Integrating temperature limit into PID / heat cartridge circuit

Thread Starter

bok_choy_boy

Joined Feb 11, 2021
5
Hello,

I'm building a heat press that uses a simple PID, SSR, thermocouple circuit to power a 300W heat cartridge circuit. My client asked that I integrate an overtemp limit controller with a separate thermocouple as added protection but I'm unsure how to do this and haven't been able to find much info online.

If anyone has advice for how to go about this, knows where I can find an overtemp limit controller, or is aware of another thread on this topic, please let me know.

Thanks so much
 

crutschow

Joined Mar 14, 2008
27,718
What does the thermocouple circuit consist of?
Perhaps another such circuit could be used, along with a comparator circuit, to shut turn off the SSR that controls the heat cartridge?
 

Danko

Joined Nov 22, 2017
1,134
If anyone has advice for how to go about this, knows where I can find an overtemp limit controller, or is aware of another thread on this topic, please let me know.
Look at this:
Thermocouple Conditioner and Setpoint Controller AD596/AD597
FEATURES
Low Cost (AD597AR_$4.88_Digi-Key)
Operates with Type J (AD596) or Type K (AD597) Thermocouples
Built-In Ice Point Compensation
Temperature Proportional Operation – 10 mV/°C
Temperature Setpoint Operation – ON/OFF
Programmable Switching Hysteresis
High Impedance Differential Input
ADDED:
Overtemp may occur because of SSR is damaged, so second controller should break power line at all.
 

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Thread Starter

bok_choy_boy

Joined Feb 11, 2021
5
Thanks for the feedback everyone.

@crutschow the thermocouple relays the sensed temp back to the PID, which then adjusts its cycle to deliver appropriate power for desired heat.

@Danko, for my application, I'm hoping to find a standalone overtemp controller than can be integrated without the need of building a circuit board. ideally something similar to a PID that, instead of delivering power to achieve desired heat, senses temp and when heat goes above set point, shuts off power.

@Alec_t it should await manual reset.

I think what I'll do is integrate an overtemp controller in series w the side of the SSR that is delivering power to the heat cartridge. When the temp goes above a set point, it shuts off power to the SSR.

If anyone is able to recommend any sources for such devices, it'd be much appreciated!

Thanks.
 

GetDeviceInfo

Joined Jun 7, 2009
1,877
So, what is your trip temp? What environment? Would a snap disk not work?
A client asked me the same thing when their glue pots flooded the elements. It turned out not to be heat, but ram pneumatics always on.
 
Something like thermal switches, switches, and snap-on discs (resettable) may work for you, which include: thermal protection in electrical equipment; protection against overloads, overheating and mechanical failures.
 

Thread Starter

bok_choy_boy

Joined Feb 11, 2021
5
Thanks for the feedback everyone. The temps we’re dealing with are quite high (+500F) and will vary depending on circumstances. I couldn’t find any snap discs/thermostats that suited our needs (tho I’m sure they exist somewhere). We’re going to use PIDs for temp control and watlow ez-zone pm express limit controllers for the over temp safety
 

Marley

Joined Apr 4, 2016
416
The reason for a second control is for safety in case a fault causes overheating. So you don't want another electronic system involving a thermo-couple or other type of sensor which could also go wrong.

You need a thermo-mechanical switch that breaks the supply to the heater. Non-resetable (a thermal fuse) - which melts at a set temperature is best, but can cause nuisance failures. Or a manually resetable or auto resetable thermal switch could be used. 500F is 260degC. So here is an auto-resetable type for 265degC. Many other types are available.
 

Reloadron

Joined Jan 15, 2015
6,061
Thanks for the feedback everyone. The temps we’re dealing with are quite high (+500F) and will vary depending on circumstances. I couldn’t find any snap discs/thermostats that suited our needs (tho I’m sure they exist somewhere). We’re going to use PIDs for temp control and watlow ez-zone pm express limit controllers for the over temp safety
That will do just fine.

Ron
 
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