I want to switch ON/OFF a 12v, 6A SMPS from a standard PID Temperature controller for regulate heat in a cartridge heater. For this switching On/Off, either I may cut the input AC 230 V supply by a relay or may cut the 12v 6A DC output by a high current relay using the SSR/Relay o/p for the controller. My question is - which process is preferable ?

 

PID Control is Proportional Integral Derivative Control and On/Off control is what the name implies. They are two different means of control and you choose the type of control based on the application. Generally you control power to the heater element and not turn a power supply on and off. For example using a SSR (Solid State Relay) to power the element.

Ron

 

How accurate does the temperature need to be? A simple thermostat might be enough.

Bob

 

How accurate does the temperature need to be? A simple thermostat might be enough.

Bob

Another good point. When choosing a type of control you need to consider everything about the application.

Ron

 

I want to switch ON/OFF a 12v, 6A SMPS from a standard PID Temperature controller for regulate heat in a cartridge heater. For this switching On/Off, either I may cut the input AC 230 V supply by a relay or may cut the 12v 6A DC output by a high current relay using the SSR/Relay o/p for the controller. My question is - which process is preferable ?

Controlling (switching off/on) the output of the SMPS would be by far the best solution.

The way in which PID temperature controllers work – is to apply full power until the actual temperature reaches close to the set temperature. The controller then monitors the system response with no power input (temperature rise/fall rate) and adjusts the required power to reach the set point accordingly.

Once at temperature, the PID will give short bursts of power to the heating element to maintain the temperature (without overshooting or allowing the temperature to fall). These short burst of power can be less than 1 second – with such a short time period, the output of the SMPS may not even be energised, causing temperature control issues.

 

Controlling (switching off/on) the output of the SMPS would be by far the best solution.

The way in which PID temperature controllers work – is to apply full power until the actual temperature reaches close to the set temperature. The controller then monitors the system response with no power input (temperature rise/fall rate) and adjusts the required power to reach the set point accordingly.

Once at temperature, the PID will give short bursts of power to the heating element to maintain the temperature (without overshooting or allowing the temperature to fall). These short burst of power can be less than 1 second – with such a short time period, the output of the SMPS may not even be energised, causing temperature control issues.

Yau have answered right to the point, I was eager to know. Thank you.

 

I would also add that, when using a PID controller, with its frequent switching cycles, it's a good idea to use an SSR (or other solid state device as appropriate) instead of a mechanical relay. Mechanical relays will get worn out prematurely by the frequent switching cycles, where solid state devices don't suffer any wear and tear during those cycles.

 

When switching the low voltage DC, you do not need an SSR. All you need is a MOSFET rated for the voltage and current needed.

Bob

 

When switching the low voltage DC, you do not need an SSR. All you need is a MOSFET rated for the voltage and current needed.

Bob

Agreed!

(My comment was more of a general truth about PID controlled switching. To be honest, I'd forgotten the exact nature of the switched voltage in this case!)

 

A little note here about PID control of a process be it temperature control, flow control, pressure control or any of a wide range of parameters to control. A PID controller can have any of several output types and is not limited to any single type of output. Among the possibilities are DC Pulse, Solid State Relays internal to the controller to drive other devices, small mechanical relays to drive other devices, and a choice of analog current such as 4 to 20 mA or an analog voltage such as 0 to 10 Volts DC. The latter outputs of current and voltage are popular for driving phase angle fired triac power controllers for heating elements. When choosing a PID controller it is important to choose a controller which affords the desired output type for the application and when choosing the power control it is equally important.

PID controllers themselves also come in a few flavors such as manually tune, auto tune and those which afford both manual and auto tune function. Each method has some good and bad points. All of the features of the control should be considered when deciding exactly what needs controlled and how tightly the control needs to be.

Introduction To Temperature Controllers is a very good read on the subject and includes a few good examples of each method and how they can function. I was also a little off base in post #2 where I cited two types of control being On/Off and PID as there is also Proportional Control less the Integral and Derivative. My bad on that note.

Ron

 

When switching the low voltage DC, you do not need an SSR. All you need is a MOSFET rated for the voltage and current needed.

Bob

As others have said, you can have the PID controlling the 12Vdc power directly via a suitable transistor. If you were to use a SSR you run the risk of it not turning off due to the current flow being dc, and the internal triac remaining on.

 

If you were to use a SSR you run the risk of it not turning off due to the current flow being dc, and the internal triac remaining on.

Yes, that would require a DC type SSR which uses MOSFETs in the output rather than a Triac/SCR.

 

As others have said, you can have the PID controlling the 12Vdc power directly via a suitable transistor. If you were to use a SSR you run the risk of it not turning off due to the current flow being dc, and the internal triac remaining on.

Not all SSRs are TRIAC-based, but you'd have to choose carefully. I'm not suggesting there's any need to use one, just that you could (playing devils advocate!)

 

There is a lot more information needed for the proper selection of the elements.

Many power supplies have a REMOTE On/OFF logic control.

You would not generally want to turn on/off the mains. Operating into an inductive load also has issues.

These https://www.omega.com/pptst/SSRDC100V.html can work for DC with the proper heatsink and/or cooling.

 

One other important consideration of your circuit is what happens if the control circuit fails, continually supplying 72W to the cartridge heater. If no fire or other hazard would result, then no safety circuit is required.

But if you determine that a hazard would result – or an unacceptable level of damage inflicted on the equipment, then having a suitably rated thermal cut-out wired directly within the 12V supply should be included within the design.

As an alternative, you could include a watchdog timer monitoring the 12V supply to the heater cartridge – and if permanently energised for longer than expected, the system could be shut down.