How the SSR, the PID and the thermocouple work together.
The wall connected AC powers the PID. Utilizing the same connections, (high voltage as a terminal connection pass through) power to the SSR. When the PID senses an error in temp it provides a DC triggering signal to the low voltage side of the SSR. The PID is generally powered by AC line voltage, but will in most configurations, will only output a low voltage DC triggering signal. When the triggering signal is received the SSR closes the “switch” on the low voltage side which in turn closes the “switch” on the high voltage side. When the high voltage “switch” is closed high voltage can flow to the load. The PID is monitoring the change in temperature through the thermocouple. When the thermocouple reading indicates the temperature change is in accordance with the selected temperature value setting (SV) the PID stops sending the low voltage to the SSR which opens the “switches” and stops the action. When the PID senses the temperature is in variance with the SV the process begins again.
A tip when using SSR units: if you connect a DC control voltage to the input side and try to hook up a meter to measure continuity on the output/control side like you would with a dry contact relay, you get no continuity on the load side even though the triggering LED is lit. This has caused some people to think their SSR was bad. SSR's need an AC voltage on the load side and a feedback loop is created within the circuitry of the SSR to "switch" the load on or off. In the end it functions like a plain old analog "switch" but there is no physical switch or moving parts in a SSR. But for all appearances a SSR will function like a simple analog Single Pole Single Throw switch (like your homes light switch) with the PID triggering signal acting like the "finger" that flips the switch.
Many thanks to dward51 of Smoking Meat Forum and Mike W1 for the clarification and the tip.
What Do The Letters Mean? This answer is from Dave M at Google Groups:
The character "R" in any of these positions means that it has relay contacts
at the output.
The character "S" in any of these positions means that it has a solid state
relay (SSR) at the output.
The character "T" in any of these positions means that it has a silicon
controlled rectifier (SCR) at the output.
The character "N" in any of these positions means that there is no output
for that position.
For example, the number for model TA4-RNR indicates that the first output is
a relay; the second output is None; and the third (Aux) output is a relay.
Here is a diagram that explains the 110V AC side:
Left side no signal from PID Right side with signal from PID
Here is another that explains how no power passes through the SSR at ANY time:
Attachment 204279
Thank you to Dward51 of Smoking Meat Forum