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I have an old Omega CN-9121 PID Controller and a 105Watts 115V Fibre heater mantle with an inbuilt K-type CHromel-Alumel temperature thermocouple also attached into it. I plan to use the PID controller to control the temperature of the heater. Looking at the manual below:

Manual for PID

I decided that I wanted to use the inbuilt SP2 relay in the PID controller. From the wattage of the heater, I know it would draw a maximum of only 1A and is going to be below that. The SP2 output from the PID has a small 5V energized relay with 3A load capacity. I would like to know if it is safe to use the heater directly with the SP2 relay terminals of should I use the SP2 relay to run another AC relay for the heater?

Heater

PID PID rear Specs Relay- SP2

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I decided that I wanted to use the inbuilt SP2 relay in the PID controller.

That's fine.

From the wattage of the heater, I know it would draw a maximum of only 1A and is going to be below that.

That's fine too.

The SP2 output from the PID has a small 5V energized relay with 3A load capacity.

I didn't read the manual (but have used Omega controllers in the past) and don't know how you figured out that it uses a 5 V relay coil internally but it's not that unlikely.

I would like to know if it is safe to use the heater directly with the SP2 relay terminals of should I use the SP2 relay to run another AC relay for the heater?

enter image description here

Figure 1. Waveform for short-period duty cycles from an SSR. Source: LEDnique.com.

You can do either. For relay operation it would be normal to set the output duty cycle period to 5 s minimum. This is to reduce relay and contact wear. If your heater's thermal response is too fast and you require a shorter time then you should use the SSd output which will provide a 5 V signal for an SSR. This you can switch much more often - even down to 0.5 s but be aware that your SSR will, most likely, use zero-cross switching and that with very short periodic times you will be switching on for only a few mains cycles.

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