Please could somebody help an electronics novice!
I am building an electronic controller for my pottery kiln; basically it is a mains powered PID temperature controller that operates solid state relays to turn on/off and regulate the high current load to kiln elements.

There are two temperature controlled (settable) alarm outputs on the PID that service low current relays which are used to feed simmerstats where the loads provide an alternate feed to the solid state relay. The purpose of the simmerstats is important to ramp the kiln loading in set stages. In principle the logic works fine.

The difficulty that I am having is the load to the simmerstats produce voltage spikes as the bimetallic strips make and break and these spikes are upsetting the PID. I've tried putting 470pf ceramics across the simmerstat feeds and that has perhaps improved things a little but not solved the problem.

Could anybody help me with a better solution? Thanks

 

Please could somebody help an electronics novice!
I am building an electronic controller for my pottery kiln; basically it is a mains powered PID temperature controller that operates solid state relays to turn on/off and regulate the high current load to kiln elements.

There are two temperature controlled (settable) alarm outputs on the PID that service low current relays which are used to feed simmerstats where the loads provide an alternate feed to the solid state relay. The purpose of the simmerstats is important to ramp the kiln loading in set stages. In principle the logic works fine.

The difficulty that I am having is the load to the simmerstats produce voltage spikes as the bimetallic strips make and break and these spikes are upsetting the PID. I've tried putting 470pf ceramics across the simmerstat feeds and that has perhaps improved things a little but not solved the problem.

Could anybody help me with a better solution? Thanks

First off are you certain your problem is indeed 'glitching' secondary to transients (as opposed, for instance, to 'bounce')?

Assuming the difficulty owes to transients you may wish to consider implementation of MOV{s}...

If, on the other-hand, the problem is issue of contact bounce and you do not have write access to the firmware, a satisfactory hardware solution is signal 'scrubbing' via a one-shot...

Best regards
HP

 

Something you can try, rather than just a capacitor across the switch contacts is a "RC Snubber" circuit.



A Google will bring up formulas for the values of R and C. Driving a resistive load like heating elements this should not be a problem. I see where you mention:

There are two temperature controlled (settable) alarm outputs on the PID that service low current relays which are used to feed simmerstats where the loads provide an alternate feed to the solid state relay.

Anything across the relay coils like a few flyback diodes? Also with good PID control in a kiln I don't understand the need for simmerstats but kilns aren't my forte either.

<EDIT> I see Hypatia's Protege is up and about this morning. </EDIT>

Ron

 

Something you can try, rather than just a capacitor across the switch contacts is a "RC Snubber" circuit.

View attachment 84864

A Google will bring up formulas for the values of R and C. Driving a resistive load like heating elements this should not be a problem. I see where you mention:



Anything across the relay coils like a few flyback diodes? Also with good PID control in a kiln I don't understand the need for simmerstats but kilns aren't my forte either.

<EDIT> I see Hypatia's Protege is up and about this morning. </EDIT>

Ron

Indeed! Addition of a dissipative element may well 'do the trick'

Best regards
HP

 

Please could somebody help an electronics novice!
I am building an electronic controller for my pottery kiln; basically it is a mains powered PID temperature controller that operates solid state relays to turn on/off and regulate the high current load to kiln elements.

There are two temperature controlled (settable) alarm outputs on the PID that service low current relays which are used to feed simmerstats where the loads provide an alternate feed to the solid state relay. The purpose of the simmerstats is important to ramp the kiln loading in set stages. In principle the logic works fine.

The difficulty that I am having is the load to the simmerstats produce voltage spikes as the bimetallic strips make and break and these spikes are upsetting the PID. I've tried putting 470pf ceramics across the simmerstat feeds and that has perhaps improved things a little but not solved the problem.

Could anybody help me with a better solution? Thanks

At what voltage does your kiln operate? You could install a TVS diode in series with a high capacity resistor, and place those components at the simmerstats feeds. That could help you get rid of those spikes

 

First off are you certain your problem is indeed 'glitching' secondary to transients (as opposed, for instance, to 'bounce')?

Assuming the difficulty owes to transients you may wish to consider implementation of MOV{s}...

If, on the other-hand, the problem is issue of contact bounce and you do not have write access to the firmware, a satisfactory hardware solution is signal 'scrubbing' via a one-shot...

Best regards
HP

Thanks, No I am not certain but think it correct; I have fitted a permanent voltage and current measure device on the overall input to the device in preparation for when I connect in the elements and what I observe is the voltage and current spiking "in tune" with the clicking that I can hear from the simmerstats and at the same point that happens the readings on the PID display go awal.

 

Don't know much about the size of your kiln, but when still working our small heat treat oven (basically a kiln) used mercury relays, because of 'point bounce' on mechanical relays.

 

Something you can try, rather than just a capacitor across the switch contacts is a "RC Snubber" circuit.

View attachment 84864

A Google will bring up formulas for the values of R and C. Driving a resistive load like heating elements this should not be a problem. I see where you mention:



Anything across the relay coils like a few flyback diodes? Also with good PID control in a kiln I don't understand the need for simmerstats but kilns aren't my forte either.

<EDIT> I see Hypatia's Protege is up and about this morning. </EDIT>

Ron

Thanks, yes I will try this. I have ordered a selection of higher voltage ceramics of different value and will also try adding a resistance as you suggest. Is this best on the live or the neutral side of the capacitor or does it not matter. I'll explain the reasoning for needing simmerstats to the response from Max below.

 

At what voltage does your kiln operate? You could install a TVS diode in series with a high capacity resistor, and place those components at the simmerstats feeds. That could help you get rid of those spikes

Thanks, Everything runs at 240V; not really sure I understand as my electronics is fairly fundamental. I'll look up TVS diodes on google and see if it makes more sense to me.

 

I must be missing something? If you are controlling a kiln with a PID loop, why the need for simmer stats?
Max.

Thanks, You need to ramp a kiln so for example you may want to heat the first 100C very very slowly with a 10% energy cycle which will take several hours, 100-350 can be ramped a bit quicker so you perhaps give it a 50% energy cycle. 350-1200 you might want it to go as quick as you can so you have it full on until it reaches 1200 at which point you want it to dwell (maintain temperature) for say an hour and then switch off.
The way I have set it is the turn on position operates from the first simmerstat until the first warning switch in the PID is triggered this sets conventional relays turning off the first simmerstat and turning on the second simmerstat this will then continue to heat until the second warning switch in the PID is triggered which again sets relays turning off both relays and instead using the standard PID heater feed.
The PID does have an energy regulation function but it is just one setting which would mean that I would have to be there and manually reset the PID each time I want to change the ramp rate which would not buy me anything. I did for a while consider buying multiple PID's but this seemed over the top; I may though have to go back to this idea if I can not get old simmerstat technology to work along side present day PID tech.

 

Thanks, You need to ramp a kiln so for example you may want to heat the first 100C very very slowly with a 10% energy cycle which will take several hours, 100-350 can be ramped a bit quicker so you perhaps give it a 50% energy cycle. 350-1200 you might want it to go as quick as you can so you have it full on until it reaches 1200 at which point you want it to dwell (maintain temperature) for say an hour and then switch off.
The way I have set it is the turn on position operates from the first simmerstat until the first warning switch in the PID is triggered this sets conventional relays turning off the first simmerstat and turning on the second simmerstat this will then continue to heat until the second warning switch in the PID is triggered which again sets relays turning off both relays and instead using the standard PID heater feed.
The PID does have an energy regulation function but it is just one setting which would mean that I would have to be there and manually reset the PID each time I want to change the ramp rate which would not buy me anything. I did for a while consider buying multiple PID's but this seemed over the top; I may though have to go back to this idea if I can not get old simmerstat technology to work along side present day PID tech.

I had a feeling this was about the ramp. What would be cool is a nice controller with ramp and soak features where you can for example say go to 200 F and take 3 hours to to it. Remain at 200 for 1 hour (Ramp and Soak). Then go to whatever and soak... . Anyway, give some of the suggestions a shot and see if they don't clean things up.

Ron

 

Something you can try, rather than just a capacitor across the switch contacts is a "RC Snubber" circuit.

View attachment 84864

A Google will bring up formulas for the values of R and C. Driving a resistive load like heating elements this should not be a problem. I see where you mention:

The TS's capacitor would've stopped the spikes if it had been a fair bit bigger than 470p, but each time the contact closing coincides with the crest of the AC wave - the contacts go "splat"!

The resistor in your snubber diagrame is absolutely vital.