature will never get hot enough for my use since the power going to the kiln is being attenuated by the infinite switch. Odd, huh?

Here is an experient I did. I connected my kiln and a small test lightbulb to the controller at the same time with a plug splitter. When the controller sends power, the light bulb lights up and the kiln also receives power. When the power "locks" on and the kiln continues to heat past the setpoint, the test light stays on with the kiln, as you would expect. Then - here's the important part - when I turn the kiln off with the infinite switch so it draws no power, the test light begins to turn off in about 1.5 seconds and fades to black over about 1 second. It does not go black instantly as if you flipped a switch in your house. So when the kiln draws no power through the controller, the circuit breaks as it should. That's why I use the terminology that the kiln "locks the power on".

Here is another weird finding. If I reach behind my controller box and turn the controller off, so that the fuji screen is totally dark. That test light I mentioned tries to light up. It is very very dim, barely noticable, but it has a flicker of power about 2 times per second in a consistent evenly spaced rythmic pattern. I'm not sure if that is a normal power analysis "ping" from the Fuji or if that should not be happening or what it might indicate. Again, the fuji screen is dark, and the controller is "off" at the time.

I am using a solid state relay in the controller and the symptoms indicate to me that the relay is probably the problem. It seems to me that the relay is not closing the ciruit when instructed by the Fuji because of the high power being drawn through it by the kiln and is probably not behaving as it was designed.

Does what I'm saying make any sense? Any ideas on what might be going on or suggestions on what to do? Also, do any of you have any advice on who I might PM or talk to about this. Who could help me? Thank you!

[Dale M.]

What sort of relay are you using with controller to do the actually turn on/off power to kiln, is it a SSR (solid state relay) or a mechanical relay the clicks while doing on/off/on/off operation....

IT sounds like you have a relay may be sticking in "on" position ( may want to monitor "relay control circuit") ... Or you have a programming error....

Also can you supply wiring diagram of system... IT may be an error in wiring that is causing a problem..

IF it were my system, I would want to monitor control circuit for relay, that is where the real indicator would be for if its a relay or controller (programming) problem...

Dale

[bhhco]

Originally Posted by Passing Glass

...sometimes the kiln continues to heat way past the setpoint even though the controller is signaling to turn the power off - it just keeps going and the power does not cycle off. The kiln will continue to heat forever. It's as if the kiln "locks" the power and just keeps the circuit open and keeps draws power even though the Fuji is signaling to stop the power (and the Fuji IS signaling to stop the power, I've checked it)... :

Well... you have taken a very logical approach. I undertand the problem is the kiln continues to heats when it should not?

The only thing that can give power to the kiln is the controller (which is a PID and a SSR). If you are certain the PID is signaling to stop power... then the SSR is not responding, and it is giving the kiln power. You did not say how you verified the PID was sending a 'off' signal to the SSR, but we will assume the PID is working as expected.

There are only 3 possibilities, and as you suspected they are all 'relay', i.e., SSR oriented.

1. The SSR is wired to the wrong terminals on the PID. You may be driving the PID with the wrong variable voltage solid state output. Not likely... but could happen. If so... see PID manual... and rewire if needed.

2. The SSR is mismatched for the PID. Possible... more likely... but not most likely. Note the SS drive currents in the PID manual and determine if the SSR matches. If not... replace SSR, and see next item.

3. The SSR gets too hot and becomes erratic. Most likely the problem. SSRs are not mechanical relays... they can get too hot if they don't have a heat sink, and if the amperage draw is too high. The SSR should be rated to carry the amperage required to operate the kiln. If not... replace SSR with properly amperage rated SSR -- with a heat sink attached.

As for everything else... the PIDs use extremely complicated algorithyms that have no resemblance to common sense. In some modes the will go off into neverland and borg the system... to 'learn' the system they are controlling... even allowing the heat to climb way past the set point... until they get what they want to know. It can be frustrating programming a component without the actual source code to know what it really does. Suggestion: Pick the simplest routine and use it exclusively.

You might do a little googling to learn how SSRs work, if your interested. They are strange little beasts.

Real men use mechanical relays .

Me

[Passing Glass]

Hey thanks guys. I think bhhco has the solution after I have talked with another person regarding the problem. I did not mount the SSR which I am using to a heatsink and I think that is causing the SSR to behave eratically due to the heat.

It is also in a project box from radio shack which has no ventilation holes in it. I plan on buying a heatsink and giving the relay some "air conditioning" and see if the problem goes away.

I would like to use a mechanical relay, but I need one that actuates or charges the coil with less than 20 milliamps, which I cannot seem to find.

I'll follow up with ya'll and let you know what the problem was.

By the way, I DO have one EMR in the project, which ensures that if there is a power failure and the power then comes back on, the kiln will not resume operation. So I'm not a total wuss.

Thank you!!

[Dale M.]

Yah... I was kind of going in the direction bhhco went, he just go there first....

Dale

[smutboy420]

How far past the setpoint is it going when set at say 900 degrees? have you run the auto tune function on the controller yet?
You din't by any chance happen to of gotten a latching relay did you? They latch on or off each time voltage is applied to it. Rather then only being on while power is being applied to it.

About the flickering light bulb.
SSR's that do not have a isolation circuit in them Can allow a tiny bit of main power to bleed threw the SSR. Even when the relay if off some power can still leak threw the ssr. its a very small amount but it can build up enought to flicker the bulb. if you have some thing that draws only a tiny tiny bit of power like an led or some thing it will stay on. where as a light bulb will not light from the tiny bit of power leaking threw. Inless it builds up and the spikes will be a fliker as the bulb burns off the electric.
If yoiu measher it with a volt meter even when that light bulb is off there is still some voltage going to it.
A macanical relay on the other hand fully breaks the contact when the relay is off and you don't get any bleed threw voltage or tramp voltage.

[Passing Glass]

Thanks for the reply smutboy. To answer your questions, when the kiln stays on past the setpoint, it will continue to raise the temp forever, presumably until the coils burn out. I have attempted to run the autotune function but I don't believe it has ever finished because of the power problem. Since the relay is not operating correctly, the autotune can't do it's job and properly learn the system. I do not have a latching relay. I know that because the relay works properly at lower power levels, turning on and off as expected.

The info you gave me rgarding my test light was VERY helpful. I was not aware of the bleed through voltage that can be present in a solid state relay like you describe. Thanks!

[Dale M.]

Another thought..... Is thermocouple the proper type for controller (programed properly) and is thermalcouple placed properly in kiln?

Dale

[Passing Glass]

The thermocouple is type K and the controller is set for that. Everything works fine at lower power levels, so I'm sure the design is correct. I think it's a component problem, the SSR specifically. I'm going to mount the SSR on a heatsink today and see if it solves the problem.

Thanks!

[smutboy420]

Any thing stampped on or writen on the relay as far as Amprage or any thing??? You want it to be atleast 30% higher amp rating then what you will be drawing.
Sounds like you diagnosed it to the relay correctly. If its staying on when the controller is not sending the output signal to the relay.

BTW did you buy your relay brand new? or could it of been a used really by any chance?

[Passing Glass]

Hi again smutboy. The relay is an Omron G3NA-2258 and it is marked as follows:

24-240 vac / 25 amp load
5-24 vdc input

My kiln should be drawing 14-15 amps.

I attached 6 small brass heatsinks to the metal plate on the back of the relay, and butted them up against one another for better heat distribution, then I ran the kiln again. Same result.

But, when I put my hand on the heatsinks while it was running, they were HOT. Damn hot. Too hot to leave my hand on it for even a second without getting burned. The relay that I have in that setup should not be getting THAT hot should it? I mean, I know it's going to get "hot" but that was really hot. Thanks again.

[Dale M.]

Originally Posted by Passing Glass

Hi again smutboy. The relay is an Omron G3NA-2258 and it is marked as follows:

24-240 vac / 25 amp load
5-24 vdc input

My kiln should be drawing 14-15 amps.

I attached 6 small brass heatsinks to the metal plate on the back of the relay, and butted them up against one another for better heat distribution, then I ran the kiln again. Same result.

But, when I put my hand on the heatsinks while it was running, they were HOT. Damn hot. Too hot to leave my hand on it for even a second without getting burned. The relay that I have in that setup should not be getting THAT hot should it? I mean, I know it's going to get "hot" but that was really hot. Thanks again.

IF heat sinks are too hot to touch, the are too small..... Get one LARGE heat sink....Also heatsink needs to be outside any enclosure so air flow can remove heat... SSR overheating can also be indication it is defective, going into thermal runaway...

Also are you monitoring output of controller to SSR?... If "control" circuit is cycling appropriately to controller instructions, then the SSR/Heatsink is problem....

Dale

[smutboy420]

Your SSR must be bad if its getting super hot like that.

[Passing Glass]

Update for those of you who answered. Just thought I would let you know that I have solved the problem. It was definitely that the SSR was overheating and malfunctioning. I got a sheet of 1-2mm aluminum that I had in the kitchen. It was originally sold as a "thawing tray" for frozen food. You put the food on it and it thaws it very quickly. It's just a giant heatsink.

Well I clamped the SSR onto it with some thermal compund in between and it runs like a champ. The aluminum sheet is about 6 times bigger than the SSR, which I hope is overkill and I can cut it down some.

So, the moral of the story is, keep those SSRs COOOL!

[bhhco]

Thanks for the update! Now we have a good data point for others who might have a similar problem. Appreciate it

Me